You didn’t eat ANYTHING for an entire week!?! That sounds terrible! Why? Aren’t you worried about losing muscle? Weren’t you hungry? Didn’t you miss food? That’s so impressive.

Everyone

I’ve received the reaction above every time I’ve told someone about my fast. For the impatient among you, here are the quick answers:

You didn’t eat for 7 days? Explain. That’s right! I drank water, coffee, and tea. I also took sodium and magnesium supplements. Otherwise, I ate nothing–no meals, no snacks, no juice, nothing.

That sounds terrible! Actually, I was totally fine 90% of the time and felt pretty bad for about 10% of the fast. Now that I’ve done this once, I know what I did wrong and how to make the experience totally fine. Plus, I made it into a fun mini-science experiment and measured a bunch of stuff like body fat percentage and ketone levels.

Why? Fasting is like exercise–it provides a host of health-span and life-span benefits (ex. Reduced cancer risk).

Aren’t you worried about losing muscle? Nope. The human body is very conservative when it comes to burning muscle for energy. That’s literally what fat stores are for.

Weren’t you hungry? Sometimes, but mostly no. Hunger actually peaks around day 2 or 3 before dropping precipitously each day after that. By day 7, I wasn’t hungry at all.

Didn’t you miss food? More than I expected, to be honest. Not because I was hungry, but because I enjoy the hedonic pleasure of eating.

That’s so impressive.  Honestly, it’s not that difficult–pretty much anyone can successfully fast.

If your curiosity is piqued, the rest of this post goes into detail on:

• Why – all of the benefits and arguments that have convinced me to fast.
• Fasting Logistics – preparing and planning for my fast.
• My Experience – observations, thoughts, and measurements from my fast.

But first, a word of warning…

Safety First

I have no formal medical training and I know nothing about your health. While fasting should be safe for most healthy adults, there are some people for whom prolonged fasting is not safe (ex. extremely low bodyfat individuals) or who require medical supervision (ex. diabetics need to change their medication).

Furthermore, fasting has a short term immuno-suppressive effect and, with a global pandemic going on, might not a great choice at the moment.

What I’m trying to say is, if you’re feeling inspired, speak to a medical professional before you start fasting.

Why

“Why fast? It sounds unhealthy.”

Everyone

This is the first question on most people’s lips when I mention my fast. The chain of reasoning seems to go something like this:

• Malnourishment is bad.
• Fasting is not nourishing.
• Therefore, fasting is malnourishing.
• Therefore, fasting is bad.

Or:

• I feel good when I’m full.
• I feel bad when I’m hangry (spelling intentional).
• My body is telling me I should always be full.
• Therefore, fasting is bad.

Two years ago, I would have made similar arguments. Our culture inundates us with the message that we’re only a few hours away from hunger, malnourishment, and ill health–and that eating is the solution.

In the sections that follow, I’m going to attempt to give you all of the arguments and evidence that convinced me that fasting is not only not bad it’s actually an essential component of a healthy lifestyle–on par with exercise, sleep, diet, and social interaction. To do that, we’ll consider:

• Healthspan and  Lifespan – the goal that makes me interested in things like fasting.
• My Journey – the resources that brought fasting to my attention.
• An appeal to your intuitions – arguments based on intuition for why fasting ought to be safe and healthy.
• Science – science-based arguments with links out to research demonstrating the benefits of fasting.

To give you a taste of the science, here’s a quote from an excellent fasting literature review:

“Fasting … reduce[s] oxidative damage and inflammation, optimize[s] energy metabolism and bolster[s] cellular protection. In lower eukaryotes, [fasting] extends longevity … by reprogramming metabolic and stress resistance … In rodents [fasting] protects against diabetes, cancers, heart disease and neurodegeneration, while in humans it reduce[s] obesity, hypertension, asthma and rheumatoid arthritis.”

Wow, anything that provides such powerful benefits across millions of years of evolution should pique our interest.

The Goal: Healthspan and Lifespan

Before discussing why fasting is beneficial, it’s worth noting why I care. Put simply, I’m interested in maximizing my expected lifespan and healthspan. That means I both want to live longer (lifespan) and, more importantly, be healthy for as long as possible (healthspan). I want to increase my chance of being like this woman–91 years old and still doing gymnastics.

