BMI

onemanpeloton wrote: »

When I look at a BMI chart, the "healthy" range for me spans 20kg. 20kg!!

Not only is that an incredibly broad range (25% of my body weight), it doesn't address body composition, which is arguably more important.

I understand its use for doctors giving very basic and very broad advice to a wide range of people who are extremely overweight and need a very rough number to aim for, but I don't see it's place in an intelligent conversation based on an individual subject and their specific needs

onemanpeloton wrote: »

TavistockToad wrote: »

mattrus1989 wrote: »

cdjs77 wrote: »

mattrus1989 wrote: »

cdjs77 wrote: »

mattrus1989 wrote: »

cdjs77 wrote: »

mattrus1989 wrote: »

ceiswyn wrote: »

Is this a reference to this thread? https://community.myfitnesspal.com/en/discussion/10682410/need-help-and-advice-to-lose-more-weight-and-achieve-a-cyclist-climber-body

not exclusively although that thread does have elements of what I'm talking about, yes

I don't think that thread is the hill to die on. Regardless of the error rate involved with BMI, it's probably pretty accurate in predicting whether or not one is underweight if they are highly active. A BMI of 18.5 for a very active cyclist is likely underweight and will probably negatively affect performance (and I say that as a cyclist with a BMI of 18.5).

As for other mentions of BMI, it's not inaccurate enough that it should be disposed of completely, and for most people coming to this website, it's probably a good thing to keep an eye on. It also wouldn't hurt to choose goal weights for most people here based on BMI, especially if they don't have another way of accurately measuring their body fat.

I think we've already concluded that it wasn't one thread in isolation, so no ones dying on any hills.

How can something with such a broad range of acceptable measurements be classed as "pretty accurate"? Using myself as the example again, I wouldnt consider a 25% range of variability to be "pretty accurate".

I'm intrigued about the relationship between BMI and being active. Why is 18.5 (a supposedly healthy level of BMI) unhealthy for a cyclist and how would it negatively affect performance?

For weight loss, it seems to me that taking measurements of key areas of the body (as MFP allows you to do) would give you a better indication of body composition vs weight.

If by measurements you mean caliper measurements done by someone who knows what they are doing? Then probably, but measurements themselves aren't particularly accurate.

I also think you and I have a different definition of what it means to be accurate. Individual BMI isn't meant to be a point estimate of someone's body fat percentage, it's meant as an estimate to see what category they fall into and their risks of having an unhealthy body fat percentage based on that category. If people come to MFP with a BMI of 35, it's pretty likely they are indeed obese. As for how can something with such a wide range be accurate, you do realize what it is trying to estimate right? The range of healthy body fat percentages is also wide (14% to 31% for women and 6% to 22% for men), in order to include them all there would also likely be a wide weight range. You're really asking how can such a wide range of weight be healthy and that is not a question for BMI. (Aside from that, statistical accuracy doesn't care about what you think about interval widths, if such a wide interval is accurate for a sample, it's likely mathematically accurate for the population, whether you like how the math works or not).

18.5 is on the verge of underweight for anyone, for someone like him who regularly cycles, it's likely underweight, meaning he likely has a very low body fat percentage (especially since he is a man and likely has a lower body fat percentage than a woman at that weight). How will BMI affect his performance? It won't, but having a lower body fat percentage and trying to lose weight will, which is what BMI is estimating. If you are active and have a low BF%, you're just sacrificing muscle by losing weight, and at his height/weight, that's likely what he's doing. He's 5'9" and 66 kilos as a man, with an FTP of 220, he likely won't gain much by dieting. I say this as a 175cm (5'9") 56 kg (125lb) woman who has been this weight most of my adult life and is also an avid cyclist.

So would you consider caliper measurements done by someone who knows what they're doing to be less accurate than BMI or more?

I'm not sure what you mean in the first few sentences of the 2nd paragraph. Is BMI meant to determine body fat or not?

I agree with you about extreme BMI numbers being "likely" to correlate with obesity, but that can still be better understood by talking about the persons body composition rather than blindly applying a number and therefore a category to them.

