Does Fasting Work? Or Does a Recent Meta-Analysis Prove That It Doesn’t?

by Nils Osmar. May 31, 2022

There have been claims that “fasting works!” and counter claims that “fasting doesn’t work!” for decades now. Hundreds of studies have suggested that it does have benefits. But could those benefits ultimately come from the fact that when we fast repeatedly, we end up taking in fewer calories?

Different analyses of the data have come to different conclusions.

One recent meta-analysis suggested that when we compare fasting with simply eating less, there are more similarities than differences in the results. Opponents of fasting, including at least one Youtube blogger, declared that this is proof that “Fasting doesn’t work!”

But is this true? According to a study called Effectiveness of Intermittent Fasting and Time-Restricted Feeding Compared to Continuous Energy Restriction for Weight Loss:

There is some evidence to suggest that the timing of meals may impact weight loss. In two weight-loss interventions, individuals who self-reported consuming more calories in the morning compared to the evening lost more weight, even though both the energy intake and self-reported physical activity were similar [55,56]. In addition, food intake at night (as seen with shift work) is linked to obesity, independent of energy intake [57,58,59]. 

To paraphrase: Changing when we eat – in this case, eating in the morning and fasting at night – can and does effect how our bodies metabolize food, and can have an impact on body composition and weight loss. It’s not all about calories, but also about taking breaks from eating.

A different meta-analysis – Effects of Intermittent Fasting in Human Compared to a Non-intervention Diet and Caloric Restriction: A Meta-Analysis of Randomized Controlled Trials – again suggests that, while both caloric restriction and fasting can be useful, there are some benefits that are unique to fasting. The authors state:

Intermittent fasting was more beneficial in reducing body weight, WC, and FM without affecting lean mass compared to the non-intervention diet.

IF also effectively improved insulin resistance and blood lipid conditions compared with non-intervention diets…

Some researchers suggested that weight loss after CR was due to the adaptation to metabolically induced reductions in FM and FFM (6768). The weight loss resulting from IF was also due to the reduction of FM and FFM.

However, the difference was that IF, in addition to metabolic adaptations, consumed the stored hepatic glycogen 10–12 h after fasting and then generated massive ketone bodies through the oxidation of fatty acids in adipose tissue, which would be used as the energy source of the whole body (6970).

Therefore, an increase in blood ketones was found after IF, but not after CR alone …

Johns Hopkins Medical weighs in

  • This article – Intermittent Fasting: Is it For You?” = published on Johns Hopkins Medicine’s website points out that fasting appears to “lower blood pressure and cholesterol, slow the progression of Parkinson’s and Alzheimer’s diseases and reduce cancer risk” and has numerous other benefits in human beings.

From the article:

“Johns Hopkins neurology professor Ted Dawson’s cholesterol was creeping into dangerous territory when his doctor recommended statins. Dawson decided to try intermittent fasting first, a restrictive eating schedule that mimics the feast-then-famine life of prehistoric hunters.

“Sure enough, it lowered my cholesterol,” Dawson says. “I never needed a statin.”

That was about 10 years ago. Dawson, now 60, still follows the regimen, consuming just 500 calories a day, twice a week. “To be honest, it’s still a pain,” he says. But his cholesterol levels keep improving, and his energy levels are high, he says.

On his low-calorie days, Dawson eats a little oatmeal or yogurt in the morning, a handful of nuts and fruit mid-day, and a veggie burger with no bun for dinner.

On the remaining days, he eats whatever he wants — in generous quantities. “My caloric intake is probably what it was when I was 25 or 30,” says Dawson, who now has to work to keep his weight up. “For health reasons, I try to stick to a Mediterranean diet, but I like ice cream and other desserts, so I’ll eat those too. I have red meat once in a while.”

A growing body of research, much of it led by Mark Mattson, adjunct professor of neuroscience, indicates the regimen offers a host of benefits, such as helping adherents shed pounds, lower blood pressure and cholesterol, slow the progression of Parkinson’s and Alzheimer’s diseases and reduce cancer risk.

In December, Mattson and Rafael de Cabo of the National Institute on Aging put much of that information in one place, publishing a widely publicized research review in the New England Journal of Medicine.

The catalyst for intermittent fasting’s health benefits, they say, is metabolic switching, which occurs when cells transition from using glucose for energy to using ketone bodies, and then back again.”

This suggests, that the switch between eating and not eating has benefits which simply reducing calories does not hae.

Problems with meta-analyses

Meta-analysis can be useful in one sense, but they can also confusing. In the case of fasting, that confusion can come in because they often jumble together different types of fasts to make their comparisons.

But as Dr. Valter Longo has pointed out, the effects of a daily 16 hour fast are completely different from those of a 24, 36 or 72 hour fast.

Longo is the Edna M. Jones Professor of Gerontology and Biological Sciences and Director of the Longevity Institute at the University of Southern California –Leonard Davis School of Gerontology, Los Angeles, one of the leading centers for research on aging and age-related disease. He is the developer of, and a key proponent of, the ProLong Fasting Mimicking Diet.

He has suggested that the differences between fasts of different lengths are so significant that breaks from eating which are shorter than 24 hours should not even be called “fasts.”

Summing up

Even if that conclusion that the benefits of fasting are all related to caloric restriction were to turn out to be accurate, this would not mean that fasting doesn’t work. It would mean that one of the key benefits of fasting is the fact that it reduces calories over time.

An advantage of fasting from that perspective would be that it gives us a way of reducing calories without constantly counting them. Intermittent fasting lets us eat what we want for a stretch of time, then take a break from eating. If most of the benefits really turn out to be from caloric restriction, fasting could then be seen as one of several ways to accomplish this restriction. But I think there’s ample proof that there are benefits that go far beyond that.

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