My Journey: From Skeptic to Believer

As I noted earlier, if you’d asked me about fasting 2 years ago, I would have argued that it’s unhealthy. So, what changed my mind?

The biggest influence has been Peter Attia’s excellent work, especially his podcast “The Drive“. He’s easily the best resource I’ve found on the topic of improving healthspan/lifespan and he discusses fasting quite frequently. On top of his podcast, he advises Zero Fasting, which is another excellent resource that is entirely focused on fasting.

Next, I’ve been following some of Rhonda Patrick’s work too. I find her content to be a bit harder to parse than Peter’s but it’s always excellent as well.

Last, I started intermittent fasting (aka. time-restricted eating) a while ago and have found that it improves my energy levels and appetite control. It has been an excellent way to reset my sense of hunger and prove to myself that I can feel great when I haven’t eaten for a while.

An appeal to your intuitions

Science is great and all but most of us lead with our intuitions. Therefore, before we jump into the science, let’s try to get an intuition for why fasting might be beneficial. Here’s a sketch of the argument:

• Our society sends the message that under-nutrition is the cause of our afflictions.
• In reality, over-nutrition and eating too often are bigger problems.
• We evolved to be capable of fasting.
• Fasting is the flip side of eating in the same way that exercise and rest or sleep and wakefulness are complementary states. Each needs to be balanced with the other.
• Fasting can be pleasant but, just like exercise, it takes practice.

Malnutrition/Under-nutrition

Our society is obsessed with under-nutrition. We’re constantly bombarded with messages like:

• Breakfast is the most important meal of the day.
• Eat 6 times per day to avoid hunger.
• Eat right after you exercise or you won’t build muscle.
• Eat more or you’ll be vitamin and mineral deficient.
• Eating several “superfoods” at each meal is the only way to stave off disease.

Here’s the problem: that’s not science, it’s advertising.

Sure, if we literally never eat, we’ll be malnourished, but that’s an issue for few people in affluent societies. In reality, the issues above are less a function of how much or how often we eat and more a function of what we eat. If all we eat is sugary junk, we will be hungry, lose muscle, and be vitamin deficient.

This isn’t a post on what to eat, however, so I won’t belabor that point. Just know that, when we’re shown a message to eat more, it’s usually because someone wants to sell us something–not because we’ll be healthier for it.

The Real Problem: Over-nutrition

The truth is this: disease in affluent societies is largely a product of over-nutrition, not under-nutrition. Obesity, Cancer, Type 2 Diabetes, Alzheimer’s Disease, Heart Attacks, Fatty Liver Disease–the list goes on.

Yes, the causes of these diseases are multi-factorial. We eat to much sugar, we don’t eat enough fiber, we don’t exercise enough, we don’t get enough sleep–again, the list goes on. I’d like to propose another, however: we eat too often.

Eating Too Often

In the last 100 years, we’ve gone from eating 2 or 3 times per day to eating 5 or 6. Furthermore, with the elimination of food scarcity, we’ve stopped fasting entirely–an occurrence that would have been quite regular in the past.

What happens when this process is reversed? Well, during WWII, food shortages resulted in a decline in diabetes–a hint that the problem is too much food.

An Argument from Evolution

I’m always cautious about making arguments from evolution–it’s easy to mislead ourselves with just-so stories. Still, as we’ll be looking at the scientific evidence later, it’s worth considering this argument too.

In the ancestral environment, we frequently would have encountered periods of fasting–only in the last 100 years have we eliminated hunger. If our bodies couldn’t handle periods without food, we would have gone extinct millennia ago. If we started wasting muscle at the first hint of hunger, we frequently would have become weak and died before finding our next meal. It makes no sense whatsoever to use protein for energy when our body has weeks worth of energy stored in body fat–tissue with almost no other purpose.

This idea alone implies that we should at least tolerate fasting well. Admittedly, needing to tolerate fasts does not imply they must be health-promoting–that’s where the science comes in and it’s coming up shortly.