I still don't understand the link youre making with cycling and BMI. Why does him being someone "who regularly cycles" have anything to do with being "likely underweight"? If we trust BMI then hes either underweight or he isnt, regardless of whether he cycles or sits on the couch all day.

You said originally that his BMI will probably negatively affect his performance. Now you say it won't......

Why would having a lower body fat % negatively affect his performance if he lowers that percentage further? He's specifically interested in climbing hills, where a good power to weight ratio is essential. If that fat isn't assisting his power up the hills then it is hindering his power/weight ratio and slowing him down. I won't comment on whether thats a healthy choice, but I'm not sure how its possible to claim that it will negatively affect his performance.

Lastly, I'm not sure why you're telling me your statistics. What difference does that make to this discussion?

Sorry, English isn't my native language so perhaps the language in my post was confusing. Yes, I think caliper measurements done by a "professional" are probably more accurate at assessing BF% than BMI (but I also don't know whether this is true, I have never looked up the study of accuracy of calipers).

BMI, itself, the number, does nothing more than put you into a category, it's the categories which are useful. I do not think BMI, itself, estimates BF% as a point estimate (by point estimate I mean it is not accurate to say a BMI of X equals X% bodyfat, point estimates are never accurate and no statistician would claim such), BMI can, however, make an estimate of whether one is over- or underfat.

I still don't understand the link youre making with cycling and BMI. Why does him being someone "who regularly cycles" have anything to do with being "likely underweight"? If we trust BMI then hes either underweight or he isnt, regardless of whether he cycles or sits on the couch all day.

I say likely because that is how statistics work. BMI is a statistical estimate. It does nothing more than give "likelihoods" and those likelihoods vary with different lifestyle factors (because the underlying population changes, if you want to know the mathematical reason). If we trust BMI then we can only say likelihood because that is all it tells us: likelihoods and risks.
18.5 is the cutoff point for a healthy BMI, taken from the whole population, with no other information, one could only say he was likely on the verge of underweight (because BMI is a likelihood estimation). If we know that he is an active cyclist, the underlying population changes. We are now not looking at BMI for the average population, we are looking at BMI for active cyclists. This distribution will look different. In this distribution, the cutoff BMI for being underfat will likely be higher.

As for BMI negatively affecting his performance, I must have worded it improperly or you misunderstood what I meant, I think having a low BF% and trying to lose weight will negatively affect his performance. I do not think BMI, itself, will negatively affect his performance, it is nothing more than an inferential statistic. If he has a low BF%, it will be hard for him to lose any significant amount of weight without also losing a lot of muscle. If he has a BF% of 20, then he can surely go for it and probably gain some power out of it. However, his w/kg is probably equivalent to a cat 4 racer, he could gain a lot more from decent training before he could gain power by losing weight. He isn't at the point yet where he can only gain power by dropping weight.

Lastly, I'm not sure why you're telling me your statistics. What difference does that make to this discussion?

Because I'm a cyclist at his desired weight/height, so I know how it affects at least my own performance. I also want to point out that I am not one of those MFPers who thinks anyone under a BMI of 20 is anorexic, as many people here seem to think.

You make a good argument, thanks.

I'm no expert in caliper measurements either, but I think it would be fair to say that body composition and fat percentage can be determined much more clearly than the BMI scale.

Your points about population changes is an interesting one, of which you no doubt have more knowledge than me. However, to me at least, its a confusing message. I thought that BMI was a universally recognised scale? Surely the BF% (which BMI claims to calculate) is healthy or unhealthy for any person, regardless of activity? Surely the official BMI numbers and thresholds dont change depending on your activity levels? Maybe I'm wrong but it seems confusing.

For the last part, I totally agree. If he can lose BF then it can still be beneficial to his climbing. That doesn't mean its beneficial to his overall health and it doesnt mean it would be easy to do. And yes, his power needs a lot more work than his weight.

BMI doesn't calculate BF% - which is exactly the issue i thought you were arguing? so now i'm just confused.

But it does assess your health and risk of obesity related diseases based on the assumed fat that your body has, does it not? Hence it categorising people as "obese" based purely on their height and weight