Intermittent fasting (IF) is the latest nutrition, health, and, in some circles, performance trend out there. Here are some thoughts, from a neutral observer without a horse in this race, based on the latest evidence and theory.
For most people, intermittent fasting reduces calories. If you don’t have the option to eat all day you’ll end up eating less. Eating less is associated with weight loss. Being at a healthy body weight is associated with reduced disease risk and enhanced longevity.
Other claims about intermittent fasting tend focus on the health and especially longevity benefits beyond the practice’s potential positive affects on weight control mentioned above. Research has shown intermittent fasting activates certain pathways in the body, but no study (yet) has gotten to the kind of real-world outcomes we care about (i.e., health, performance, longevity).
An outcome study is challenging because you’d essentially have to have two groups of people eat the exact same diet and daily calories but one group only eats in certain windows. And you’d have to track participants for years—virtually impossible to do in a rigorous experimental setting. Nutrition science is hard!
If you zoom out a little, it is true that the body responds well to stress—so long—and this is a big so long—that the stress is at the right dose. The theoretical benefits of intermittent fasting (again, beyond weight control) all center around its role as a stressor to which the body adapts.
This productive stress theory is sensible, and there are related areas to which we can look for patterns. Physical activity is great and well-studied example. Without physical activity, you get sick. With too much physical activity, you also get sick (or injured).
The key to physical activity, for heath anyways, is to stress yourself just enough over and over again. If you believe intermittent fasting offers stress-related benefits, it’d probably be wise to consider the “just enough over and over again” approach here too. In other words: be a little hungry most days.
For most people, being a little hungry most days translates to something like don’t eat for 12 to 13 hours. For example, stop eating at 8 PM and have breakfast between 8 and 9 AM. This kind of hunger stress feels equivalent to what would be healthy workout stress: a little hard but not too hard, very much repeatable.
The beauty of this approach is that you get some stress and you won’t be grumpy and miserable because you are always hungry. Also, you probably won’t develop an eating disorder, something that sadly becomes increasingly common the longer you intermittently fast in the name “health.” (A big disclaimer to all of this is that folks in recovery from eating disorders should probably steer clear of all intermittent fasting, or at the very least talk to their therapist extensively before trying any kind of new nutritional approach.)
Now, you may be wondering, what about the very hard workouts you do once in a while? Well, those are as much for your mind as for your body. Doing hard things every so often is good. But you better do them with knowledge or coaching, otherwise you get hurt.
A full day fast once or twice a year? Fine. But don’t think that full day fast is doing anything for your physical health. If fasting is how you want to experience a challenge, however, go for it—again, so long as you have proper knowledge and coaching. I know I’d personally rather just train capital HARD a few times a year.
The bottom line is this: intermittent fasting is a way to control calories. Any other benefits are still theoretical. The main theories center around positive stress-related adaptations, such as those you see with exercise. Perhaps, then, the best way to approach intermittent fasting is similar. Make it hard but not too hard. Be a little hungry most days.
Longer periods of fasting carry risks such as burning out; binge eating in the windows you do eat; and, by far the most dangerous, eating disorders. If you are going to go in this direction, understand the risks, potential benefits, and be deliberate.