Eating and Fasting are like Resting and Exercising or Sleeping and Waking

As the science below shows, fasting is the flip side of the coin that is eating. Just like rest and exercise or sleep and wakefulness, fasting and eating are complementary states that need to be balanced.

When we eat, our body enters into a fed state. As with sleep and exercise, however, it’s entirely possible to have too much of a good thing. Our bodies need to enter into a fasted state to balance the fed state. When fasted, our body initiates essential processes that are turned off by eating–cleanup of cellular waste, breakdown of old cells, renewal of important bodily systems, and more.

Without these processes, waste products build up (Alzheimer’s), cells turn malignant (cancer), and systems breakdown (immune degradation). Without fasting, our body never has a chance to perform these essential functions–and we suffer for it.

It can actually be pleasant

Some people will counter that fasting is uncomfortable–when we’re hungry, our body is telling us to eat and we should listen to it.

Well, the same could be said of someone new to exercise. When they exercise for the first time, it’s uncomfortable–but we all accept that it’s actually good for them

As anyone who exercises frequently will tell us, once we’re used to it, exercise is pleasant–it can even be euphoric.

Guess what, so can fasting. Fasting isn’t like being hangry for 7 days, it’s a completely different experience from what we feel after not eating for 8 hours and, like exercise, it gets easier the more we do it.

Bring on the Science!

Alright, enough intuition, next up, let’s look into the science of fasting–science that is overwhelmingly positive. Before we begin, some caveats (because I love nuance):

• I’m not a scientist or a doctor. I’m bound to get at least some of the interpretation below wrong–please don’t treat my words as gospel.
• The distinction between intermittent fasting (12 to 36 hours) and prolonged fasting (2 or more days) is a blurry one. I cite studies for both in the content below. Some of the benefits of fasting accrue in both, while others are unique to either intermittent or prolonged fasts.
• Human fasting studies are expensive and, therefore, less common than animal studies.
• Animal studies are harder to interpret. They’re also harder to classify as intermittent or prolonged. Some studies call 24 hours of fasting for a mouse “intermittent” but mice die after 3 days without food while humans can survive for 3 or more weeks. If one day is “intermittent” to a mouse, then one week might be “intermittent” to a human.

With that out of the way, let’s start with fasting’s effect on insulin and glucose.

Fasting improves insulin sensitivity, preventing obesity and type 2 diabetes

Obesity and type 2 diabetes are both largely diseases of excess insulin. Through fasting, we can reduce our insulin levels and improve our insulin sensitivity.

Insulin

When we eat, our body breaks down food and transports glucose into our bloodstream. In response, our pancreas produces insulin–a hormone with the following essential functions (there are  more, these are just the ones we’re interested in):

• Insulin moves glucose from our bloodstream into our cells (including muscles and the liver)–it’s how our cells get the glucose they need to burn for energy and how our body stores glucose for later use.
• Insulin tells our body to convert excess glucose into stored body fat.

Insulin Sensitivity – Combatting type 2 diabetes

Insulin sensitivity tells us how effective insulin is at moving glucose into cells and the liver. If we are insulin sensitive then our blood glucose levels are well-controlled because a relatively small amount of insulin moves a large amount of glucose out of the bloodstream and into our cells. This is important because high blood glucose levels are strongly correlated with all-cause mortality (how likely we are to die from any cause).

We can estimate insulin sensitivity by measuring how much glucose is in our blood when we haven’t eaten for 24 hours. In the following chart, we can see the all-cause mortality hazard ratio of blood glucose levels (a proxy for insulin sensitivity).  Let’s break that down:

All-cause mortality=Death from any cause.
Blood glucose levels=A measure of the amount of glucose in a person’s blood.
Hazard ratio=A multiplier indicating how much risk is associated with a given measurement.
All-cause mortality hazard ratio of blood glucose=A multiplier which tells us how much more likely one person is to die than another, based on their fasted blood glucose level.

The chart below indicates that someone with a fasting glucose of 130 mg/dL is 1.4 times as likely to die (from any cause) as someone with a fasting glucose of 80-94 mg/dL. As fasting glucose rises (reduced insulin sensitivity, increased insulin resistance), the chance of death goes up dramatically.

