Who’s afraid of being fat?
A new study suggesting that being a bit chubby is harmless has left anti-obesity activists spitting out their salad.
The usual New Year spate of diet stories has been unusually lively this year, from a report by the UK Royal College of Physicians (RCP) making explicit parallels between obesity and smoking, to a call by the shadow health secretary, Andy Burnham, to regulate the amount of sugar in breakfast cereals.
But perhaps the most controversy was generated by a dry piece of meta-research in the Journal of the American Medical Association (JAMA) – alluringly entitled ‘Association of All-Cause Mortality With Overweight and Obesity Using Standard Body Mass Index Categories’. (Body mass index – BMI – is the most common measure of obesity and is defined as weight in kilograms divided by the square of height in metres (kg/m2).) The researchers used 97 existing studies, covering nearly 2.9million individuals, to find the statistical relationship between the standard weight category individuals fell into – underweight, normal, overweight, obese – and how likely they were to die within the timescale of the study.
Sounds provocative, no? Did we really need another piece of research to tell us that fat people die faster? Why the fuss?
Because, as previous studies have already shown, fat people don’t die sooner than others. Very fat people – those with a BMI of 35 or more – die younger on average than ‘normal’ people (BMI 18.5 – 25). Underweight people also die faster. But people in the ‘overweight’ category (BMI 25 – 30) are less likely to die than those of ‘normal’ weight. And people with ‘grade 1 obesity’ (BMI 30 – 35) have roughly the same lifespans as ‘normal’ people.
This is not, I repeat, a surprising or new result. Like many things we are warned against for our own good, putting on weight has a J-shaped relationship with our odds of dying. The graph looks like a tick: above the optimum BMI, your likelihood of death within a given period rises steadily. Below that ideal weight, it rises swiftly. And the optimum weight is near the top of the ‘normal’ band.
The ‘normal’, healthy BMI category used by medical professionals and policymakers is a poor fit with what is observed in real life. Being at the lower end of normal, with BMI between 18.5 and 20, corresponds to the same chance of dying as somebody with BMI around 30. So if your attitude to food is ruled by your desire to minimise your statistical risk of death, nudging the ‘overweight’ label is your safest bet.
Raining on the anti-obesity parade
If the JAMA study had drawn the conclusion that BMI determines date of death, and therefore most people should put on a few pounds and only the truly immense should bother losing weight, there are several criticisms that could be levelled.
For starters, BMI is a crude measure of health, equating the heavily muscled athlete with the lardy couch potato, and the supermodel with the emaciated invalid. Health researchers now point to other measures, including waist size or type and distribution of fat, as more reliable predictors of fat-related health risks.
So BMI is a crude measure when applied over a population and almost useless for an individual as a guide to health. But it happens to be the measure widely used to give us figures such as ’65 per cent of men and 58 per cent of women are either overweight or obese’, which are cited as reasons for interventions into our lives for our own good. These interventions might include a lecture from your GP when you visit with problems unrelated to your weight or apparently serious proposals to dock your housing benefit if you don’t stick to your medically prescribed exercise regime. Or, as mentioned above, the plan for the government to control how sugary our sugar-frosted cereals can be, or how large a fizzy drink we’re allowed to take into the cinema.
So it’s rather disingenuous to criticise one paper using standard BMI-based categories, while applauding policy measures using exactly the same categories.
What that study did say
In the JAMA study, the overweight were found to be six per cent less likely than the normal to die while being studied, or in technical terms to have a hazard ratio (HR) of 0.94 against 1 for all-cause mortality (ACM). That has been criticised as being too small to be significant, especially as for most age groups you’re very unlikely to die of anything, so a six per cent increase (or decrease) in that already small risk can only be measured at all in large groups of people.
For an individual’s lifespan, such a small risk is unlikely to add (or subtract) even the time it would take you to read the calories on a McDonalds menu. Interestingly, among the over-65s (the age group that around nine in 10 of us will have reached when we die) both the overweight and the grade-1 obese groups had HR of 0.9 (that is, were 10 per cent less likely to die than those in the normal group). So a few extra pounds in retirement have more of an impact on your lifespan, but it’s still a small difference. Statistically, you’d get more benefit from a bit of exercise and a nightly tot of alcohol than whether or not you carry a few extra pounds or not.
