written by Marsha Seidelman, M.D.
Intermittent fasting has become popular both for weight loss and health improvement. People have adapted this to be anything from alternate day fasts, drastically restricting calories for 2 days each week(a 5:2 plan), or restricting eating to a certain number of hours per day, usually 8-12 hours. Is there any scientific reason to believe it works?
A recent review article in the New England Journal of Medicine gives some basis, at least in theory, as to why it may be beneficial. My own opinion is that there is interesting metabolic information available, but we need long term studies in humans to know whether it is worthwhile. Most of the studies to date have been in mice, rats and monkeys, but MANY studies are ongoing to see if the benefits could be applied to humans as well.
A look at clinical trials.gov will show current trials for how decreased calories or intermittent fasting can affect: breast, colorectal, lung, gynecologic, prostate and certain brain cancers; diabetes and pre-diabetes; obesity and hyperlipidemia; stroke, traumatic brain injury and other etiologies of cognitive impairment and biomarkers of aging; rheumatoid arthritis, and competitive runners performance. So clearly, there are many researchers who feel there is potential benefit related to how much and when feeding takes place in humans. It will be several years before we can definitively say what is most helpful.
To be clear, the limited calories should be with the advice of a dietician or nutritionist to ensure that nutritional needs are met. Also, the progression from the baseline diet to any one of these regimens should be gradual. Some people experience irritability and decreased ability to concentrate with restricted eating, which may or may not abate over time. I’ll mention more reasons below why these regimens won’t be workable for everyone.
I’d also like to stress that there are many ways to change one’s nutrition for better health. Most widely accepted and confirmed over the years is to move more toward a plant-based whole foods diet, for many of the benefits listed above.
What is the scientific basis for even considering intermittent fasting? When we use up available sugar stores during a fast, our bodies start using fatty acids and ketone bodies instead. Ketone bodies influence many molecules that have to do with general and brain health and aging. During the fast, cells have a coordinated stress response that is beneficial, regarding DNA repair, mitochondrial formation (there are our cells’ powerhouses), and anti-inflammatory responses. So that’s the theory. But in practice, it’s not that clearcut. For instance, in rats, one study showed that the average life span can be increased by up to 80% by alternate-day feeding, but this is influenced by sex, diet, age and genetic factors. When multiple studies were evaluated over decades, the effect was less than 45%, and in mice it was under 27%. There was no improvement in mortality in monkeys.
Human studies have involved small numbers of people, and/or observed them for short periods, usually up to 6 months. Some showed that blood pressure, HDL and LDL cholesterol levels, and insulin sensitivity were improved, and that waist circumference decreased more with intermittent fasting than with equivalent weight loss with other methods, but this was not universal. Intermittent fasting reduces markers of inflammation that are associated with atherosclerosis, the etiology of heart attacks and strokes. Regarding cancer, fasting may suppress tumor growth for glioblastoma, a devastating type of brain tumor; many other studies are ongoing for other cancer types, as noted above. So, again, in theory this may prove to be very helpful; but the data is not in yet.
Weight loss by any means reverses some of the harmful effects that obesity, diabetes and brain inflammation have on memory, executive function and spatial learning. In animals, excess calorie intake in midlife increases the risks of stroke, Alzheimer’s and Parkinson’s. Alternate-day fasting may be able to delay the onset and progression of these. Especially in the absence of medications that help memory and brain aging, it will be worthwhile to pursue how dietary changes can influence memory loss.
Some studies have shown that concentrating caloric intake to the earlier hours of the day, may allow for weight loss compared to eating the same number of calories through the day into the evening hours. This is attributed to the natural Circadian variation in hormones and metabolism over each 24 hour period. If you are having difficulty losing weight, this may be something to consider.
A regimen of intermittent fasting may be difficult for many people to adhere to, given our culture of eating three meals per day plus snacks. People may experience irritability, and more importantly serious episodes of hypoglycemia. Those with diabetes should not consider this without consulting their physician first. Any of these regimens should be approached gradually, with coaching as to how to maintain full nutrition with fewer calories. It is essential also to maintain good hydration, and time the calories to safely achieve your exercise goals.
I think it’s important to understand what all the hype about intermittent fasting is about, but I’m not convinced that in the long run, it’s any better than daily caloric restriction with healthy nutrition combined with daily activity, preferably 150 minutes per week, if that is a safe option for you. There will be much more information to digest in the years ahead.
de Cabo, R and Mattson, M. Effects of Intermittent Fasting on Health, Aging and Disease. NEJM 381; 26 (pp 2541-2551)Dec 26, 2019.
Parsons, J, et al. Effect of a Behavioral Intervention to Increase Vegetable Consumption on Cancer Progression Among Men With Early-Stage Prostate Cancer – The MEAL Randomized Clinical Trial
Dr. Michael Greger multiple videos regarding time restricted eating- if you enjoy these videos, please consider contributing, as they are informative and require funding from the readers to keep them going!