Type 2 diabetes is a disease of insulin resistance–it’s the right side of the chart above. Anyone over 100 mg/dl is pre-diabetic and anyone over 126 mg/dl is fully diabetic–a diagnosis with a host of negative health consequences including hypertension, vision loss, and more.

Even if we don’t have diabetes or pre-diabetes (though 1 in 3 Americans do), we should be motivated to keep our glucose levels in check as they tend to worsen with age. Furthermore, the effects of hormones always fall on a curve–126 mg/dl is an arbitrary cutoff and 125 is essentially just as bad. At the end of the day, the more insulin sensitive we are, the better–our arteries will be healthier, our energy levels will improve, and we’ll live longer.

Clearly, we want to be insulin sensitive–and this wouldn’t be a post about fasting if I didn’t tell you that fasting increases insulin sensitivity (1). A thorough discussion of possible mechanisms would take a few paragraphs but a likely one is simply that, when we fast, we deplete our cells of stored glucose. Then, when we eat again, there’s more room in our cells so less insulin is needed to bring in glucose.

Fat Burning – Reducing obesity

On top of promoting the storage of new fat, insulin prevents the breakdown of existing body fat. This fact implies that, if our insulin levels are constantly elevated we won’t be able to lose weight. Furthermore, every meal we eat raises our insulin levels for 1 to 3 hours–3 hours during which our body will not be burning fat.

In other words, if we eat every 3 hours, our insulin levels will constantly be elevated, our body will never switch over to burning fat, and we’ll never lose weight. Is it any wonder that eating 6+ times a day doesn’t improve weight loss?

This is where fasting comes in. When we’re fasting our insulin levels drop below 50 pmol/L and our body starts burning fat for energy. As discussed above, however, if we’re eating, our insulin levels will be elevated and our body will not burn fat.

“But”, you reply, “I’ve been told that eating 6+ small meals each day will reduce my hunger levels, which means I’ll eat fewer calories, which means I’ll lose weight.”

Unfortunately, that doesn’t work for most people. As we’ve discussed, when we eat frequently, we constantly produce insulin. When we constantly produce insulin, not only do we become insulin resistant, we also become leptin resistant. That’s important because leptin signals to our brain that we are not hungry–if we’re leptin resistant we’re constantly hungry. As a result, eating more often actually makes us hungrier, resulting in more calories consumed, not less.

Fortunately, fasting also improves leptin sensitivity. If we restrict the amount of time we are eating (fasting) our insulin levels drop and we become more sensitive to both insulin and leptin. Thanks to improved leptin sensitivity, our appetite is suppressed. Furthermore, because our insulin levels are low, our body will burn fat for energy and we will lose weight.

Fasting stimulates autophagy, protecting us from neurodegenerative diseases and cancer

Our cells are constantly producing waste products and being damaged as a side-effect of normal metabolism. As we age, these waste products and cellular damage accumulate in our bodies and, eventually, impair important cellular processes.

Waste products cause a variety of inflictions but among them is the most feared disease of all: Alzheimer’s. Cellular damage contributes to another dreaded disease: cancer. While it’s impossible to eliminate all risk, fasting is a powerful preventative measure for both of these problems.

Fasting stimulates a process called autophagy. When autophagy is activated, our cells collect cellular waste and damaged components into the lysosome. There, they are broken down and recycled in a process of cellular renewal.

With the waste cleared away, our cells are healthier and better able to perform their function which, for brain cells, manifests as improved cognition. Similarly, once a damaged cellular component is recycled, our cell creates a new healthy replacement. The end result is a cell that is less likely to turn cancerous the next time it is damaged.

Unfortunately, even a small amount of food prevents autophagy. Once again, fasting is the solution–and, in this case, intermittent fasting isn’t sufficient. When we don’t eat, our body reduces mTOR (a growth hormone), increases AMPK (a catabolic enzyme), and depletes our liver glycogen (stored energy which takes 24+ hours to deplete). All of these need to happen to maximize autophagy and it can take up to 3 days without food to fully ramp up the process.