However, that doesn’t change the fact that the overweight and mildly obese are not, as we are so often told, on the fast track to an early grave.
Any study that links only one variable with short-term likelihood of dying inevitably misses out other significant factors. This criticism has been levelled at the JAMA paper. Smokers tend to weigh less, for example, but are at higher risk of several significant causes of death. People who are already ill may have lost weight and be over-represented at the underweight end of the scale. Are these factors skewing the results?
Possibly – but the difference can’t be explained in those terms alone. Studies that include only non-smokers and those with no diagnosed disease at the start of the study have shown a weaker correlation between lower weight and earlier death, and a stronger effect from obesity. But the overall pattern still holds, as noted in detailed comments by the JAMA study’s authors.
Since being overweight is now regarded as a risk factor in itself, some have argued that overweight people are getting better due to earlier medical intervention, and that this compensates somewhat for their less healthy state. This is possible, though it’s not necessarily more convincing than suggestions that a little body fat is a handy reserve in times of illness, or vital padding to protect older people against the effects of falls. It also raises the question of whether just looking at somebody’s weight is even helpful to doctors trying to predict heart disease or other illness.
However, the paper’s authors make no argument at all that their research means the current obesity panic should be shelved, and we should all relax about what we eat and drink. In stark contrast to the willingness of some public health researchers to extrapolate from one study to sweeping policy proposals, the conclusions are one paragraph of bare statistical analysis:
‘Relative to normal weight, obesity (all grades) and grades 2 and 3 obesity were both associated with significantly higher all-cause mortality. Grade 1 obesity was not associated with higher mortality, suggesting that the excess mortality in obesity may predominantly be due to elevated mortality at higher BMI levels. Overweight was associated with significantly lower all-cause mortality. The use of predefined standard BMI groupings can facilitate between-study comparisons.’
The authors make no suggestions for policy measures, or personal lifestyle choices, based on these conclusions. They meticulously list the limitations of their work, including the crudeness of BMI as a measure and the fact it addresses only mortality and not ill-health. In fact, the paper is a model of sober rigour that other public-health researchers would do well to follow.
Epidemic? What epidemic?
The reason this unassuming paper drew howls of outrage was the same as the reason the benefits of moderate alcohol intake are never noted without criticism: it spoils the headline health message that Fat is Bad.
Even worse, it blows the cover on the great myth – that an epidemic of Bad Fatness is sweeping the developed world. By including the dangerously obese, the innocuously tubby and the healthily plump in one category, ‘overweight including obese’, 60 per cent of the English population are labelled as potentially At Risk.
Being At Risk means these people need guidance and protection from their own vulnerable state, from the temptations of our obesogenic world and the frailties of their own sugar-addicted brains. At such a time of national peril, no measure is too extreme.
But less than a quarter of English adults are obese, according to new figures released just before Christmas, a fraction almost unchanged since 2007. And the ‘morbidly obese’ category – BMI over 40, the ones for whom it really might be worth shedding a few pounds, medically speaking – also remains steady since 2009 at 2.5 per cent of the UK population.
If only one in 40 of us is in significant weight-related danger, why do the other 97.5 per cent of us need to be protected by the state against sugary cereals and fizzy drinks? Could it be because only a few of us have fallen, but all of us are in peril? Weak, foolish and easily led astray, we need to be frightened back on to the right path. Thus Tam Fry, spokesman for the National Obesity Forum – who has called for children to be monitored from birth for signs of obesity – told the Independent: ‘If people read this and decide they are not going to die… they may find themselves lifelong dependents on medical treatment for problems affecting the heart, liver, kidney and pancreas – to name only a few.’
So there we have it. Those extra post-Christmas pounds aren’t going to kill you. If you’re approaching an age at which there’s any real prospect you will die, they probably have a tiny protective effect. But if you’re told the truth, suggest the obesity obsessives, you’ll gorge yourself into a disgusting ball of flab.
Nothing in this study is worth changing your lifestyle for, or revising your New Year diet plans. But you might want to bear it in mind next time somebody proposes a policy to protect us all from the deadly tide of flab.