Howard Flaks, a specialist in obesity treatment in a private practice in California, USA, replies: A certain amount of body fat is essential to cushion the joints, protect the organs and help regulate body temperature, as well as to store vitamins and help the body sustain itself when food is not available. Therefore, it's possible to have a body fat percentage that's too low.
Numerous athletes, some of the healthiest people around, have five per cent fat and are extremely healthy. But the important aspect is what you do to achieve a low body fat percentage. Lowering your levels by starving, dehydrating yourself or using laxatives can be very dangerous.
But if you do it in a healthy way, using proper diet and exercise, I believe some people can go as low as five per cent, but certainly not below that. Some researchers feel that with elite athletes, strength and mental concentration deteriorate below seven per cent body fat. As well, loss of muscle can occur below five per cent, as the body will turn to muscle for fuel in the absence of fat.
It's much healthier to have lower body fat percentages than it is to have elevated levels. Obesity actually suppresses the immune system, so the heavier you are, the more susceptible you are to colds and infections.

Bonnie Modugno, a nutrition consultant, replies: Having less than ten per cent body fat isn't necessarily unhealthy. Generally speaking, seven per cent to ten per cent body fat is found on elite athletes such as long-distance runners. Eleven per cent to 13 per cent is considered exceptionally fit; 14 per cent to 18 per cent is considered average. These numbers are for men in their 20s; there are no figures for categorising body fat/fitness levels for men in their 30s, 40s, 50s or beyond.
While body fat is a component of measuring fitness, it's only a small piece of the puzzle. Bulging muscles and lean body mass are often associated with fitness, but some people with this appearance may not be fit at all; they may have risk factors such as high blood pressure or elevated cholesterol levels that are indicative of poor general health.
Overall, though, the concept of fitness needs to be expanded beyond body composition to take into consideration cardiovascular fitness and metabolic fitness - how your body metabolises energy.
It's more important to ensure that your body is using energy effectively so you have a sense of well-being and enough energy for the tasks you ask your body to perform.