Fasting regenerates the immune system, inhibiting cancer and improving viral response

As we age, our immune system becomes less effective. That’s a big deal because it curbs our ability to fight off viral infections–turning an illness that is trivial for a young person into a potentially life-threatening event. If our immune systems didn’t degrade with age, the flu wouldn’t kill tens of thousands of Americans each year.

Furthermore, our immune system is an essential component of cancer prevention. When cells turn cancerous, our immune system can destroy them before they proliferate and turn into tumors. As with viral vulnerability, a weakened immune system is one of the reasons that age increases cancer risk.

Continuing with the trend, fasting can help by renewing the immune system. When we don’t eat, our body reduces the production of the growth hormone IGF-1 and the enzyme PKA which triggers the transformation of blood stem cells into new immune calls–with the end result being a healthy, diverse, responsive immune system.

Fasting lowers blood pressure, reducing risk of heart attack

We’re all aware that we’re supposed to have a blood pressure below 120/80 but nearly half of Americans have hypertension.

As with every other condition we’ve looked at, fasting can help. In one study, 13 days of fasting brought subjects blood pressure down by 20/7 mmHg–bringing over 80% of the participants below the standard 120/80 cutoffs. In another study, ~10 days of fasting brought subjects blood pressure down by an average of 37/13 mmHg.

These improvements could translate into as much as 3 years of extra life for individuals in the former group and 7 years for individuals in the later. (1)

And so much more

If all the benefits above aren’t enough, fasting also:
Stimulates Apoptosis–destroying potentially cancerous cells (1,2)
Stimulates BDNF, increasing neurogenesis and reducing neurodegenerative disease risk
Reduces inflammation
Increases gut microbiome diversity
Reduces gut inflammation and permeability

I’m convinced, are you?

I hope that all of the arguments and evidence above have you thinking differently about fasting. While it’s unlikely that every study cited above will hold up to replication, it’s even less likely that all of them are wrong. At this point, when I consider all of the arguments and evidence, I find it overwhelmingly likely that fasting is a powerful, health-promoting therapeutic.

This is not to say that I am without doubts.

My foremost concern is the challenges of interpreting health studies. In particular, few studies take individuals who are already doing everything right (exercising, eating a balanced diet, sleeping well, forming strong relationships, etc) then ask them to add fasting. I speculate that they would still see long term benefits but I wonder if the benefits would be significantly reduced because they’re already doing everything else right.

The next question I have is what is the optimal fasting dose? Should I fast for 7 days twice per year? 5 days 6 times per year with intermittent fasting the rest of the time? No one knows–and it’s going to stay that way for the foreseeable future. For now we can only speculate and do our best with the available evidence…which brings us to the logistics of my fast.

Logistics

Before starting my first prolonged fast, I had a lot of questions. There’s really only one rule: don’t eat. Nothing in life is so simple, however. This section answers the questions I had about how to fast.

No eating, got it. But is there anything I *should* consume?

My first question was if there was anything I needed to continue consuming to stay healthy. As it turns out, there are a few essentials.

Water

I quickly discovered that I not only needed to continue drinking water while fasting, I actually needed to increase my water consumption. (FYI, dry fasting without water is dangerous.)

Normally, we get 20% of our water from food. As I wouldn’t be eating, I needed to add 20% more water to make up for what I would be missing. On top of that, my kidneys would be excreting extra water due to lowered insulin levels so I would need to drink even more water to make up for that.

Fortunately, hydration levels are easy to monitor so, while it was good to be aware, I didn’t actively think about this during my fast.

Electrolytes – Sodium and Magnesium

The other important supplement I learned about was electrolytes. If my fast was less than 3 or 4 days my body would have enough stored. As I was doing 7, however, a lack of electrolytes might cause headaches, dizziness, and lethargy.

As a result, I would need to consume sodium and magnesium–two important electrolytes that are easy to supplement safely. I bought two kinds of magnesium pills and decided, for some reason, to just add table salt to lime water for sodium (honestly, not great, I’ll use salt pills next time).

Next question: what *can* I consume without breaking my fast?

If you ever research fasting, the first thing you’ll notice is the number of people asking “But, can I have X?” or “Does X break my fast?”.

