Whither BMI?

Body Mass Index is a much-maligned measure of fatness, yet it remains the standard measure of weight-related health risk among the medical and scientific community. In this post, I'm going to try to explain why this is the case.

Before BMI standards were widely adopted in the 1980s, fatness was largely determined by height/weight charts. Those early BMI standards considered men with a BMI >=27.8 and women with a BMI >=27.3 to be "overweight." Then in 1998, the National Institutes of Health lowered their BMI cutoffs to match the World Health Organization standards. Under the new guidelines, a BMI of 25 or more was considered "overweight," and 30 or more was considered "obese." It sounds a little convenient that these categories happen to fit such nice round numbers, doesn't it? But this is pretty much the basis for these categories:


And also, to a lesser extent, this:



(These images courtesy of Obesity Online).

The relative risk of diabetes and heart disease goes up at a BMI of 25, and the line of BMI vs. risk seems to change slope at around BMI = 30 (references are on the images). And BMI is cheap and easy to measure, all you need is a scale and a yardstick, or even just a telephone--you can just do a survey and ask people their height and weight. In contrast, to actually measure fat mass accurately, you need a machine called a DEXA scanner which costs $20K-$80K depending on how fancy your machine is.

So those are the main reasons why BMI is so popular among researchers and clinicians. Now, on to the criticisms.

The main criticism I hear of BMI is that it is simply a height/weight ratio and does not take into account muscle mass. While this is true, most people (women especially) are not going to be much affected by this unless they are body builders or professional-caliber athletes. Take, for example, Cruiserweight boxing champion David Haye, pictured here:



A very muscular guy, obviously, yet his fighting weight BMI clocks in right at 25, the border between normal and overweight (he's 6'3" and the Cruiserweight limit is 200 lbs. I chose a boxer for this example, because I figure their weights must be pretty accurate). The point here being that while muscle mass is one of the more common criticisms people have of BMI, I think that for the most part it's not that big a confounding factor in most cases.

One element that does seem to be a major confounding factor is race. Several studies have shown that people of Asian descent tend to be fatter than Caucasians with the same BMI, and consequently, they tend to develop weight-related illnesses at lower BMIs (1-4). Latinos also seem to be more susceptible to diabetes at lower weights, although less so than Asians (4). For this reason, some people now think that waist circumference is a better indicator of risk of overweight-associated illness, and other researchers suggest that using the two measurements in tandem provides the best estimate of risk (5-7).

In addition to racial differences, some investigators feel that BMI cutoffs should be different for women and men, as men tend to have more muscle mass due to their higher testosterone levels. As you can see in the graphs above, men have a lower relative risk of diabetes vs. BMI compared to women, but the risks for cardiovascular disease are similar.

Another major problem with using BMI as a determinant of health risk is the fact that several recent studies have shown that people with overweight BMIs have a lower risk of mortality than people with normal BMIs (8-9). (Of course, everyone's risk of mortality is 100% eventually, but this was as measured within the period of the study). This finding was perplexing to researchers, as it would seem to counteract the information in the graphs above, findings which had been replicated many times. Some have suggested that these findings could be a result of some people in the "normal" group experiencing weight loss due to undiagnosed illness that later contributed to death. Others pointed to the so-called "obesity" paradox: the finding that while overweight people are more likely to be diagnosed with heart disease and renal failure, they also have a survival advantage over normal-weight people with this disease (10). (There's a lot more to say about the obesity paradox, but I'll save it for another post!) At any rate, the overall picture is still quite murky.

In summary, I would say that BMI is a somewhat useful tool for determining whether a person is statistically at heightened risk for certain complications of obesity. But it should be kept in mind that this increased probability is not at all a predetermined fate. After all, even a 45-year old woman with a BMI of 30-35 still has less than a 50% chance of developing type II diabetes (11). So, as they say on the internet, YMMV.

1. WHO Expert Consultation.

Appropriate body-mass index for Asian populations and its implications for policy and intervention strategies. The Lancet. 363: 157-163, 2004.

