Why Fast?

Long gone are the days when the benefits of fasting are ascribed to some vague nonsense about “detox” or “cleansing.” The problem with claims of “detoxification” is that no one can ever identify what toxins are being eliminated. Instead, fasting has evidence-based benefits related to rebooting the immune system, lowering blood sugar, improving insulin sensitivity and slowing the aging process. It’s sustainable, because it saves resources, completely free, and possible to fit into any schedule or diet. Finally, it functions for me as a beneficial exercise in self-discipline.

I’d like to first offer a disclaimer: I am not a doctor and in many cases you should consult one before fasting. If you are on medication for diabetes, fasting may cause your blood sugar levels to drop to dangerous levels. Pregnant women and children should never fast. Anyone with a history of eating disorders should probably never fast. Partly for this reason, I don’t promote fasting for weight loss (this will disappoint some, but it’s too controversial for me). Finally, I’d advise caution and medical supervision on long fasts. Be especially aware of the danger of depleted sodium if you are fasting for a week or longer (bone broth is usually advised in these cases, but for obvious reasons, I’d stick to vegetable broth).

There are many types of fasting and many fasting regiments. I’m interested in water-only fasting because it’s the most straightforward, although disregard that for long fasts, which, as mentioned, require a sodium source. Juice fasts don’t make too much sense to me for reasons that will become clear. There are many patterns, such as intermittent fasting (only eating for a set number of hours each day), the 5:2 diet (dramatically reducing calories for 2 days a week), alternate day fasting and extended fasting (which has been defined as fasts longer than 42 hours¹). My own fasting regimen involves fasting for 3 days once a month. I like extended fasting because it’s simple and particularly effective in producing health benefits.


Much of the benefits of fasting are related to a process called autophagy, literally Greek for “self eating.” Dr. Jason Fung, a nephrologist and authority on fasting, explains that “autophagy is a form of cellular cleansing: it is a regulated, orderly process of breaking down and recycling cellular components when there’s no longer enough energy to sustain them. Once all the diseased and broken-down cellular parts have been cleansed, the body can start the process of renewal” (par. 3, sec. “Slowing Aging,” Ch. 7). Along with autophagy, fasting also promotes aptosis, a programmed suicide function for older cells to clear the way for newer cells. This is a fundamental part of healthful regeneration of the body. Dysregulation of this system causes cancer and it’s certainly plausible that fasting is protective against cancer².

Dr. Fung explains why autophagy doesn’t occur when we eat:

Increased levels of glucose, insulin, and proteins all turn off autophagy. And it doesn’t take much. Even as little as 3 grams of the amino acid leucine can stop autophagy. Here’s how it works: The mammalian target of rapamycin (mTOR) pathway is an important sensor of nutrient availability. When we eat carbohydrates or protein, insulin is secreted, and the increased insulin levels, or even just the amino acids from the breakdown of ingested protein, activate the mTOR pathway. The body senses that food is available and decides that since there’s plenty of energy to go around, there’s no need to eliminate the old subcellular machinery. The end result is the suppression of autophagy. In other words, the constant intake of food, such as snacking throughout the day, suppresses autophagy. (par. 5, ibid.)

This is why juice fasting is unappealing to me. You are spiking your blood sugar and defeating much of the purpose of fasting.

Immune System

Research related to the immune system is a major part of what inspired my 72 hour fasting regimen. White blood cells actually decrease during the fast, but upon eating again, stem cells start producing an army of new white blood cells. Combined with the beneficial effects of autophagy in getting rid of old or damaged cells, this essentially produces a brand new immune system. This is particularly useful for cancer patients undergoing chemotherapy.


A little-touted benefit of fasting is that it saves on resources. I fast once a month for 3 days. Assuming a 30 day month, that’s theoretically a 10% reduction of my food intake. Granted, I will eat more than normal after the fast is over, but I will not even come close to making up for calories lost. Furthermore, those 3 days provide no opportunity for me to create food waste by failing to clean my plate. So long as I’m responsible about not letting food in my fridge spoil, I’m doing right by the environment while fasting.

