Crash diets may be tempting but can make weight loss harder and mess up your metabolism, a scientist warned

• Crash diets that seriously restrict calories can be tempting if you want to lose weight fast.
• But they can backfire by making you put on more weight long-term, a physiology expert said.

After the indulgences of the holiday season, it might be tempting to go on a crash diet and try to lose a few pounds fast.

But dropping calories too low could actually make losing weight and keeping it off harder, Christopher Gaffney, a senior lecturer in integrative physiology at Lancaster University, UK, explained in an article for The Conversation.

Gaffney said that losing weight does require being in a calorie deficit, and the number of calories a person should aim for depends on how much they weigh to begin with and how physically active they are. Tools such as the National Academy of Sports Medicine's calorie deficit calculator can be a helpful starting point.

But while cutting calories too low — Gaffney gave the range of between 800 to 1,200 as an example — can indeed cause rapid weight loss in some, people tend to put all the weight back on and then some.

Extreme diets are not good for our metabolisms

Gaffney explained that crash diets can lower what is known as the metabolic rate, or the number of calories burned just by living.

"In the long term, crash diets can change the hormone makeup of our bodies. They increase our stress hormones, such as cortisol. And over an extended period of time, typically months, high cortisol levels can cause our body to store more fat," he wrote.

Levels of the hormone T3 can also drop, potentially leading to hypothyroidism and weight gain in the long-run, he said.

The healthiest way to lose weight is to do it gradually

"If you're trying to lose weight, the best strategy to use is following a long-term, gradual weight loss diet," Gaffney said. The CDC advises aiming for between one to two pounds a week.

Such diets are more sustainable and less likely to negatively affect a person's metabolic rate than crash diets, Gaffney said. He added they also help keep our mitochondria — known as the energy factories of our cells — working, creating a "greater capacity for burning calories even after we finish dieting."

Gaffney's advice chimes with the many dietitians and nutritionists that BI has spoken to about sustainable weight loss.

BI's Rachel Hosie recently spoke to a fat loss coach and dietitian who explained how to use calories and macronutrients — proteins, carbs, and fat — to lose weight. They said eating enough protein is particularly important as it helps a person feel full while preserving their muscle mass as they lose fat. Carbs are an important sources of energy, while fats have benefits including keeping hormones functioning properly.

Gaffney also stressed the importance of eating protein, which increases the metabolic rate more than diets high in carbs or fats.

"While it may be tempting to crash diet if you're trying to lose weight fast, it could have long-term consequences for your metabolism. The best way to lose weight is to slightly reduce the number of calories you need per day, exercise, and eat plenty of protein," he said.