Should we be fasting?

Although my main area of expertise is in osteoporosis and other forms of bone disease, as an endocrinologist, I can’t help but be concerned about the increasing rates of obesity and type 2 diabetes. Although the causes of both of these diseases are numerous and complex, I think most of us would agree that what we eat plays a role.

It’s becoming clear that when we eat might also be very important. Not just for things like weight and diabetes, but also cancer and mortality. Specifically, ‘fasting’ or giving our bodies a big chunk of time without any significant fuel to break down, absorb, and store seems to promote changes to our metabolism that not only allow us to lighten up some of our fat stores but also perform ‘housekeeping’ functions that might help our cells function better (for longer).

When I first started reading about fasting, it was in the context of a strategy for weight loss, and I was skeptical. A large (as far as diet trials go) study that compared alternate-day fasting with limiting calories every day showed that, over 6 months, people lost similar amounts of weight on both diets, and not even all that much of it (about 5-6% of the body weight they started with, which is typical for weight loss studies). After the diet, people gained back a similar amount of weight regardless of whether they had been in the fasting group or not.

As someone who would be miserable going without a proper meal for more than 24 hours (or, cough, more than 12), this weight loss trial did not compel me to start fasting, or to think about recommending it to my patients, beyond including it as one of many possible lifestyle strategies for weight loss (keeping in mind that some strategies work for some people and not others).

However, at the end of 2019, a great review article by Rafael de Cabo and Mark P Mattson was published in the New England Journal of Medicine, and it has started to really change my perspective on fasting.

I’ll share three of the major points I took away from this article:

1. When thinking about health benefits, two different types of fasting are generally recognized: intermittent or alternate-day fasting (usually this involves two to four days a week on which only a small meal of <500 calories or nothing is consumed), and time-restricted feeding (in which calories are only consumed during a certain timeframe each day, often this is 4, 6, or 8 hours…corresponding to a fasting period of 20, 18, or 16 hours). Usually, people eat what they want during the ‘non-fasting’ period in both intermittent/alternate-day fasting and time-restricted feeding.
2. When we fast, we usually deplete our blood of glucose and liver of glycogen (long-ish term storage for carbohydrates) within 8-12 hours, at which time our bodies start to break down fatty acids (hijacked from fat cells…corresponding to fat loss) and make ketones* to fuel the body and the brain. When we eat again, the body ‘flips’ back to using glucose for fuel. Switching back and forth between the use of glucose and ketones is (intuitively) called ‘metabolic switching‘. In animals, the process of metabolic switching enables most (or possibly all) organs in the body to overcome the challenge of fasting and then get back to business as usual (with the scientific name for business as usual being: homeostasis). Repeated exposure to fasting (aka ‘flipping the switch on and off and on and off and…you get my drift) help cells respond quicker and better over time. Almost like a muscle.
3. More data in humans is needed, but current research suggests that fasting might improve lifespan, or at least health span.  Specifically, some studies have indicated that fasting regimens might improve memory, executive function, and global cognition, may reduce the risk of obesity and type 2 diabetes (although don’t forget that study I mentioned above), and could possibly lower the likelihood of developing cardiovascular disease and cancer.

It will take a lot more data to convince me that fasting is the magic bullet that will make all of us live to 100. However, given research suggesting that the average North American’s daily eating window is more than 14 hours, I don’t see a lot of downsides to trying to reduce this window, if done with common sense. Plus, it seems likely that even cutting down our eating window to 12 hours would facilitate some metabolic switching action. As I read the paper, I thought: even I could do that!

And so, for the next four weeks, I’m going to test it out: a 12 hour eating window each day. I’m not going to track any metrics on this (terrible scientist, I know), except for a general sense of how hard I’m finding it to do, and whether I feel any different. Another possibility, for someone thinking of dabbling in the world of time-restricted feeding, would be to consider an 8 hour eating window two or three days a week (on the weekends perhaps). If you were into tracking metrics, you could follow: sleep patterns, hunger, mood, energy levels, calorie intake (and perhaps food quality), weight, and waist circumference. Or you could just try it and see how things go.

*Side note: when ketone levels become high enough, the body is considered to be in ‘ketosis’. People who follow a ketogenic diet strive to remain in this state 24/7, whereas people who follow a fasting regimen will go in and out of ketosis as they consume