Intermittent Fasting: Are 5:2, 16:8 and ADF Diets Really the Healthiest Ways to Eat?

Growing evidence suggests that intermittent fasting has a host of health benefits. Getty Images

Live longer, lose weight, boost your health and eat what you want most of the time. The promises of much-lauded intermittent fasting diets are undeniably attractive, and growing evidence suggests that these on-off regimes could be healthiest way to feed ourselves. But are they too good to be true?

Intermittent fasting is an umbrella term for various regimes where food is consumed during set times. The popular 5:2 diet, for instance, involves dramatically restricting calorie intake for two days of the week and resuming recommended levels the rest of the time. With 16:8, food is consumed in a daily eight-hour window, while followers of alternate-day fasting eat normally every other day.

Studies have linked fasting to weight loss as well as major health benefits, from cutting the risk of cardiovascular disease and cancer to boosting life expectancy. The latest research indicating the benefits of intermittent fasting comes from researchers at the University of Alabama at Birmingham (UAB). In their small, pilot study on eight men with prediabetes, participants followed an early time-restricted feeding (eTRF) regime, where eating was restricted to a six-hour window and the final meal was consumed by 3 p.m. After a two-week reset period, they swapped to a more conventional pattern of eating spread across 12 hours. Unusually for such trials, the participants were observed while eating every meal for the duration of the four-month study, even using Skype when necessary.

The eTRF diet appeared to improve how their bodies processed excess glucose, and made the pancreas more efficient at supplying insulin. The participants' blood pressure and oxidative stress levels also dropped—all without them losing weight.

Dr. James H. Catterson of the Institute of Healthy Aging at University College London, who was not involved in the study, commented that although the study was small, it was the first to test the relatively new eTRF regime on humans, and was carried out in an impressively rigorous and controlled manner, which is unusual for nutritional studies.

The UAB study followed a long line of studies that report beneficial effects of intermittent fasting, he told Newsweek, which have been exhibited in a number of organisms. "Bacteria, yeast, nematode worms, and mice (and many others to mention) exhibit beneficial effects on health and longevity when placed on an intermittent fasting diet," he said.

At a time when the World Health Organization regards obesity as one of the greatest public health challenges of the 21st century, scientists and clinicians are particularly excited by intermittent fasting, Dr. Catterson said. Currently, over a third of U.S. adults and one in six children and adolescents are obese. As overeating is harming our health, intermittent fasting provides a useful guideline for restricting our diets in a way that is considered easier to stick to than traditional diets, he explained.

"Following a plan like this is likely to leave you feeling less deprived as 'dieting chunks' only last for just 24 hours at a time," chimed Linia Patel, a qualified dietitian and a spokesperson for the British Dietetic Association. "This type of eating pattern may also prevent your metabolism from slowing which is what happens with a constant low-calorie intake."

Willpower is tested to the extreme with plans such as fasting-mimicking diet (FMD), where calories are restricted for five days, with fewer that 1100 calories consumed on day one, and around 500 calories day two to five, resulting in similar outcomes to regular fasting, according to Dr. Catterson. This approach, researched by experts at the University of Southern California, is attractive as it suggests a healthy person would only need to perform two or three five-day fasts per year, and still reap the protective benefits of intermittent fasting.

So why aren't health bodies rushing to recommend that we adopt such lifestyles?

"So far, the consensus seems to be 'let's wait until more rigorous studies, with larger sample sizes that adjust for confounding lifestyle behaviors, have been performed before we conclude anything prematurely'," said Dr. Catterson.

Currently, intermittent fasting can't be seriously regarded as anything more than another form of dietary restriction, which is admittedly especially beneficial in people who are overweight to the point of ill health, he said.

Take the American Heart Association's recent article acknowledging that studies have suggested intermittent fasting has short-term heart benefits, as does eating smaller, frequent meals throughout the day. But the long-term effects haven't been thoroughly investigated, they warned.

"It makes sense that eating more earlier during the day and less at night is more healthful, but the studies aren't available," Dr. Penny Kris-Etherton, a nutrition professor at Penn State University who co-penned the article said in a statement.

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Part of the problem comes down to the difficulty of conducting experiments which result in trustworthy evidence, as studies rely on participants being consistently honest about what they eat.

"Indeed, the data in animals is much more convincing, but this likely due to the highly controlled environment within which animals are kept, and the larger sample sizes," explained Dr. Catterson. "There are also many studies that either report no overall effect, or sometimes the opposite. And this is where it gets a bit murky, as it is very difficult to put proper controls in place when performing studies to do with nutrition."

And even if such diets are proven to be healthy, following them is easier said than done. Although some may find intermittent fasting less daunting and easier to stomach than seemingly never-ending restrictions, so-called "feast days" shouldn't be regarded as an open pass for gorging on the high-saturated fat, high-sugar foods we know to avoid.

"If you spend your day eating junk food or very large portions you won't get the benefits on the fasting days," warned Patel, advising that an unhealthy diet could lead to inadequate intake of vitamins and minerals in the long term.
"It is important that you opt for nutritious foods on both the fast and feast day," she added.

Terminology such as "fast" and "feast" is problematic in itself, Dr. Catterson suggested, as it risks pathologizing food and arbitrarily categorizing items as "good" or "bad," which can trigger restrictive and bingeing eating disorders in the most vulnerable.

Overall, focusing on foods such as vegetables and fruit, wholegrains, pulses, lower fat diary, protein rich foods such as chicken, fish, lean red meat and eggs and food fats like olive oil, avocado and tree nuts, is the wisest way to eat, Patel said.

The key to long-term weight and health management is to find an eating plan that suits you and your lifestyle, she added. Of course, anyone adopting a new diet is advised to check with their doctor, particularly if they have diabetes, indigestion, or have suffered with eating disorders.

Ultimately, then, while the jury is out on the benefits of intermittent fasting, it seems the best diet advice to follow is the most obvious: Eat your veggies and avoid junk.