Fat or Fiction

Summary

More or Less (BBC Radio 4)

Episode: Fat or Fiction

Date: January 28, 2013

Host: Ruth Alexander
Produced by: BBC World Service

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Episode Overview

This episode explores the reliability and intended use of the Body Mass Index (BMI) as a measure of healthy weight. The hosts question BMI's effectiveness, especially for individuals, discuss a newly proposed formula to "improve" BMI, and examine how BMI categories were created. With expert opinions from mathematician Nick Trefethen, medical statistician Tim Cole, and public health researcher Katherine Flegal, the program questions whether BMI is truly a sound measure of health, or simply a blunt tool that is misunderstood in both personal and public contexts.

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Key Discussion Points and Insights

1. Host's Personal Skepticism about BMI (00:13–01:38)

• Ruth Alexander introduces her personal experience: according to BMI, she's always been "underweight," yet she feels healthy.
• Memorable moment: On-air BMI calculation: Ruth's height (1.74m) and weight (54kg) yield a BMI of 18, making her "underweight."
  • Wesley Stevenson: "Would you like a donut?" (01:30)
  • Ruth: "Not really…But the point is, I refuse to believe I’m unhealthily thin." (01:32)
• Immediate skepticism about BMI’s accuracy when evaluating individuals.

2. Limitations of BMI and a New Proposal (01:38–03:14)

• Flaw in BMI:
  Wesley Stevenson: "It's not always that good a measure when applied to individuals." (01:38)
• Introduction of a new formula by Professor Nick Trefethen (Oxford University), meant to correct for BMI’s peculiarities:
  • New formula: 1.3 × weight (kg) / height (m)^2.5
  • Prof. Trefethen: "My proposed alternative is not based on careful epidemiology... it's a proposal of a possibility." (02:26)
• Wesley Stevenson explains: Tall people previously called overweight may now be classified as normal, while some short people now shift from normal to overweight. (02:54)
• Ruth’s recalculated BMI becomes 17.8 (even lower), still underweight by either method. (03:08)

3. Expert Critique from a Medical Statistician (03:14–05:20)

• Professor Tim Cole (University College London) provides a professional perspective:
  • BMI, as a population measure, uses regression analysis to adjust weight for height.
  • "The fact that the power is rounded to 2 makes it easier to handle, but there’s a fair degree of uncertainty in what the power is and it’s not critical." (03:35)
  • Trefethen's formula tweaks the index but doesn’t address BMI’s underlying flaws.
  • "You can tinker about with the power of height...it shouldn’t be expected to do what it can’t do—which is to measure fatness." (04:21)
  • Key point: BMI knows nothing about body composition (fat vs. muscle), so it’s imprecise for individuals, but useful for large populations.

4. Origin & Arbitrary Nature of BMI Categories (05:20–07:09)

• The traditional BMI boundaries (underweight, normal, overweight, obese) were suggested in the 1980s by Garrow and Webster, and have "just kind of stuck."
  • Wesley Stevenson: "For example, Brad Pitt when he was muscle bound for Fight Club, was overweight according to his BMI..." (05:37)
• Some countries adjust categories for their populations (e.g., in India, overweight = BMI over 23).
• Professor Cole: "Trying to draw a line somewhere on that spectrum and say, below this cutoff is healthy and above this cutoff is unhealthy...is obviously a rather arbitrary exercise." (06:21)
• Discussion of recent US studies suggesting that people categorized as overweight might have lower mortality than those in the "normal" range.

5. BMI and Mortality: The Research (07:09–08:09)

• Dr. Katherine Flegal (US National Center for Health Statistics), whose research found that "mortality in the overweight category was slightly reduced...and the mortality in the obesity category was slightly increased relative to the normal weight category." (07:23)
• Wesley Stevenson: "That sounds to me as though your BMI might give you a slightly confusing picture about whether you're more or less likely to die an early death." (07:42)
• Dr. Flegal: "BMI categories are weight categories and they're not exact health categories...Weight is not synonymous with health, and our study indicates at least it's not synonymous exactly with mortality." (07:54)

6. What BMI Means for Individuals (08:09–08:54)

• Wesley Stevenson asks Flegal if Ruth is simply an outlier.
• Dr. Flegal: "On a population level, it’s probably pretty good, but it’s only one indicator...There are people who eat well and exercise probably at every weight level." (08:34)
• No personal advice offered; implication is that BMI is only one piece of the puzzle and may not accurately represent individual health.

