Does it matter when we eat? Intermittent fasting and time restricted feeding

Intermittent fasting has taken on a life of its own in popularity in the ‘dieting’ world, and nary a day goes by that I am not asked about this by either a patient or health care colleague. I was really excited to see this important topic get some attention at the recent American Diabetes Association meeting. To help disseminate this knowledge, here I have summarized some of the learnings and key messages from this excellent symposium.

First off, some definitions. ‘Intermittent fasting’ can take several forms, with the main ones being:

Alternate day fasting: 24 hours of fasting every second day (the fasting days can be full fasting or semi-fasting, eg eating around 500 calories on the semi-fasting days)
5:2 – 2 days of semi-fasting per week (eating about 500 calories on each of those days); the other 5 days are regular days
fasting 3 days per week
Time Restricted Eating (TRE) aka Time Restricted Feeding (TRF): limiting food intake to 6-10 hours daily

Why does time matter? As blogged previously, we have body clocks in our brain, and in every other cell in our body, that govern our metabolism. The clock in the brain is regulated by light, and the clocks in our other tissues are regulated by food, activity, and sleep. These clocks are evolutionarily built to work with a regular routine of eating during daytime while awake, and fasting and sleeping in the night.

Eating outside of this circadian rhythm is associated with many health issues, as has been seen, for example, in studies of shift workers and flight attendants. These include:

higher body weight, blood pressure, CRP (a marker of inflammation), and insulin resistance
a higher risk of cardiovascular disease, cancer, type 2 diabetes, and metabolic syndrome

So what happens if we try to take advantage of this circadian rhythm by concentrating our eating into daytime hours, and a shorter number of hours per day? This is what we call Time Restricted Feeding. The data suggest that a lower number of hours per day of eating (greater time restriction) is associated with better metabolic health – better blood sugars, blood pressure, lower CRP, and so forth. Setting these eating hours earlier in the day seems to be better than later for these health benefits, likely because this is the best fit with the human hormonal biorhythm. The benefits of time restricted eating may give a little weight loss (1-4%) by reducing hunger, but the health benefits seem to be independent of weight loss. For a person who chooses to follow time restricted eating, an eating window of 8-10 hours per day seems to be best, to balance best metabolic effects and the likelihood of being able to adhere to it long term. It was stressed that the studies are small, and larger (and longer) trials are needed before drawing definitive conclusions.

Intermittent fasting has also been studied, again in short and small trials. A meta-analysis of studies comparing intermittent fasting (IF) to daily caloric restriction (CR) showed that IF results in more weight loss in the short term (studies of 3 months or less), but there was no difference in weight in longer trials (>3months). In alternate day fasting studies, people tended to eat more on their low calorie days as the trials went on (meaning the fasting was tough to stick to). There may be metabolic benefits to fasting independent of weight loss, favoring a rise in mitochondrial biogenesis (production of our cellular energy factories) and cell repair. It was again stressed that larger and longer trials are needed to understand more about intermittent fasting and any potential health benefits.

BOTTOM LINES: Based on the available (and very limited) data:

Eating for a shorter number of hours (eg 8-10 hours), in the earlier part of each day, seems to be the best of all of these approaches for metabolic health benefit. (Remember that weight management is about improving health, not about the numbers on the scale!)

No form of intermittent fasting nor time restricted feeding has much effect on weight.

My thoughts: No ‘diet’ works if it is temporary, for health benefit nor for weight management. Think of nutrition as a permanent lifestyle plan, not as a diet. For some people, if they are able to adhere to one of the above strategies long term, it may work for them to manage their weight and health. But for most people, these approaches will be tough to stick to over the long run.

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