Why fad diets can actually lead to weight gain

A fad diet often promises rapid weight loss or other health benefits like a healthier immune system or longer life. Often made popular by celebrities, these diets - such as Keto, SlimFast, Atkins, Low-Carb and diet pills - encourage weight loss through cutting out food groups or severely restricting calorie intake.

According to the NHS, the average man needs to eat 2,500 calories a day and the average woman needs 2,000. It also recommends a diet balanced with at least five servings of fruit and vegetables daily, as well as carbs, proteins and fats.

Katherine Kimber, a registered dietician and spokesperson for the British Dietetic Association, says anything significantly less than that can damage our health.

"Consumers may be thinking 'there can't be any harm' in taking 'natural' pills, or cutting out groups of foods, but there really can be," she says.

"Fad diets can result in dehydration, weakened immune system, fatigue, and constipation. Specific ingredients such as fat-burning pills can be devastating."

There has been significant interest in recent years in very low-calorie diets (VLCDs) for people with type 2 diabetes. But while some studies have shown marked improvements in blood glucose control with these diets, the research has involved close medical follow-up and 'behaviour modification' to address harmful long-term habits while people are restricting their intake.

Weight gain

Aside from the adverse effects fad dieting can have on your health, it can also make you gain weight - hardly the desired effect when you decide to try one.

That's because a fad diet is unsustainable in the long term.

"Difficulty in keeping the weight off can be due to a multitude of factors: from the discontinuation of the diet due to unsustainability, genetic conditions, and something called the set-point theory. This is the theory that every unique individual has a weight range their body likes to be at," Kimber says.

"In addition, if crash diets are repeated frequently, this constant yo-yoing of weight has been associated with more weight gain, less exercise, and higher rates of binge eating."

One of the other main problems is that if you adopt an extreme diet and then come off it once you've lost weight, you're likely to go back to your old patterns of eating - which were precisely what caused the weight gain in the first place. This is in marked contrast to more gradual changes which allow you to re-evaluate your long-term relationship with unhealthy foods and move to a healthier way of life.

Some studies have identified that up to 55% of women experience weight yo-yoing due to fad diets, Kimber explains.

"There is also some strong evidence that shows weight yo-yoing can increase visceral fat accumulation - the fat around our internal organs. This type of fat can put people at higher risk of heart disease and diabetes, regardless of what the individual's weight is."

If that wasn't enough to put you off, fad diets can also cause lasting damage to your metabolism - your body's natural process of converting food into energy.

"There is a risk of slowing down the metabolism. While we know weight loss is possible in the short term, the body has strong physiological mechanisms to bring the weight back up," Kimber says.

"A 2016 study of contestants of 'The Biggest Loser' competition identified that not only does our metabolism slow down when people diet, but it also remains suppressed when people regain the weight they lost."

Put simply, your metabolism becomes slower, meaning it doesn't burn food into energy as quickly as it used to. Therefore you're likely to gain weight faster than you did before the fad diet.

There's a dangerous aspect of causing this kind of damage to your metabolism as well. A slower metabolism puts you at greater risk of metabolic diseases like heart disease, stroke and type 2 diabetes.

You may well lose weight rapidly if you opt for a fad diet, but some of this is down to your body losing water weight, Kimber explains.

For every gram of energy your muscles store, they store three grams of water. So when you're in an energy deficit, caused by several restricting calories, your muscles lose water quickly.

But don't be tricked into thinking this means the diet is working.

"As with any fad diet that puts the body's energy balance into a deficit, we may see some initial drop in muscle mass and body fat, which can also result in us seeing a dip on the scales," Kimber says.

Instead, losing weight and keeping it off for good both need to come from behavioural changes in what we eat and how much exercise we do.

"However, not all hope needs to be lost. We know that health can be improved through behaviour change, regardless of changes in weight," Kimber adds.

"My advice would be to remove the focus from the number on the scales and to focus on what you can do to feel good and improve your health. Can you find ways of adding more fibre into your diet? Can you find ways of moving your body that are pleasurable and enjoyable?"

And most importantly, focus on eating a healthy, balanced diet. The old 'energy in, energy out' mantra is true. Eating carbs isn't bad for you; eating the occasional piece of chocolate cake also isn't bad for you; but cutting out food groups and restricting your calories to the point where you're hungry is.