As with all general questions, the answer is that “it depends”. Depending on the goals of a fast, the things that can be consumed will vary. In my case, I decided that the only things I would be consuming were:

• Water
• Saltwater with lime (to mask the saltwater flavor)
• Magnesium pills
• Coffee
• Tea

Some people will consume broth or MCT oil. I tried making broth but was concerned that it had too many calories so I didn’t actually drink it.

I have to say, thank goodness for coffee and tea, they got me through the toughest moments of my fast.

Do I still exercise? If so, how much?

Recall that I wasn’t worried about losing muscle during my fast. Exercise is a big reason why.

There are two powerful ways to promote the preservation of lean muscle mass. The first is eating a high-protein diet (not allowed while fasting) but the second is exercise. I have a detailed workout regimen and paired my fast with a de-load week.

What should I track?

I turned my fast into a mini-science experiment by tracking a bunch of stuff along the way. I recorded:

• Body weight
• Body fat %*
• Muscle %*
• Water %*
• Blood pressure
• Resting heart rate
• Heart rate variability
• Everything else my Oura ring measures
• Blood Ketones**
• Blood Glucose**

*As measured by my scale each morning. These are of such dubious accuracy that I’m not convinced the values are informative.

**As measured by a keto-mojo: morning, pre-workout, post-workout, bedtime. Ketones are created from fat for use as energy in the brain–they were a helpful indicator of how deeply I was into my fast. Blood glucose is the normal source of cellular energy while in a fed state.

Fasting

Alright! We know why I fasted. We know the plan. It’s about time I tell you how it went!

Friday – 0 Days of Fasting

I started fasting Friday morning at 9:30 am after eating a large breakfast.

Friday was, frankly, effortless. I regularly go 16 hours without food for 16/8 intermittent fasting so skipping food for one day wasn’t unusual. I worked all day, enjoyed a light workout in the afternoon, and took advantage of all the time I wasn’t eating by poking myself in the finger to measure my glucose and ketone levels.

Despite ending the day in good spirits, I have to say: it was daunting to go to bed knowing I wouldn’t be eating for another 6.5 days.

Saturday – 1 Day of Fasting

Day two started off pretty typical. I drank a large, delicious mug of coffee and had excellent energy levels.

It wasn’t until early afternoon, just before my workout, that the first sign of change arrived – when I checked my ketones I found they’d jumped to 1.6 mmol from their typical 0.2-0.6. I’d arrived in ketosis and I still wasn’t hungry!

Working out changed that quickly, however. I normally eat post-exercise and, after a light 1.5 hours, I was absolutely ravenous. It took an effort of will to reach, not for the fridge, but for my ketone meter-which had an unexpected result. I’d dropped from 1.6mmol to 0.5 mmol–so much for being in ketosis…

Honestly, the remainder of Saturday was a struggle. I was sluggish. I was unfocused. I spent an inordinate amount of time contemplating food. Still, I’d made it 1.5 days–the longest I’d ever gone without food!

Sunday – 2 Days of Fasting

Sunday started with a few interesting changes:

• My weight was down significantly from 160 lbs to 158.6 lbs for a total of 1.4 lbs, with a corresponding drop from 11.7% to 11.2% body fat.
• My blood glucose, which normally hovers in the high 80’s, was down to 62. A huge drop and a sign that my body had depleted its stored glycogen. As expected, my morning blood glucose hovered in this range for the entire fast.

Based on everything I’d read, I expected this to be the most difficult day of the fast. I have to admit, I was so hungry on Sunday that I found it distracting. I was doing well in the morning–my ketones had jumped up to 2 mmol–but, as soon as I worked out, they crashed to 0.2 mmol and I was left starving.

As it turns out, ketone levels and exercise interact in a complex manner but, in my case, the trend was apparent for the entire week. Every time I exercised, they dropped significantly–and that’s a bummer because elevated ketones are one of the things that make you feel good while fasting.

My ketone levels dropped by 1 to 2 mmol with every fasted workout. The exceptions on this chart (Workout 0 and 7) were completed within a few hours of eating.

With my ketone levels bottomed out I was, once again, starving, sluggish, and unfocused. Fortunately, my appetite began to subside by evening–though my constant musing about food was not helped by friends talking about it incessantly on a video chat.

