The truth about BMI
When it comes to working with folks and determining health status, there are many things that we can calculate and measure, but … Body Mass Index (BMI) should not be one of the things.
Let me explain.
Currently, BMI is used to determine a person’s level of “obesity” which, from an anatomical and metabolic perspective, refers to excessive fat (triacylglicerols) accumulation. BMI is also used to assess mortality risk across different BMI categories (1).
There are problems with this. Here’s why:
BMI cannot tell us whether a person has high or low body fat mass, meaning that someone who has minimal fat may still have a high BMI and similarly, someone who has a high fat mass, may have a low BMI (2).
BMI does not tell us the location of where the fat accumulates, which is important in assessing chronic disease risk (1).
BMI does not tell us whether someone is eating fruit & vegetables or enough fibre or enough dairy or anything at all about dietary quality.
BMI does not tell us about the types of physical activity a person undertakes ie: their cardiovascular fitness, strength, muscularity, lung capacity or flexibility.
BMI also does not tell us whether someone is getting enough sleep or their stress levels or their smoking status or alcohol consumption or their family medical history or their socio-economic status … all factors that we need to consider when determining health status.
What this means is that someone who has poor dietary quality, is sedentary and has a family history of chronic disease is grouped in the same BMI category with someone who has high dietary quality, undertakes physical activity every day, has no family history of chronic disease if they simply share the same height and weight!
What BMI can be useful for is as a population measure to look at possible disease risk in different population groups. But we have to be careful using that data, because it merely shows correlation, not causation.
That’s literally it!
So why the “obesity epidemic” hysteria?
When we hear about the “obesity epidemic” the data usually combines both the overweight and obese categories. This is not only harmful at a societal level and perpetuates the thin ideal, it is also not accurate.
The reality is, the average BMI in Western population studies is generally between 24 to 27 (2, 3), which is higher range of the “normal weight” category (18.5 - 24.9) and the lower range of the “overweight” category (25 - 29.9).
Yes, it is true that weight has increased in some populations. Data from the USA shows that the average weight of men aged 20 – 74 years increased by about 11kg (about 250 grams per year – unknown if fat mass or lean mass) between 1960 – 2002 (1). They’ve also grown about 2.5cm (1). But on top of growing bodies, data also shows that we’re living longer, have higher quality diets, a greater availability of food, a reduced number of people smoking and reduced incidence of chronic and serious infectious diseases (1).
Yes, it is also true that there is a link between higher BMI and chronic disease (4), but this does not represent causation. It is an assumption that weight drives the development of disease, but there could be a range of other factors that could influence the disease process, such as:
Environment – exposure to trauma and stress as well as our socio-economic status are very significant determinants of our health.
Discrimination & weight stigma – for folks who experience weight stigma and discrimination within the health care setting, they are far less likely to see health professionals for regular check ups and therefore have diagnoses missed or treatment not continued.
Body dissatisfaction – this can result in less likelihood of engaging in self-care activities & is associated with binge eating and other eating disordered behaviours, lower levels of physical activity and increased weight gain over time (6).
Chronic psychological stress – this can increase cortisol levels which can lead to a chronic inflammatory response by the body which can lead to the development of chronic disease.
So, it is not as simple as trying to change weight and/or someone’s BMI to reduce the risk, because we know that intentional weight loss is not sustainable for 97% of the population (7). It also causes a myriad of harmful side effects, including disordered eating, weight gain, binge eating, lowered self-esteem and decreased mental health (8). More on this here.
However we do know that health behaviours such as eating 5+ fruit and vegetables each day, regular physical activity, not smoking and drinking alcohol in moderation that can increase health, rather than intentionally trying to reduce someone’s weight (Matheson et al, 2012).
The reality is, BMI is a tenuous measure of a person’s health. So much that in 1998 the USA decided to change their BMI categories which resulted in millions of Americans who were one day in the “healthy weight” category and suddenly in the “overweight” category the next because the category cut off points changed! (click here for more on the history of BMI if you’re a history buff).
In short, BMI is not an effective way to measure a person’s health and the hysteria that we’re “living in an obesity epidemic” is harmful.
Let’s look beyond BMI and dive deeper to explore what’s really going to focus on the things that are modifiable and not going to cause further harm.
References:
(1) Nuttall, F.Q. 2015. Body Mass Index - Obesity, BMI, and Health: A Critical Review. Nutrition Research, 50(3), 117-128. doi: 10.1097/NT.0000000000000092
(2) Wellens, R.I., Roche, A.F., Khamis, H.J., Jackson, A.S., Pollock, M.L., & Siervogel, R.M. (1996). Relationships between the Body Mass Index and body composition. Obesity Research, 4 (1), 35-44. doi: 10.1002/j.1550-8528.1996.tb00510.x
(3) Ogden, C.L., Fryar, C.D., Carroll, M.D., & Flegal, K.M. (2004). Mean body weight, height, and body mass index, United States 1960-2002. Adv Data, 27(347), 1-17.
(4) Australian Bureau of Statistics (ABS). (2015). National Health Survey First Results Australia 2014-2015. Retrieved from https://www.ausstats.abs.gov.au/Ausstats/subscriber.nsf/0/CDA852A349B4CEE6CA257F150009FC53/$File/national%20health%20survey%20first%20results,%202014-15.pdf
(5) Matheson, E. M., King, D. E., & Everett, C. J. (2012). Healthy Lifestyle Habits and Mortality in Overweight and Obese Individuals. The Journal of the American Board of Family Medicine, 25(1), 9-15. doi:10.3122/jabfm.2012.01.110164
(6) Neumark-Sztainer, D. (2009). Preventing Obesity and Eating Disorders in Adolescents: What Can Health Care Providers Do? Journal of Adolescent Health, 44(3), 206-213. doi:10.1016/j.jadohealth.2008.11.005
(7) Anderson, J. W., Konz, E. C., Frederich, R. C., & Wood, C. L. (2001). Long-term weight-loss maintenance: a meta-analysis of US studies. The American Journal of Clinical Nutrition, 74(5), 579-584. doi:10.1093/ajcn/74.5.579
(8) Bacon L, Aphramor L. Weight science: evaluating the evidence for a paradigm shift. Nutr J. 2011;10:9.