Strange Bedfellows: Who Encourages Fasting and Why?

Fasting as a vegan can make for strange bedfellows. True, there is some support for fasting in the loopy, pseudo-scientific contingent of veganism. However, fasting has more widely been taken up by the low-carb and paleo communities. I already referred to the popularity of bone broth in the extending fasting community, which says something about the place of veganism there. Some of the benefits of fasting rest on an insulin-centered view of weight gain and other ailments. Fasting drops your blood sugar levels, thereby limiting the need for insulin and increasing insulin sensitivity. Dr. Fung uses fasting to help type 2 diabetics wean off of insulin, believing it to be the cause of obesity. To him, giving diabetics insulin is like fighting fire with gasoline.

When we consume sugar, insulin sends a signal for the glucose molecules to enter our cells. The problem of insulin resistance occurs when cells are already overstuffed with glucose. The traditional approach for treating type 2 diabetes has been ever increasing doses of insulin, forcing even more glucose into already saturated cells. Fasting takes the opposite approach by depleting the cells of glucose, which means insulin doesn’t have to do so much work to push glucose into the cells. (Fung, par 1-7, sec. “Why Fasting Works for Type 2 Diabetes,” Ch. 6).

I am swayed by some of these arguments, which puts me opposite the traditional vegan stance in a major debate on nutrition. Mainstream nutrition, along with most vegans, is devoted to the energy balance hypothesis–asserting that weight gain is simply a math problem–if calories consumed are greater than calories burned, you will gain weight and vice versa. Increasingly critics of the energy balance hypothesis have claimed that this narrative is missing something crucial: the role of hormones, especially insulin, in weight gain. Critics such as Gary Taubes, Dr. Fung and most of the paleo community advise limiting carbohydrates because they increase blood sugar, thereby increasing insulin, which in turn sends a signal to put on weight. Dr. Fung states bluntly that insulin causes weight gain and that fasting should be used as a weapon against the obesity epidemic. This a very basic explanation of why low-carbohydrate diets are in such vogue. However, the story of obesity, nutrition and insulin is really complicated and the value of fasting for weight loss is disputed by mainstream sources.

If the body’s glycogen stores are depleted during fasting, what does the body use as a fuel source? After 2 to 3 days fasting, the body goes into ketosis, the so-called “fat-burning” stage of metabolism. In my opinion, fasting is the most painless way of entering ketosis. A 3 day fast has the virtue of allowing your body to enter ketosis for a brief period, without extensively monitoring your carbohydrate intake. Traditional ketogenic diets are meat-centric, often low in fiber and high in cholesterol. A vegan ketogenic diet may be possible, but it’s very difficult. If not for fasting, going through the trouble of entering ketosis simply wouldn’t be worth the effort.

 A Final Note on Difficulty

Most of the time when I tell people I fast, I get the same reaction I get when I tell them I’m vegan: “Oh, I could never do that.” As amazing as the health benefits of fasting are, many of them remain abstract to me. I’m lucky enough to have no actual problems with insulin resistance or compromised immunity. A significant reason I was attracted to fasting is that it’s an exercise in self-denial. When people tell me that fasting is too hard, I respond that for me that was part of the point. After all, have you ever been proud of doing something easy?

Like anything else, fasting gets much easier with practice. A good starting point is to simply skip breakfast and lunch, in which case you’ll have completed a 24 hour fast from dinner to dinner. I have found through my experiences with fasting I have become a person who is more disciplined and better at listening critically to my body. Combined with the benefits of autophagy, boosted immunity, insulin sensitivity and environmental sustainability, fasting is a powerful solution to many problems.


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¹ Fung Jason. The Complete Guide to Fasting. Victory Belt Publishing, 2016. E-book.

² See the section “Fasting and Cancer” for a detailed discussion of the research supporting this hypothesis: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3946160/

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