2. Deurenberg-Yap M., Deurenberg P. Is a re-evaluation of WHO body mass index cut-off values needed? The case of Asians in Singapore. Nutrition Reviews 61:S80-S87, 2003.

3. Huxley R, James WP, Barzi F, Patel JV, Lear SA, Suriuawongpaisal P, Janus E, Caterson I, Zimmet P, Prabhakaran D, Reddy S, Woodward M, Obesity in Asia Collaboration. Ethnic comparisons of the cross-sectional relationships between measures of body size with diabetes and hypertension. Obesity Reviews 9:53-61, 2008.

4. Shai I, Jiang R, Manson JE, Stampfer MJ, Willett WC, Colditz GA, Hu FB. Ethnicity, obesity, and risk of type 2 diabetes in women: a 20-year follow-up study. Diabetes Care 29: 1585-1590, 2006.

5. McCarthy HD. Body fat measurements in children as predictors for the metabolic syndrome: focus on waist circumference. The Proceedings of the Nutrition Society 65: 385-392, 2006.

6. Deurenberg P, Deurenberg-Yap M. Validity of body composition methods across ethnic population groups. Forum of Nutrition 56: 299-301, 2003.

7. Koster A, Leitzmann MF, Schatzkin A, Mouw T, Adams KF, van Eijk JT, Hollenbeck AR, Harris TB. Waist Circumference and Mortality. American Journal of Epidemiology April 15 Epub ahead of print, 2008.

8. Flegal KM, Graubard BI, Williamson DF, Gail MH. Excess deaths associated with underweight, overweight, and obesity. Journal of the American Medical Association 293: 1861-1867, 2005.

9. Flegal KM, Graubard BI, Williamson DF, Gail MH. Cause-specific excess deaths associated with underweight, overweight, and obesity. Journal of the American Medical Association 298: 2028-2037, 2007.

10. Schmidt DS, Salahudeen AK. Obesity-survival paradox--still a controversy? Seminars in Dialysis 20: 486-492, 2007.

11. Narayan KMV, Boyle JP, Thompson TJ, Gregg EW, Williamson DF. Effect of BMI on Lifetime Risk for Diabetes in the U.S. Diabetes Care 30: 1562, 1566, 2007.

This blog is not dead

I know the last bunch of entries here garnered a lot of new traffic thanks to links from Big Fat Deal and Elastic Waist, so I've been feeling awfully guilty about not posting anything since then! But life has been hectic, and unfortunately, the blog has to take a back seat to things like my real job and eating and sleeping. But I am working on something new to go up hopefully later this week or possibly early next week about the history, utility and limitations of BMI. Thanks for sticking with me despite my inconsistent posting schedule!

The politics of weight control

In doing research for my last post, I happened to come across an article written by Joanne Ikeda and her colleagues (1), critiquing the National Weight Control Registry program. Ikeda is a member of the board of directors of the Association for Size Diversity and Health, which promotes the Health at Every Size program. Their paper recycles the claim (twice) that 95% of persons who lose weight regain it (without providing any references), and basically makes the argument that since weight loss efforts are generally futile, HAES should be promoted instead of weight loss.

This is an argument that has been picked up by many in the Fat Acceptance movement, and I can understand their motivation. After all, if you say that significant permanent weight loss is impossible, then obese people are not "culpable" for their obesity, because it is utterly beyond their control. And if you say that it is possible, then you can rightfully accuse obese people of sloth and gluttony, right?

Obviously, I'm utilizing some hyperbole here, so you can probably tell that I personally don't agree with these viewpoints. I think the evidence strongly suggests that some people can lose weight permanently and others can't. It may even be the case that some people are able to lose weight at certain times in their lives and not at others. And it's impossible to make a judgement about any one individual's ability to lose weight.

So, I don't agree with the FA movement when they say that nobody should try to lose weight. And I don't agree with fat bashers who say that all obese people should try to lose weight. Having a "normal" BMI takes a different amount of effort for everyone, and people should be left to make their own decisions about how much energy to invest in regulating their body weight without being judged or criticized by others (except, obviously, in extreme cases involving eating disorders).