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Notable Quotes & Memorable Moments

• On BMI’s Limitations:

  • "It's not always that good a measure when applied to individuals." — Wesley Stevenson, (01:38)
  • "BMI is an imperfect index...it shouldn’t be expected to do what it can’t do—which is to measure fatness." — Prof. Tim Cole, (04:21)

• On the Arbitrary BMI boundaries:

  • "Trying to draw a line somewhere on that spectrum and say, below this cutoff is healthy and above this cutoff is unhealthy...is obviously a rather arbitrary exercise." — Prof. Tim Cole, (06:21)
  • "Brad Pitt…was overweight according to his BMI, as was England rugby player Johnny Wilkinson..." — Wesley Stevenson, (05:37)

• On Research Findings:

  • "Mortality in the overweight category was slightly reduced relative to the normal weight category…" — Dr. Katherine Flegal, (07:23)
  • "Weight is not synonymous with health, and our study indicates that at least it’s not synonymous exactly with mortality." — Dr. Katherine Flegal, (07:54)

• On Individual Variation:

  • "There are people who eat well and exercise probably at every weight level. So it's not a perfect match." — Dr. Katherine Flegal, (08:34)

• Humorous Aside:

  • "I checked your BMI with Dr. Flegal and asked her whether you were just an outlier or whether the underweight category was in the wrong place." — Wesley Stevenson, (08:26)
  • "I’m not going to give her any advice." — Dr. Katherine Flegal, (08:51)

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Important Segment Timestamps

• 01:10–01:26: Ruth’s on-air BMI calculation
• 01:38–02:54: Introduction and discussion of the proposed new BMI formula
• 03:14–04:21: Prof. Tim Cole explains BMI’s statistical background and limitations
• 05:20–07:04: Origins of the BMI categories and their arbitrary nature
• 07:09–08:09: Discussion and summary of Dr. Flegal’s mortality and BMI study
• 08:21–08:54: Individual assessment: BMI’s value and shortcomings for personal health

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Conclusion

The episode illustrates that while BMI is useful for assessing entire populations, it is flawed and potentially misleading for evaluating individual health. The introduction of a mathematically "improved" formula does not solve BMI's core problem: it cannot distinguish between fat and muscle or account for body composition. The boundaries which separate "normal" from "overweight" are historically arbitrary and may not accurately signal health risks, as even recent research finds some "overweight" people live longer than "normal" weight individuals. Ultimately, BMI should be regarded as just one tool among many in evaluating health, not a definitive measure.

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For Further Listening

Other episodes can be found at bbcworldservice.com/moreorless.
Contact the program at [e-mail].

Transcript

[BBC Announcer]

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[Ruth Alexander]

hello and welcome to More or Less on the BBC World Service. I'm Ruth Alexander, tipping the scales in favour of numbers that make sense. This week we're looking at the Body Mass Index, the internationally agreed measure, which, according to the Centre for Disease Control in the United States, is Australian, a reliable indicator of body fatness for most people and according to the National Health Service in the uk, is a good way to check if you're a healthy weight. Now, I must declare an interest right at the top because I personally have never found the BMI to be reliable. My BMI has always put me as underweight, and yet I think I'm fit and healthy. Now. The last time I was told this by my doctor was more than 10 years ago. So I'm going to calculate it again with Wesley Stevenson helping me.

[Wesley Stevenson]

Hi, Ruth. Yes. So what I need you to do is to step on the scales.

[Ruth Alexander]

Okay?

[Wesley Stevenson]

Yeah. Both feet, please. I'm going to give you a weight out on the radio. Is that okay?

[Ruth Alexander]

Go on, then.

[Wesley Stevenson]

It's 54 kilograms. And I also need your height. BMI, as it's known, is calculated by dividing your weight by your height squared.

[Ruth Alexander]

Okay.

[Wesley Stevenson]

So what's your height?

[Ruth Alexander]

1.74 meters.

[Wesley Stevenson]

1.74 meters. So you have a BMI of 18.

[Ruth Alexander]

Well, looking at the chart, that means I'm still underweight.

[Wesley Stevenson]

Would you like a donut?

[Ruth Alexander]

Not really, because I wouldn't mind, actually. But the point is, I refuse to believe I'm unhealthily thin.

[Wesley Stevenson]

And this is the controversy with bmi. It's not always that good a measure when applied to individuals. And a new formula has been proposed which apparently irons out some of the flaws. It's been put forward by mathematician Professor Nick Trefethen from Oxford University in the uk, and it's had a lot of coverage worldwide.

[Professor Nick Trefethen]

The formula I have recommended as something to explore changes from weight over height squared to 1.3 times your weight in kilograms divided by your height in meters to the 2.5 power, when you were

[Wesley Stevenson]

trying to recalculate it and come up with the better figure. Did you go back and look at sort of samples of people in order to try to calculate what would be the best?

[Professor Nick Trefethen]

No. The truth is that my proposed alternative is not based on careful epidemiology. And that's why I'm trying to emphasize that it's a proposal of a possibility. There is a literature there that suggests that it should be a power of more like 2.5 or even a 2⅔, whether that matches medical data. There's been some puzzlement about.

[Ruth Alexander]

Well, I'm not sure I'm much clearer as to why this new BMI formula might be better, but what effect does it have?

[Wesley Stevenson]

Well, on the whole, it means that some tall people previously deemed overweight are now normal and some short people who were normal are now overweight.

[Ruth Alexander]

And as I'm tall, presumably I've become even more underweight. According to this new measure.

[Wesley Stevenson]

Yes. Calculating your BMI with the new method, your score drops from 18 to 17.8.

[Ruth Alexander]

Even though I still feel perfectly fit and healthy?

