Wednesday, 23 September 2009

Terrible article on weight

I read BBC News a lot, and in general, my thoughts about its stories are more about how bad the events described are, not how bad the reporting is. This is not one of those times.

BBC News's science (and related fields) reporting is one of the weakest aspects of the operation, and this article about perceptions of obesity is a good example of this.

Let's start at the top

The poll, carried out by YouGov for Slimming World, found just over a quarter of 2,000 people questioned had measurements which would place them squarely in the obese camp.

So, no reason that a survey about weight for a magazine that exists to promote diet plans might in any way be slightly biased. This potential for bias never even gets a mention in the rest of the article.

Over half of those deemed morbidly obese believed they ate a healthy diet, while more than a third of the overweight said they had never tried to shed the pounds.

Given that it's extremely possible to be fat and eat a healthy diet (or indeed, as I was as a teenager, very underweight despite eating numerous large meals), that's not as unlikely as it's being framed as. I also wonder what proportion of that third are women, since "overweight" BMI ranges are more socially acceptable in men (and arguably less noticeable at all, given that the cultural images of women in the UK are generally extremely thin).

It also doesn't say, though I expect the survey collected it, how many of the "underweight"1 people have also dieted in an attempt to get thinner still.

While our life expectancies have increased at the same time as our weight, the consensus now is that cases of diseases such as diabetes and even cancer could be reduced if everybody strove to be within the "normal" Body Mass Index (BMI) range

Okay, so life expectancies have increased as weight increases (and bear in mind that the increase in weight averaged across the whole population has been small). Therefore, we should reduce the weights of people to increase life expectancies even more.

That's not particularly convincing. In the US, there's a study called NHANES, which is a huge survey of health and diet. It's been carried out three times, and in each case, the people studied have been the subject of a later follow-up study. One of the basic checks in the follow-up study is whether or not the person is still alive.

There's a 2005 paper in the Journal of the American Medical Association called "Excess Deaths Associated With Underweight, Overweight, and Obesity" (copy via "Alas, a blog"). It takes the 18-25 BMI category as a baseline, and compares that (controlling for age and other obvious factors, such as smoking) with four other categories:

  • Less than 18 ("underweight")
  • 25-30 ("overweight")
  • 30-35 ("obese")
  • More than 35 ("also obese, but more so")

It's a simple test: check what proportion of the 18-25 BMI range have died since the original study, and then see what proportion of the other ranges have died. If the arguments about fat being dangerous are true, then the heavier categories should have more deaths - the "Excess deaths" referred to in the title of the paper. For shorthand, I'll refer to categories as being "more dangerous" or "less dangerous", if they have excess deaths or fewer than expected deaths respectively.

There are some useful graphs at the bottom of the second PDF, but in summary:

  • In adults between 25 and 59 years old, there is no statistically significant difference between the 18-25 range, the 25-30 range, and the 30-35 range. The 35+ range is associated with an increase in the number of deaths (though not for people who have never smoked). The 18- range is also not statistically significant.
  • Between 60 and 69 years old, the 18- range becomes statistically significantly more dangerous. The 35+ range remains statistically significantly more dangerous than the 18-35 range, but by a lower margin.
  • At 70 years old and over, all ranges except 18- are statistically the same as the baseline. 18- is again more dangerous

It's worth noting further that even the statistically significant changes are relatively small - even the most dangerous category (BMI 18-, age 60-69), is only between 2 and 3 times more dangerous. On a similar measure, smoking is around 20 times more dangerous. Conversely, probably the safest category (BMI 25-30, age 25-59) is between baseline and 33% safer.

The study also notes that the older data from NHANES 1 has a higher death rate. The more recent data from NHANES 2 and NHANES 3 shows no statistically significant increase in death rates for any category except 18- (in NHANES 3 only. Conversely, NHANES 2 shows a statistically significant decrease in death rates for the 25-30 category.

So, essentially, differences in BMI are unlikely to change your chance of dying in a given time period much at all, especially if you don't smoke (unless you get old, in which case it starts to get somewhat dangerous if it drops too low, but even then not by much).

This bit of the last excerpt needs more comment.

[...] if everybody strove to be within the "normal" Body Mass Index (BMI) range

There are multiple major problems with this sub-sentence alone:

  1. BMI is a horrible measure of healthiness-of-weight. It was designed as a statistical measure of weight within populations, not as something that could be applied to individuals. The "normal" category comes from the late 19th century, which had far more problems with malnutrition than the present day. It doesn't distinguish at all between fat and muscle mass (most athletes are "obese" according to a BMI scale). It doesn't do anything different for body type - it's designed for white male Europeans of average build.
  2. No-one has managed to find a way to change the long-term weight of an individual to a different selected weight. Short-term, starvation will reduce it and overeating (and we're talking 10,000 calories a day, not just an extra portion of chips) will increase it. Long-term, this changed weight will revert back to the previous norm for that person in almost all cases. This is like saying "if everyone strove to be taller, we wouldn't need to buy so many stepladders". It's not true, and even if it was, it wouldn't mean people could get taller.
  3. Diets and other weight-loss interventions can be very bad for health in and of themselves. "Yo-yo" dieting (a natural consequence of telling people that they have to lose weight or else
  4. "normal" has been picked arbitrarily based on what was average over a century ago. You'd expect that with malnutrition being reduced in European countries since then that the mean and median weights would rise... in fact, it would be worrying if they didn't.
  5. (I've probably missed at least one, but this is enough...)

The focus on the extreme in television documentaries about the very large but also in the pictures that are chosen to illustrate articles about obesity have also been held up as another potential culprit.

A few points to the BBC for identifying the "headless fatty" pictures as not really depicting what the 'average' "obese" person looks like. Minus several million points for going ahead and using one at the top of the article anyway (it cuts out quite a lot that's not the head, too).

The article goes on to criticise the negativity around fat people, and mentions the abuse that fat people (including children) receive, but only identifies this as a problem because:

"All the discussion around overweight children is so negative that it is not surprising parents find it difficult to acknowledge there is a problem. It's a defence mechanism," says Dr Susan Jebb of the Human Nutrition Research Laboratory of the Medical Research Council.

So, it's only a problem that fat people are subjected to abuse for being fat, and told that being fat is bad, because it means that we can't tell them that being fat is bad in a detached, emotionless, scientific manner. Glad that's sorted, then.

It terrifies me that this is the quality of the debate that's going on about the (probably non-existent) "obesity crisis". So far, the only actual outcome all this worry has appears to be a reduction in the quality (but not the length) of life of fat people from all the abuse. Oh, and a great profit for the dieting industry. Keep commissioning those surveys, then.

Finally, missing from the article, any opposing view at all. The BBC science reporting does have a tendency to reprint press releases uncritically. Not every article needs to have an opposing view - and the media more generally certainly has a problem with looking too hard for them - but this is a news organisation that's run positive reports on research like the research I've quoted.

1 "underweight" and "overweight" are terrible terms in themselves, since they imply an "ideal" or "normal" weight and are generally used to imply that this "ideal weight" is the same for everyone (after 'correcting' for height). More on "underweight" and "overweight" at Alas, though there doesn't seem to be any consensus on what the "unmarked" category should be called. "medium-weight" or some variation sort-of works, though the variations on "government-mandated standard weight" make the point that normal/unmarked/medium has already been set arbitrarily.