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Why Intermittent Fasting Is Gaining Support

Some evidence supports intermittent fasting—but is it practical long-term?

Ketogenic diets—diets that switch the body from using glucose for energy to burning ketones—have been promulgated for many years and in many forms. The idea was actually first tested as a treatment for epilepsy in the early 20th century, for which it remains today as a medically-prescribed intervention for children with medication-resistant focal seizures.

The classic ketogenic diet for weight loss involves eating foods high in fat and low in carbohydrates. The Atkins diet is an example. Although studies show that ketogenic diets do result in weight loss, they have been controversial because of fears that high levels of ketones might be harmful, and the difficulty people have staying on them for long periods of time in order to sustain weight loss.

Another approach to switching the body’s metabolism from carbohydrates to ketones is intermittent fasting. Recently, a review in the New England Journal of Medicine (NEJM) by scientists from the National Institute on Aging and Johns Hopkins medical school suggests that intermittent fasting may in fact be a safe and effective method not only to lose weight, but to increase lifespan as well.

How does intermittent fasting work? When we eat three meals a day plus snacks—the usual American diet—our bodies preferentially burn carbohydrates in the form of glucose. But with either the ketogenic diet or fasting, carbohydrates become unavailable and the body switches to turning stored fatty acids into substances called ketone bodies (see illustration), which are used for fuel by many organs, including the brain. This results in lower glucose levels and in depleting fat stores, hence weight loss.

According to the NEJM review, there is abundant evidence from animal models that intermittent fasting improves signs of “obesity, diabetes, cardiovascular disease, cancers, and neurodegenerative brain disease.” In addition, authors Rafael de Cabo and Mark Mattson write “studies in mice and nonhuman primates [i.e. monkeys and apes] show consistent effects of caloric restriction on the healthspan [i.e. the length of time an individual lives in relatively good health].”

Ketones Have Many Positive Actions

It turns out that weight loss may not be the only way that switching to metabolizing ketones from sugars may suppress disease. Ketones apparently have important physiological roles of their own; they “regulate the expression and activity of many proteins and molecules that are known to influence health and aging.” These include proteins and molecules involved in inflammation, cardiovascular and brain function, and glucose utilization. The NEJM review does not discuss any possible harms from high ketone levels and for the most part, these do not seem to be clinically significant for most individuals.

In humans, de Cabo and Mattson review studies reporting that “intermittent-fasting interventions ameliorate obesity, insulin resistance [the problem in type 2 diabetes], dyslipidemia [such as high cholesterol levels], hypertension [high blood pressure], and inflammation.” It is claimed that intermittent fasting can have a positive or preventative effect on a very wide range of disorders, including asthma, brain tumors, rheumatoid arthritis, and cognitive decline. De Cabo and Mattson make recommendations about how to initiate intermittent fasting in order to reach a goal of 5:2—five days a week eating and two days fasting. Other fasting regimens are also possible.

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The NEJM encomium to intermittent fasting appears to give support to a growing fasting-for-health fad. It has, however, has been met by some pushback from experts. Perhaps the major concern is whether people will, in fact, be able to continue the intermittent fasting regimen over long periods of time. So far, most of the studies in humans are relatively short term, usually not more than months, and even those can have high dropout rates. According to Guy Mintz, a New York City cardiologist, some studies show patients eating more than recommended amounts of food on fasting days and less on feast days. Mintz also cautions that for some people, like those with diabetes and the elderly, intermittent fasting could be dangerous.

Indeed, not all studies of intermittent fasting in humans have yielded positive results. As noted in the NEJM review, a 12-month study showed no differences in weight loss or other measures of health between an alternate day fasting regimen and daily calorie restriction. There was a 38 percent dropout rate in the fasting group versus 29 percent and 26 percent respectively in the calorie reduction and control groups.

Other Health Diets May Be More Manageable For Longer Periods

There certainly are no data that can indicate whether intermittent fasting would make people live any longer, healthily or otherwise. That would take following people for decades who do or do not practice intermittent fasting to see who lives longer, and even that kind of observational study would be hard to interpret given the many other variables that would affect the groups’ outcomes. The only definitive way, of course, to know if it extends lifespan would be to randomize people to either an intermittent fasting group or some other diet group. Such a study would be very expensive, so we are skeptical it will be pursued any time soon.

There is, then, irrefutable animal evidence that intermittent fasting has health benefits and evidence from some, but not all, short term human studies that it may be beneficial. The real question, however, remains whether significant numbers of people will be able to do it on their own (that is, not as part of a monitored study) for many years. We don’t know the answer to that yet, but there is reason to think it may be very difficult to spend two twenty-four-hour periods a week without eating for many years.

It is curious in the current fervor to consider intermittent fasting that other, possibly more easily adhered to, diets are not simultaneously being kept in the public’s mind. For example, a recent study randomized 6,874 men and women aged 55 to 70 years with metabolic syndrome but no cardiovascular disease to either a reduced-calorie Mediterranean diet or an unrestricted calorie Mediterranean diet. The former group also received encouragement for physical activity and behavioral support.

At the one-year analysis, 96 percent of the study subjects were still adherent to the diets to which they were assigned, with greater adherence in the intervention than the control group. The Mediterranean diet has also been shown by numerous studies to have a wide range of health benefits, including many of the same ones claimed for intermittent fasting. What we really need to know at this point is which are the healthy diets that people will actually be able to sustain for years.

It is abundantly clear that the American diet is generally not a healthy one. We eat too many highly processed and heavily advertised foods, consume too much sugar, and exceed recommended calorie levels. The result has been an epidemic of obesity and diabetes. All nutrition experts agree that we need to improve our diets and increase our quota of daily exercise.

We also know, however, that instituting changes in diet and exercise is very difficult for most people and the best of intentions often last for only a few days or weeks. If it could be shown that intermittent fasting is something that a lot of people can really maintain for life, it is becoming increasingly clear that it might be a very good strategy to improve health and, maybe, increase lifespan.

We remain skeptical, however, that this will be adopted by a meaningful proportion of the American population. Therefore, we believe that too much hype about things that are impossible to achieve distracts our attention from what might actually work. We need both more research and enhanced public health messaging about practical, sustainable strategies for improving what we eat.

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