Lancaster:Those trying to kick-start their weight loss or perhaps wanting to lose a few pounds before a big event or holiday may be tempted to try a crash diet. While it's true that in order to lose weight you need to eat fewer calories than your body uses each day, in reality crash diets may actually work against you and may make weight loss more difficult.
Crash diets have been around for years, but have stayed popular more recently thanks to influencers and social media. Typically, these diets involve drastically reducing calorie intake to 800-1,200 calories a day for a few weeks at a time. Proponents of these diets claim it can lead to rapid weight loss, which may explain why they have such a significant appeal.
Indeed, research has shown these diets can actually be very effective for certain people. In a study of 278 adults with obesity, a 12-week crash diet of 810 calories a day led to greater weight loss after 12 months than people who only reduced their calories by portion control. The crash diet group lost an average of nearly 11kg versus only 3kg in the moderate diet group.
Similarly, one study showed that very low-calorie diets may be beneficial for people with type 2 diabetes. The researchers found that 60% of participants who ate 600 calories a day for eight weeks were able to put their type 2 diabetes into remission. They also lost around 15kg on average.
A follow up at 12 weeks showed participants put around 3kg back on but, importantly, their blood sugar levels remained similar. But while these diets may lead to short-term weight loss success in some people, they can have the long-term consequence of damaging your metabolism. This may explain why around 80% of diets fail with the person ultimately putting all the weight they lost back on, or even gaining more weight than they lost.
Crash diets and metabolism
Your metabolism is the sum of all chemical reactions in the body. It's responsible for converting the food we eat into energy, and storing any surplus energy as fat. Your metabolism is affected by many things including diet, exercise and your hormones. Crash diets affect all these components.
With a crash diet, you consume far less food than normal. This means your body doesn't need to use as much energy (calories) to digest and absorb the foods you've eaten. You also lose muscle. All of these factors lower metabolic rate meaning the body will burn fewer calories when not exercising.
In the short-term, crash diets can lead to feelings of tiredness, which makes doing any activity (let alone a workout) challenging. This is because less energy is available and what is available is prioritised for life-sustaining reactions.