And I hope people will feel free to share their own views on the matter in the comments...

1. Ikeda J, Amy NK, Ernsberger P, Gaesser A, Berg FM, Clark CA, Parham ES, Peters P. The National Weight Control Registry: A Critique. Journal of nutrition education and behavior 37: 203-205, 2005.

Losing weight and keeping it off

As I mentioned in my last post, there is significant debate over the rate of success in losing weight and keeping it off without the use of surgery or drugs. However, it is definitely possible for some people, and there has been great interest in characterizing the behaviors of those people.

Many, but not all, of the studies I will mention in this post use subjects in the National Weight Control Registry (which I will refer to as the NWCR). They have a pretty detailed website, so if this subject is of interest, you might want to check it out. While this is a tremendously valuable resource in the study of successful weight loss maintenance, in general, I would say that the studies that come out of this cohort are slightly less reliable, because the analysis is retroactive in nature, meaning that the people's habits are being studied after they have lost weight and kept it off. Other studies, in contrast, have looked at people's habits immediately after losing weight and then looked to see what lifestyle factors ended up being associated with maintaining that loss. The former approach is more susceptible to something called recall bias, meaning that the fact that the people being studied have already successfully maintained their weight loss might affect their memories and perceptions of their own habits as compared to people who did not maintain weight loss. With that caveat (and you can tell which studies are NWCR-related by the authors...Rena Wing and James Hill are the founders of the registry), here is a summary of some of the major findings.

Diet--The following dietary habits have been found to be associated with successful maintenance of weight loss:
-Eating five or more servings of fruits and vegetables per day (1, 2)
-Not eating at fast food restaurants (as compared to those who eat fast food at least twice a week) (1)
-Using low-calorie pre-packaged meals (2-4)
-Practicing portion control (2, 11)
-Moderating intake of fat (2, 11)
-Eating breakfast (11)

Exercise: Multiple studies have found that exercise improves one's chances of maintaining weight loss (1,2, 5-8). There is some debate, however, over how much exercise is necessary. Several studies have suggested that an hour of exercise each day is needed to improve chances of weight loss maintenance (1, 2) but at least one other study found that any amount of exercise was beneficial (5)

Weight loss as a percentage of starting weight: Multiple studies (5, 9, 10) have shown that basically, the more weight you lose, the harder it is to maintain. For this reason, many public health officials are now recommending that obese people strive to lose 10-15% of body weight and keep it off rather than trying to achieve a "normal" BMI and then rebound.

"Screen time": Television and computer use (outside of work) has been shown to be positively correlated with weight regain (5)

Consistent Monitoring of Weight: One of the characteristics identified in subjects in the NWCR was frequent weighing (11). 44% of these people said they weighed themselves daily and 31% weighed themselves weekly (11). A more detailed study also showed that subjects in the NWCR who started weighing themselves less frequently gained more weight than those who maintained their strict weighing schedule (14). However, it's not clear whether weighing itself somehow affected these people's motivation to maintain their weight or whether people who knew they'd "slipped" a bit had more trepidation about stepping on the scale regularly.

Online Support: A recent study published in JAMA (12) looked at people who had lost weight through a 6-month program of monitored diet and exercise and then looked at the efficacy of three different types of counseling in helping them to maintain their weight loss over 30 months. Following the initial weight loss period, the "self directed" group were given some literature with recommendations for diet and exercise and met with a counselor after 12 months. An "interactive technology-based intervention" group was given unlimited access to a web site which assisted them in monitoring their weights, monitoring their caloric intake and physical activity and charting their progress, as well as giving them access to a message board where they could interact with other members of that group. The third group received monthly personal contact with a counselor either over the phone or in person. While all three groups regained some weight, they found that the technology-based intervention group regained less than the self-directed group, and the personal contact group regained the least.

