With the holidays over and those belts a little tougher to tighten, you may be looking at one of the many so-called “fasting” diets for a quick fix. So, do they work? And how do they compare with other weight-loss strategies?

Intermittent fasting – also known as “time-restricted feeding” or “food timing” – is an umbrella term for a dietary regime that involves restricting eating to set times. The best known of these feasting-and-fasting programs are the 5:2 diet, also known as “The Fast Diet” (you eat normally for five days a week and fast on the other two days) and the 16:8 diet (you fast for 16 hours a day and eat within an eight-hour window). There’s also the Alternate Day Fasting Diet (you eat 500 calories every second day, and as you normally would on the other days), and numerous other variations on the “you are when you eat” theme.

The basic idea is that by fasting for periods of time you give your body a break and forces it to dip into its stored fat reserves. While there is good evidence that the intermittent fasting diets help people lose weight, it’s wise to keep things in perspective. You might have heard, for instance, that intermittent fasting has been shown to extend lifespan and to improve age-related diseases – it has, in rodents not people.

In fact, there’s little evidence to show that a fasting diet is any better than a conventional calorie-restriction diet. The largest study to compare the two was published this year in the American Journal of Clinical Nutrition. The researchers recruited 150 overweight and obese participants, with one third following a conventional calorie-restriction diet that reduced daily calorie intake by 20 percent; a third following the 5:2 dietary plan reduced calorie intake by 20 percent over the whole week; and a control group, which followed no specific diet plan but was advised, like all other participants, to eat a well-balanced diet.

Both groups following the dietary regimes showed health improvements; they lost weight and reduced unhealthy body fat. Every kilo counted; those who reduced their body weight by only five percent lost about 20 percent of dangerous visceral fat and more than a third of fat in the liver – and it didn’t matter which diet they were on. The researchers didn’t find any difference between the two methods, or their effect on the metabolic values, biomarkers and gene activities associated with disease that were being investigated.

There’s little evidence to show that a fasting diet is any better than a conventional calorie-restriction diet.

Numerous studies have shown that intermittent fasting will reduce your risk of type 2 diabetes – but so will any diet that results in weight loss. Losing a substantial amount of weight can even reverse the disease, as shown by Roy Taylor and his team at the University of Newcastle and their landmark trial published in the Lancet in 2017. The study included 298 adults aged 20-65 years who’d been diagnosed with type 2 diabetes in the previous six years. After being put on a very low-calorie diet (less than 850 calories a day) for three to five months, nearly half of the participants that followed the weight management program could stop taking medication after a year, compared to only 4 percent in the control group.

According to Taylor, when the body is forced to rely on its own stores of fat, it first dips into fat stored in the wrong parts of the body – inside the liver (which has an important role controlling blood sugar) and the pancreas (which controls the release of insulin which brings down blood glucose levels). Getting rid of the fat in those organs allows them to function normally.

While the study has sometimes been described as a fasting diet, it was really a very low-calorie diet (known as LCVD). This is typically prescribed for an obese person who wants to lose weight quickly. Importantly, the diet prescribed in Taylor’s study, made up of liquid protein and vegetables, was carefully designed to be nutritionally complete. It was also followed by the reintroduction of healthy food and long-term support to help people keep their weight off. Taylor has also noted that any diet that leads to substantial weight loss over a period of time is going to be just as effective.

Some people might find it easier to fast for two days a week, and others might prefer to eat a little less every day and maybe cut out the biscuits.

Research on intermittent fasting is increasing and showing promising results. However, just as it’s not yet clear if they are any better than conventional dieting, it’s not yet known if one intermittent fasting regime is better than another – whether the length of the fasting interval matters, or the number of fasting days per week, or how many calories you should eat on fasting days and on the non-fasting days. It’s also not clear how sustainable such diets are either – are people really likely to keep to such diets in the long term?

It’s important to note that for some people intermittent fasting could be dangerous. Those with medical conditions, including diabetes, should talk to their doctor before trying it. Pregnant and nursing women should avoid it, as should anyone with a history of eating disorders – any diet that makes people focus on what they’re eating can exacerbate an already problematic relationship with food. Intermittent fasting hasn’t been studied in children, the elderly or underweight people, and could be harmful to them.

If you’re overweight, losing weight is going to be good for you. But different strokes for different folks; some people might find it easier to fast for two days a week, and others might prefer to eat a little less every day and maybe cut out the biscuits. There are many paths to weight loss – the trick is finding a diet that is well-balanced, suits you and your lifestyle, then sticking with it for good.

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