Fad diets - the truth behind the headlines

When you think of diets, you probably think of the plus side - a svelte new bikini body and no more muffin top. But dieting is hard work, and there is no quick fix.

When you think of diets, you probably think of the plus side - a svelte new bikini body and no more muffin top. But dieting is hard work, and there is no quick fix. You may be tempted by one of the many crash/fad diets on the market, from the Atkins to the Dukan to the 'blood group' diet, spurred on by celebrity tales of miracle success. But while these diets may help you shed the pounds in double quick time, they're very, very dull, and often very restricted.

Side effects from limiting the foods you eat are much more common than if you reduce your calories in a more balanced way - constipation, headache, bad breath and muscle cramps all affect far more people on low-carb diets. (1) They don't work any better in the long term, either - while you may be lighter after six months with a low-carb diet than you would be on a 'conventional' weight- reducing diet, you're just as likely to have regained the weight you lost by a year. (2,3)

The Atkins diet was hugely popular in the 1990s, and I still see advocates of it today. Most of them believe firmly that there is some sort of magic about cutting out carbs - that this is the key to losing weight. In fact, if you look at the evidence, people lost more weight in the first six months on the Atkins diet than other diets because they ate fewer calories - there is no magic. (4) There are lots of theories about why they ate fewer calories, but one obvious one also explains them doing less well in the longer term - a very restricted diet is dull so you don't eat so much of it.

Today, my surgeries are full of patients singing the praises of the 5:2 diet (two days a week of 'fasting' with 500 calories for women and 600 for men). It does work for many people, and there is some scientific basis for believing it may protect against type 2 diabetes, heart disease and cancer. But the diet hasn't been around for long enough to see if these claims really hold up. Fasting can cause tiredness and problems with concentration, along with bad breath and constipation. And of course, cutting food intake on two days isn't going to help you lose weight if you go overboard on the others.

Perhaps the biggest problem doctors have with diets is that sensible advice - lose weight slowly by watching portion size and increasing your level of exercise - is boring. With at least two thirds of British adults and more than one in four obese, any miracle news story on weight loss is guaranteed to boost sales. Old wives' tales are given a modern twist in the headlines, backed by 'scientific studies'. 'Lose weight by eating a big lunch and fasting in the evening' (lunch like a prince, dine like a pauper) was the gist of one headline last year, based on a study of variations of sensitivity to the hormone insulin during the day. (5) In fact, the study was in mice - but does their bedtime equate to ours?

Last summer, a national newspaper ran a banner headline about evidence from brain scans linking high GI foods (rapidly absorbed into the system) with addiction - basically suggesting they were as addictive as heroin or cigarettes. The number of people in the study? Twelve. (6) While the results made interesting reading, every scientist knows that the smaller the study, the more likely it is that positive results will be due to chance. The overall message - avoid high GI foods like crisps, biscuits and pastries and substitute lower GI alternatives like wholegrain foods and beans - is sound. But I think we knew that already.


1) Yancy, W. S. et. al. Ann Intern Med 2004;140:769-777

2) Stern, L. et. al. Ann Intern Med 2004;140:778-785

3) http://www.ceb-institute.org/fileadmin/upload/refman/arch_intern_med_2006_166_285_nordmann.pdf

4) http://www.nejm.org/doi/full/10.1056/NEJMoa022207

5) http://www.nhs.uk/news/2013/02February/Pages/claim-weight-loss-is-linked-to-mealtimes-unsupported.aspx

6) http://www.nhs.uk/news/2013/06June/Pages/brain-scans-link-high-GI-diet-to-food-cravings.aspx

Dr Sarah is unable to provide medical advice or respond directly to questions concerning your health. If you have health concerns we recommend contacting your GP.