While personal monitoring of weight maintenance may not be widely feasible, this does suggest that support provided through the internet may assist in weight loss maintenance. However, at least one other study (13) had different findings that a (different) website they set up to support maintenance of weight loss after a 4-month weight loss program did not make any difference in maintenance of weight loss. The authors attributed the lack of effect to the fact that many of the participants in the study were not comfortable using the internet.

Other: Interestingly, 2 found that unsuccessful maintainers were more likely to follow popular diet books than those who were successful at maintaining weight loss.

Anyway, I doubt any of these things are shockers, but it may be a helpful collection of information for some people!


1. Kruger J, Blanck HM, Gillespie C. Dietary Practices, Dining Out Behavior, and Physical Activity Correlates of Weight Loss Maintenance. Preventing Chronic Disease, 5:A11, 2008.

2. Befort CA, Stewart EE, Smith BK, Gibson CA, Sullvan DK, Donnelly JE. Weight maintenance, behaviors and barriers among previous participants of a university-based weight control program. International Journal of Obesity, 32: 519-526, 2008.

3. Heymsfield SB, van Mierlo CA, van der Knaap HC, Heo M, Frier HI. Weight management using a meal replacement strategy: meta and pooling analysis from six studies. International Journal of Obesity and Related Metabolic Disorders 27: 537-549, 2003.

4. Ditschuneit HH, Flechtner-Mors, M. Value of structured meals for weight management: rsk factors and long-term weight maintenance. Obesity Research 9: 284S-289S, 2001.

5. Weiss EC, Galuska DA, Khan LK, Gillespie C, Serdula M. Weight Regain in Adults Who Experienced Substantial Weight Loss, 1999-2002. American Journal of Preventative Medicine, 33: 34-40, 2007.

6. Anderson JW, Konz EC, Frederich RC, Wood CL. Long-term weight-loss maintenance: a meta-analysis of U.S. studies. American Journal of Clinical Nutrition 74: 579-584, 2001.

7. Jeffery RW, Epstein LH, Wilson GT, Drewnowski A, Stunkard AJ, Wing RR, Hill DR. Long-term maintenance of weight loss: Current status. Health Psychology 19 5-16, 2000.

8. Grodstein F, Levine R, Troy L, Spencer GA, Colditz GA, Stampfer MJ. Three-year follow-up of participants in a commercial weight loss program: can you keep it off? Archives of internal medicine 156: 1302-1306, 1996.

9. McGuire MT, Wing RR, Klem ML, Lang W, Hill JO. What predicts weight gain in a group of successful weight losers? Journal of Consulting Clinical Psychology, 67: 177-185, 1999.

10. Vogels N, Westerterp-Plantenga MS. Successful long-term weight maintenance: a 2-year follow-up. Obesity 15: 1258-1266, 2007.

11. Klem ML, Wing RR, McGuire MT, Seagle HM, Hill JO. A descriptive study of individuals successful at long-term maintenance of substantial weight loss. American Journal of Clinical Nutrition, 66: 239-246, 1997.

12. Svetkey LA, Stevens VJ, Brantley PJ, Appel LJ, Hollis JF, Loria CM, Vollmer WM, Gullion CM, Funk K, Smith P, Samuel-Hodge C, Myers V, Lien LF, Laferriere D, Kennedy B, Jerome GJ, Heinith F, Harsha DW, Evans P, Erlinger TP, Dalcin AT, Coughlin J, Charleston J, Champagne CM, Bauck A, Ard JD, Aicher K for the Weight Loss Maintenance Collaborative Research Group. Comparison of Strategies for Sustaining Weight Loss. Journal of the American Medical Association, 299: 1139-1148, 2008.

13. Cussler EC, Teixeira PJ, Going SB, Houtkooper LB, Metcalfe LL, Blew RM, Ricketts JR, Lohman J, Stanford VA, Lohman TG. Maintenance of Weight Loss in Overweight Middle-aged Women Through the Internet. Obesity, advance online publication, 2008.

14. Butryn ML, Phelan S, Hill JO, Wing RR. Consistent Self-monitoring of Weight: A Key Component of Successful Weight Loss Maintenance. Obesity 15: 3091-3096, 2007.