Monday – 3 Days of Fasting

After a day of distraction on Sunday, Monday brought welcome relief. I woke with higher ketone levels than the previous morning and felt pretty good. All in all, Monday was one of my best days in the entire fast–I actually felt pretty good.

Interestingly, my ketone levels continued to increase throughout the entire fast.

My last meal for the week was at 9:30 AM on day 0. My next meal was at 9:30 AM on day 7 (before the measurement shown here).

This trend wouldn’t continue forever but I thought it was interesting that, even after 7 days, I hadn’t reached a peak in ketone production.

Not only did I feel good on Monday, I also felt no hunger for the entire day. Thanks to high energy levels and a lack of appetite, I had a solid workout and was super focused while working.

Fun fact: Excellent focus is one of the short term benefits many people experience while fasting. In my case, I found fasting and work to be synergistic. Deep work meant I wasn’t thinking about eating and being in a fasted state made it even easier to focus on deep work!

Tuesday – 4 Days of Fasting

Brutal. Brutal is the only way to describe Tuesday. While I wasn’t hungry and didn’t think much about food, I was really lethargic and a bit woozy. I felt exhausted trying to workout and figured I must be doing something wrong.

A few minutes of research informed me that I was likely under-supplementing sodium. I’d been taking ~2 grams per day–likely sufficient if I wasn’t exercising–but I was exercising. I immediately bumped my daily consumption to nearly 4 grams and, by evening was feeling a bit better.

Interestingly, my scale detected virtually no drop in body fat % from Tuesday to Wednesday despite a big drop from Monday to Tuesday. This makes me wonder if consuming extra sodium on Tuesday made me retain more water which the scale detected as less fat loss. If that’s the case, it doesn’t increase my confidence in the scale’s bf% accuracy.

Day to Day	Body Fat % Reduction
Friday to Saturday	0.2
Saturday to Sunday	0.3
Sunday to Monday	0.3
Monday to Tuesday	0.5
Tuesday to Wednesday	0.1
Wednesday to Thursday	0.5
Thursday to Friday	0.2

Tuesday brought on another big change: my cold tolerance plummeted. I normally only wear shorts to exercise but I had to wear sweatpants and a sweater to stay comfortable–and I was still a bit chilly!

This wasn’t a surprise–the thyroid modifies the production of hormones regulating body temperature in response to fasting.

My chill continued into the evening and, when I went to bed I grabbed an extra blanket. My wife, who often complains about the temperature in our bedroom exclaimed, “now you know how I feel!”

Wednesday – 5 Days of Fasting

Wednesday was another really good day. With my sodium levels corrected, I felt focused, wasn’t lethargic, and even managed a decent workout. Lesson learned.

While I felt very little hunger at this point, I was starting to miss food. There’s something to be said for the pleasure of eating and I was really missing that at this point. I actually think this was exacerbated by Covid–my days are a bit boring, at the moment, punctuated with few “simple pleasures” outside of eating and exercising so the loss felt more pronounced.

Thursday – 6 Days of Fasting

My last day of fasting! I woke up this morning to my lowest resting heart rate ever: 39 BPM. I typically sit at around 45 BPM and this measurement confirmed a trend I’d noticed over the week–the deeper my fast, the lower my resting heart rate.

At first, I wasn’t sure if this low heart rate was due to fasting or the fact that I was on a de-load week. That answer arrived on Saturday morning–my RHR had jumped back up to 46 BPM after refeeding even though I still took Friday’s exercise easy.

I was a bit more lethargic on Thursday than I had been the day before but I still felt way better than I did on Tuesday. My cold tolerance, on the other hand, reached an all-time low. I drank several cups of tea and kept blankets on my office chair to keep warm throughout the day. If this is how my wife feels all the time, we might have to turn up the heat!

In the evening, with my next meal only 12 hours away, I found myself musing about food again. I still wasn’t hungry but I missed food. I missed the daily dopamine hits. Even more, I missed the way I normally feel. I normally feel excellent and have lots of energy for the entire day and, while I didn’t feel bad while fasting, I didn’t feel excellent either. I knew eating would bring that back.

