I’ve done a lot more reading on the science behind intermittent fasting in general and time-restricted eating in particular, and the evidence of the health benefits is sound, including reduced insulin resistance (which leads to overall metabolic health), improved triglycerides, and improved cholesterol [1,2,3]. Ref 4 is a great review article on the topic, for which full access is available here. The “metabolic switch” the authors refer to occurs when insulin levels are low, and the body has sufficiently depleted its liver glycogen stores so that it is having difficulty producing enough glucose for the brain. It then starts entering the state of ketosis by producing ketone bodies to replace the missing glucose. This occurs, according to the authors, after about 12 hours of fasting, and increases until about 18 hours of fasting, after which the benefits continue during longer fasting. The benefits include the aforementioned metabolic benefits, and the health-enhancing and anti-aging benefits of autophagy (cellular clean-up) that happen after the “switch” is flipped. This implies that to get the benefits of time-restricted eating, the eating window has to be less than about 12 hours, with shorter probably being better.
A good book that covers the science of intermittent fasting in detail is The Obesity Code, by Dr. Jason Fung. Dr. Fung is an expert in kidney disease and became frustrated treating the end stages of the disease when he felt it could be preventable through lifestyle change (both obesity and type II diabetes are major risk factors). This led him to develop expertise in diet and lifestyle changes, which have worked well with his patients. In the book, he gives evidence that a major factor in the health benefits of fasting, including time-restricted eating, is keeping insulin levels low. He noted an important piece of the puzzle of what has changed since about 1970, after which percentages of the population becoming clinically obese have steadily climbed. In the same period, the number of people with metabolic disorders like prediabetes and type II diabetes has also steadily climbed. Lots of authors have speculated on what changes in our diet and activity levels might have led to this. But the missing factor Dr. Fung pointed out is that many people snack more nowadays. “Three square meals” a day was previously the norm, but now it is more common to snack throughout the day and into the evening. And many health authors even advise this, supposedly to keep on blood sugar on an even keel. Dr. Fung thinks this is a serious mistake because it leads to chronically elevated insulin levels. He gives evidence that this leads to increased insulin resistance and other metabolic issues. My own experience in that time period is in tune with this. Growing up we seldom had between-meal snacks, which my Grandma would admonish “would spoil our appetite for dinner”. And snacks in the evening were rare, perhaps as a special treat while watching a movie on Friday night. I did not start to indulge in more frequent evening snacking until about my 30s.
One thing that might be controversial about Dr. Fung’s book is that he is also a staunch advocate of a low-carb diet. It is controversial whether restricting all carbohydrates is necessary to avoid excess insulin levels and other health issues, or whether it is sufficient to restrict just overly refined carbs. I follow the latter, and combining that with time-restricted eating has worked well for me. But other people may do better with restricting all carbs.
One final area I find fascinating is the role of time-restricted eating in becoming fat-adapted. This has benefits for both health and athletic performance. I’ll cover that in an upcoming post.
- Sutton, E, et al, “Early Time-Restricted Feeding Improves Insulin Sensitivity, Blood Pressure, and Oxidative Stress Even without Weight Loss in Men with Prediabetes”. Cell Metab., 2018. online here.
- Cabo, R, and Mattson, M, “Effects of Intermittent Fasting on Health, Aging, and Disease”, NEJM, 2019, online here.
- Patterson, R, and Sears, D,”Metabolic Effects of Intermittent Fasting”, Ann Rev Nutr, 2017, online here.
- Anton, S, et al, “Flipping the Metabolic Switch: Understanding and Applying Health Benefits of Fasting”, Obesity, 2018, online here.