Fast weight loss? Or just another crash diet?

When patients eat only 800 calories a day, it can be hard to make sure they get all the micronutrients they need

A diet where patients cut their food intake to just 800 calories a day is the key to dropping fat, lowering cholesterol and reducing blood sugar, some GPs say.

The British author of the popular 5:2 diet, Dr Michael Mosley, claims his new diet regime, known as the fast800, will allow patients to drop up to three kilos of weight a week.

The fast800 is a three-stage dieting and lifestyle regime. Individuals start by consuming no more than 800 calories a day for eight weeks, or until they have lost the amount of weight they have been targeting.

The second stage mirrors the 5:2 diet, where individuals eat 800 calories on two days a week and stick to a healthy Mediterranean-style diet for the remaining five days of the week.

The third stage is for individuals to maintain their reduced body weight by eating a portion-controlled, healthy Mediterranean diet, no longer fasting, but continuing to avoid snacking through the day.

“Our philosophy is to educate patients about food; provide practical support via weekly shopping lists and recipes; and, critically, engage with them in an online forum where professionals are available to offer the support many people need to achieve their goals,” the diet authors said.

But Professor Clare Collins, a nutritionist and dietician in the Faculty of Health and Medicine at the University of Newcastle, said when patients eat only 800 calories a day it was hard to make sure they were getting all the micronutrients they needed.

“The evidence is definitely there that rapid weight loss can help you achieve big weight loss long term (but) these people are at a higher risk for developing gall bladder disease and getting constipated from a lack of fibre,” she said.

In addition, the authors of the new diet warned that fasting diets were not suitable for patients on medication such as insulin, gliclazide or warfarin.

“If patients want to read about it or buy the book that’s fine, but they shouldn’t start it without a health check from their GP. You want to know what your blood pressure is, what your blood sugar is, [and] your cholesterol levels, so you can judge whether it’s improving your health or not,” Professor Collins told The Medical Republic.