Friday – 7 Days of Fasting – Refeed

When I awoke on the day of my refeed I discovered that I’d lost 9.6lbs, increased my muscle percentage by 1.1%, and decreased my body fat percentage by 2.1% (according to my scale, anyway). Quite the change!

I ended my fast at 9:30 AM with a few nuts and some protein powder–I made sure to start slow as I knew my stomach had shrunk. Overeating after a fast can be uncomfortable or even outright dangerous if done too aggressively.

I have to say, I have never tasted such delicious almonds in my life. My sense of taste was heightened beyond recognition after a week without food.

I ate again at 10:30–some hummus and veggies this time, expanding my food volume a bit. Then, at 11:30 I had my standard breakfast… and I crashed. By 12:30, I was falling asleep at my desk and needed a 30-minute nap to finish out the day.

Unsurprisingly, a few meals were insufficient to bring my energy levels back to normal. Still, I went to bed with food in my belly for the first time in a week–I was content.

Saturday – 1 Day Since Refeeding

I woke up feeling completely normal on Saturday. I didn’t feel a strong desire to overeat, my energy levels were back (I had a great workout), and I was generally productive.

As expected, my weight bounced back significantly–3.8lbs from the morning before. This is not to say that I put on 3 pounds of fat–most of the weight lost during a fast is water. Once I started eating again, my body started storing more glycogen, which means storing more water, which means a big rebound in total body weight.

My scale seemed to think I increased my body fat % by 0.6% from Friday to Saturday–a change that works out to nearly 1 full pound of fat. I find that pretty hard to believe (I didn’t eat enough food to create an entire pound of fat), which confirms my skepticism of the bf% changes measured by this scale.

Day X

Remember that precipitous weight drop? Well, it rebounded quickly to around 158.5lbs for a total weight loss of 1.5lbs. That’s in line with my expectations of 0.5lbs per day of fasting minus a bit of overeating on the weekend.

My bodyweight dropped by 9.6lbs then rebounded quickly.

Similarly, my bodyfat rebounded up to 11.2%, for a total reduction of 0.5% from its original 11.7%. That seems a bit low in comparison with my weight loss but is at least directionally correct.

My bodyfat % dropped by a few percent then rebounded quickly.

In case you’re still wondering about muscle loss… here’s my muscle percentage across the fast.

My muscle % is the inverse of bodyfat, which is what you would expect if my body was burning fat, not muscle.

As a final observation, I was surprised by how much healthier my gut felt post fast. I normally have low-level IBS every day but, after fasting, I’m less bloated, less gassy, and my stool quality has significantly improved. I haven’t felt bloated in the entire month since my fast and, honestly, that alone is worth one week without food.

But will you do it again?

Absolutely. In fact, I’ve already done another 5-day fast in the month it took me to publish this post.

I’m mostly convinced that fasting-both IF and prolonged-are on par with exercise, sleep, diet, and social interaction in terms of their importance for healthspan and lifespan. I’m more than willing to skip a couple of meals if it means significantly improved health throughout my life.

I don’t plan on doing a full 7-day for at least 6 months but I’m planning on making 5-day fasts a regular part of my life–on the order of one every 2-4 months. There are two big reasons I’m switching to 5-day fasts:

• Duration: 5 days is less than 7 (big surprise there) which makes it less daunting and less disruptive to normal life.
• Timing: I can fit a 5-day fast into the week (start Sunday evening, end Friday evening) while still enjoying my weekends (good workouts, go to a restaurant, eat with friends, etc).

Finally, the most important benefits of prolonged fasting (autophagy, deep ketosis, glycogen depletion, etc) start on day 3 so I should still get most of the benefits, even if I’m sacrificing a bit of “depth” in the fast.

For what it’s worth, I’ve read that prolonged fasting gets easier each time–something I’ve certainly found to be true with intermittent fasting. It may be the case that, as I do more prolonged fasts, I’ll find them easier and start extending to 7 days once more.

Questions

Did I miss any of your burning questions? Let me know, I’d be happy to answer anything I know the answer to.

Safety Last

As a final reminder, while fasting is safe and healthy for most adults, there are people who either should not fast or who require medical supervision while doing so. Talk to a qualified medical professional before you dive in.