But beyond the short-term misery, could crash dieting do long-term damage? In the research world, the closest comparison is the VLCD (very-low-calorie diet), which involves eating 800 calories (3347 kilojoules) or fewer per day – usually as a liquid diet shake with added vitamins and minerals. VLCDs are only recommended under medical supervision, and for 12 weeks at most. Spoiler alert: they lead to weight loss – but that’s not all. In a 2016 review of VLCDs by the Obesity Science & Practice journal, researchers reported fatigue, dizziness, cold intolerance and hair loss as key side effects, as well as an inflated risk of developing more serious issues, such as gallstones. Long-term complications can also arise from a lack of basic nutrients.

RELATED: How Diet Became The Dirtiest Word In Wellness

Getty Images

What can go wrong when you crash diet?

As for your metabolic rate, this is determined by your age, gender, height and, yes, weight and body composition. While any weight loss does reduce it (a smaller body burns fewer kilojoules), it’s thought crash diets exacerbate the drop due to ‘adaptive thermogenesis’ (AT). This means your body defends itself against weight loss by reducing kilojoule burn so it runs more ‘efficiently’. Research in the journal Future Medicine suggests that the more severely you restrict your kilojoules, the harder AT kicks in, thus making crash diets counterproductive. We’re not yet sure exactly when AT kicks in, but one 2015 study published in The American Journal of Clinical Nutrition found that if extreme dieting was short term (less than three weeks), AT could be reversed within a couple of weeks of normal kilojoule intake.

Why long term habits offer better results

That said, let’s check out another study, from the journal Obesity, which looked at participants in weight loss TV show The Biggest Loser (they trained daily for four to six hours on 4200 to 5000 kilojoules). Researchers discovered their resting metabolic rate dropped by around 2000kJs more than might be expected by the eventual change in body composition. Six years on, it remained about 2000 kilojoules lower than predicted, even though most participants regained a lot of the weight. Crash diets aren’t a long-term fix, and can backfire – compared with a standard low-KJ diet, there’s minimal difference in weight loss after a year. Aiming to lose 0.5 to 1kg a week should limit AT. If you’ve crashed in the past, exercise (especially strength training) is your friend. Science also indicates that alternating a sensible low-kJ diet with regular breaks is more likely to keep you lean long term. Bingo.

RELATED: How To Lose Weight Without Strict Diets Or Intense Exercise