Will A Crash Diet Wreck My Metabolism?

·3-min read
Photo credit: Martin Barraud - Getty Images
Photo credit: Martin Barraud - Getty Images

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Kira from Sterling asks:

‘Is it true that crash diets wreck your metabolism?’

Expert: Laura Tilt, registered dietitian and founder of tiltnutrition.co.uk

In the research world, the closest comparison to crash diets is the VLCD (very low calorie diet), which involves eating 800 calories or fewer per day - usually as a liquid diet, with added vitamins and minerals.

VLCDs are only recommended under medical supervision, and for a maximum of 12 weeks. But while they do result in weight loss, that’s not all.

In a 2016 review of VLCDs, UK researchers reported fatigue, dizziness, cold intolerance and hair loss as the main side effects, as well as an inflated risk of developing more serious issues, like gallstones.

Long-term complications can also arise from a lack of protein, vitamins and minerals – in some cases proving fatal.

As for your metabolism, factors that determine metabolic rate (the number of calories your body uses to power all bodily functions) include age, gender, height and, yes, weight and body composition.

While weight loss does reduce your metabolic rate (a smaller body burns fewer calories), it’s thought that crash diets exacerbate the drop, due to a phenomenon dubbed ‘adaptive thermogenesis’ (AT).

This is the idea that your body defends against weight loss by reducing calorie burn via several clever mechanisms, which cause the body to run more ‘efficiently’.

According to the research, the more severely you restrict your calories, the harder AT kicks in, making crash diets counter-productive.

We don’t know exactly when AT kicks in. In one 2015 study, researchers found that if extreme dieting was under three weeks, AT could be reversed within a couple of weeks of normal calorie intake.

But in a study of the TV show The Biggest Loser, researchers found 24-hour resting metabolic rate dropped by approximately 500 calories more than what could be explained by the change in body weight at the end of the contest.

Six years on, it remained about 500 calories lower than predicted, even though most of the participants regained weight.

Bottom line? Crash diets aren’t a long-term fix, and could have the opposite of the desired effect. Compared with a standard low-cal diet, there’s little to no difference in weight loss at 12 months, because weight rebound post-crash diet can be greater.

Instead, aiming for 1 to 2lb weight loss per week should limit AT.

If you’ve done a crash diet in the past, exercise (especially strength training) is your friend. A big part of the adaption in metabolic rate is down to moving less and losing lean mass, which can be counteracted.

New research also shows that alternating a sensible low-cal diet with regular rest periods (consuming a normal amount of calories) is more likely to keep you lean long term. Here’s what else you can try:

  • Strength-train: Do so during dieting and you’ll hold on to more of your lean muscle mass, one of the biggest contributors to calorie burn.

  • Eat more protein: One of the downsides of dieting is an increase in hunger hormones. Adding more protein can help quash them. Aim for 20-30g with each meal.

  • Switch it up: Aussie researchers found that alternating two weeks of dieting with two weeks of maintenance calories resulted in greater fat loss than a continuous diet.

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