Premature birth
If conception does occur, several studies suggest that being underweight before pregnancy raises the risk of going into labour prematurely. In 2004, research by the Yamagata University School of Medicine in Japan also found that "the risks of a low-birthweight infant and hospitalisation of the infant were elevated significantly". BLISS, the premature baby charity, agrees.
Premature ageing
"The fat cells in the face are much more sensitive than those in the body," says cosmetic surgeon Jean Louis Sebagh at the French Cosmetic Medical Company in London. "If you lose too much weight you can destroy them. Then even if you do regain weight elsewhere, you can't restore the volume to your face. And volume is youth. You need that padding to create highlights rather than shadows, which make you look old. And, of course, if you lose padding underneath lines and wrinkles, they look deeper too."
Loss of libido
The body is a miracle of biological efficiency, but not necessarily very bright. If it feels it is entering a period of low-calorie privation, it will prioritise ruthlessly regardless of whether or not the famine rations are self-imposed. And, whatever the internet may tell you, sexual congress is not a priority. As Heat magazine pointed out to Victoria Beckham this week during its enumeration of the perils of extreme dieting, without sufficient calorific fuel, the flames of sexual desire fade. No doubt the proximity of a chiselled sporting hero in the marital bed is a good way to stir the dying embers, but for most of us this is not an option.
Organ failure
The skin is only the most visible organ to be affected by lack of fat. Like any other fat deposits, those surrounding the internal organs shrink during dieting and provide less of the cushioning necessary for smooth working between them. When this is coupled with inadequate nutrition, it affects the intake and balance of salts and minerals in the body, and the heart (a big potassium fan), liver (a rapacious devourer of vitamin C) et al come under immense strain and, in extreme cases, organs can fail.
Infertility
In addition to the problem of a vanishing libido, being underweight can affect a woman's fertility. Women who are too thin frequently have disordered hormonal systems, and in particular produce less oestrogen, which reduces the chances of conception. The body needs fat reserves to draw on before it will contemplate growing a new individual - if a woman's body fat percentage is too low, her periods will usually stop.
Happiness
Researchers at Bristol University followed 3,000 people for 16 years and found a correlation between an increased body mass index (BMI) and a decreased risk of serious depression. For every five-point rise in a subject's BMI, the risk of suicide was reduced by about 15%. Inevitably publicised by some of the press as "proof" of the old saw that fat people are jolly, the results were actually surprising, and perhaps even more meaningful than the raw statistics suggest, given the prejudice and discrimination endured by those who are seen as even slightly overweight at a time when the body ideal is an unprecedentedly thin one.
Stronger bones
More women die from hip fractures due to osteoporosis than die from cervical, uterine and breast cancer combined, according to the UK's Osteoporosis Society. "The heavier you are, the less likely you are to suffer from the demineralisation of the bone that is involved in osteoporosis," says Glenn Gaesser, professor and director of kinesiology at the University of Virginia. The likely reasons for this are that fat cells retain oestrogen, the hormone that militates against calcium and bone loss; that fatter people tend to take in more of the nutrients necessary for healthy bones; and that bones that habitually bear greater loads tend to maintain a higher density.
Longevity
It is not an excuse to dive headfirst into your pie collection, but some solid research does suggest that there is not the strong linear relationship between extra pounds and certain early death that some would have us believe. In 1996, a study by the National Centre for Health and Statistics and Cornell University analysed data from more than 600,000 people. It found that the lowest mortality rate for non-smoking white males existed among those with BMIs of 23-29, which overlaps with the range officially classified as overweight.
Lowered lung cancer risk
In a review of the literature of the past 30 years, Professor Gaesser noted 35-40 studies that showed a lower incidence of and mortality from certain cancers among those with greater body weights. The most famous of these is research sponsored by the National Cancer Institute, which studied 3,607 lung cancer patients and a control group of 9,681 and found that women with a BMI of less than 22 had a 190% higher risk of developing the disease and those between 22 and 24.9 a 140% higher risk than those with a BMI of 28 or more. (Between 18.5 and 24.9 is the ideal BMI for most women.) All this applies only to non-smokers, of course, since no amount of adipose tissue will protect you against the effects of sucking carcinogens into your lungs.
No diet damage
Those, like Nigella Lawson who are comfortable with their weight, even if it involves a few excess pounds (but not obesity), will avoid the multitudinous injuries incurred by yo-yo dieting. While high blood pressure and certain types of heart disease are caused by being overweight, there is also evidence - from John Stanley of Oxford University among others - to suggest that these conditions can also develop as a result of the strain caused to the body by the repeated piling on and starving off of weight.