[Wesley Stevenson]

Yeah, afraid so. But as Nick Trefethen has pointed out, he's a mathematician and doesn't have a medical background. So I asked for a second opinion from Tim Cole, professor of Medical Statistics at University College London in the uk. But first I wanted to know, what is BMI and what does it actually tell us?

[Professor Tim Cole]

If you imagine a group of people and you've got their weight and their height, and some people are tall and some are short, but you want to be able to compare the weight of the tall ones and the weight of the short ones, so you can use a statistical technique called regression analysis to adjust for height. And so what you're left with is an index, which is weight adjusted for height. Now, if you do that calculation, you end up with an index very similar to BMI. And the fact that the power is rounded to 2 makes it easier to handle, but there's a fair degree of uncertainty in what the power is and it's not critical.

[Wesley Stevenson]

So, looking at Professor Nick Trefethen's new equation for bmi, what do you make of it?

[Professor Tim Cole]

Well, it's one of a long line of papers that have been published talking about whether BMI is the right index. I think he's aware of the deficiencies of bmi, but I don't think his index goes anywhere to addressing those shortcomings, because BMI is an imperfect index and you can tinker about with the power of height as much as you like, but it shouldn't be expected to do what it can't do, which is to measure fatness, because it doesn't include fatness, it's just got weight. So BMI can't do better than make a guess, as it were, about how much fat there is. And of course, people's body composition varies enormously the amount of fat they have. Varies, and BMI doesn't know anything about that. So it's a very imprecise index when applied to individuals, but it is useful when it's applied to populations. The population does mean that you get a more precise estimate of BMI simply by averaging over large numbers.

[Ruth Alexander]

So this new equation isn't going to fix things. And actually, I'm probably just an outlier. But the BMI chart is separated into sections. Isn't it underweight, normal, overweight and obese? What I want to know is how did it get decided where the boundaries should be drawn?

[Wesley Stevenson]

Well, I asked Professor Cole that, and he said they were proposed in the mid-1980s by two scientists, Garrow and Webster, and it just kind of stuck. So Underweight is a BMI under 18 and a half, normal is between 18 and a half. And 25, between 25 and 30 is overweight and above 30 is obese. Now, some countries have moved the boundaries to better reflect their populations. In India, for example, anyone over 23 is overweight and over 25 is obese. But the boundaries do throw up some controversial results. For example, Brad Pitt, when he was muscle bound for the filming of Fight Club, was overweight according to his bmi, as was England rugby player Johnny Wilkinson when he was in his prime. So are the boundaries drawn in the right place?

[Professor Tim Cole]

There's a question. Well, the answer is that they are to some extent based on health premises, as understood by Garrow and Webster in 1985, when the world looks rather differently now. But also they are a statistical construct, and trying to draw a line somewhere on that spectrum and say, below this cutoff is healthy and above this cutoff is unhealthy. It's obviously a rather arbitrary exercise. I mean, there's a lot of debate now in the States whether people who are overweight are actually more unhealthy than those who are normal weight. As a suggestion, the mortality is lower in the overweight group.

[Ruth Alexander]

Gosh, overweight Americans might be in better shape than people who are normal weight.

[Wesley Stevenson]

Yes. What he's referring to there is a study that hit the news earlier this year that said being a little overweight might mean that you live a little longer than if you're normal. The study was done by Dr. Katherine Fleagle from the US National center for Health Statistics.

[Dr. Katherine Fleagle]

Well, we did a systematic review and meta analysis of all the literature we could find that used the standard body mass index categories. What we found overall is that the mortality in the overweight category was slightly reduced relative to the normal weight category, and the mortality and the obesity category was slightly increased relative to the normal weight category.

[Wesley Stevenson]

That sounds to me as though your BMI might give you a slightly confusing picture about whether you're more or less likely to die an early death.

[Dr. Katherine Fleagle]

Well, I think the moral of the study, if there is one, is that these BMI categories are weight categories and they're not exact health categories. And that's what it really comes down to. I don't think weight is synonymous with health, and our study indicates that at least it's not synonymous exactly with mortality.

[Ruth Alexander]

So the categories don't always tell us much about how healthy we are or might be in the future.

[Wesley Stevenson]

That's right. And you look pretty robust to me, Ruth, it has to be said. But just in case, I thought I'd check with another expert.

[Ruth Alexander]

Who's that?

[Wesley Stevenson]

My mum. And she says she should just eat bigger dinners.

[Ruth Alexander]

Okay.

[Wesley Stevenson]

Yeah. No, no, really. I checked your BMI with Dr. Fleegle and asked her whether you were just an outlier or whether the underweight category was in the wrong place.

[Dr. Katherine Fleagle]

Well, I don't know how we look at how this works for an individual. On a population level, it's probably pretty good, but it's only one indicator. I mean, there are people who eat well and exercise probably at every weight level. So it's not a perfect match.

[Wesley Stevenson]

So she shouldn't be too worried at this stage, then?

[Dr. Katherine Fleagle]

Well, I'm not going to give her any advice.

[Ruth Alexander]

Very wise and diplomatic. Katherine Fleagle talking to Wesley Stevenson. If you want to hear other editions of More or less, you can go to our website, bbcworldservice.commorealless and you can get in touch with the program via email or More or [e-mail] There are

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