Which? magazine, published by the Consumers' Association, asked experts in nutrition, exercise and psychology to assess the most popular diet books on the market.
Those which provided balanced nutritional advice, stressed the importance of exercise and spelled out who should not follow the diet were praised.
But Which? expressed concern about fashionable "low carbohydrate" diets.
The problem with cutting down on carbohydrates in the diet is that the body cannot break down fat as effectively.
This can lead to a condition called "ketosis", which can cause bad breath, nausea and a bad taste in the mouth.
Which? warns followers of these kind of diets can also lead to a lack of fibre and too much protein, compared with a healthy diet.
The magazine said one book gave "potentially dangerous advice on doing a vigorous workout on an empty stomach and with low carbohydrate intake, which could lead to dizziness and fainting".
Sticking to very low-fat diets, the cabbage soup diet and eating according to blood-type could also cause health problems, said Which?.
Realistic advice call
Editor Helen Parker said a balanced diet and regular physical activity was the best way to keep fit and healthy.
But she said for those who did need a diet guide, a good book "should clearly point out groups of people who should not follow the diet without seeking medical advice".
"It should also set realistic weight loss targets that consider health benefits and encourage lifestyle and attitude changes, rather than a quick-fix."
A spokesman for the British Nutrition Foundation said: "We are concerned with the large number of fad diets currently on the market, some of which do not stand up to scientific scrutiny.
"Despite the hype, key advice on healthy eating has changed very little over the past few decades.
"A diet that includes a wide variety of foods with plenty of fruit and vegetables, modest amounts of fat and alcohol combined with regular activity will help maintain a healthy weight."