Body Mass Index (BMI)
One of the most widely used tools for calculating healthy weight estimates is the body mass index (or BMI for short), which relies on the ratio of weight to height measurements.
How to determine your body mass index:
- Divide your weight in pounds by your height in inches.
- Divide the answer by your height in inches.
- Multiply the answer by 703.
Now that you know your BMI, what does it mean?
- A healthy weight is one that equates with a body mass index of less than 25. Overweight is defined as a body mass index of 25 to 29.9, and obesity is defined as a body mass index of 30 or higher.
Dozens of studies have shown that a body mass index above 25 increases the chances of dying early.
- A meta-analysis published in the New England Journal of Medicine focused on the relationship between BMI and mortality. (14) The study showed a clear relationship between BMI and mortality, with both underweight (BMI <18.5) and overweight and obese (BMI >25) BMIs causing an increase in mortality. The lowest death rate from any cause was associated with the BMI range between 22.5 and 24.9.
- Experts believe that this study was strong because it was able to exclude smokers, individuals with cancer and heart disease, and individuals over the age of 85 who may be in the normal BMI range but may be suffering from frailty or other age-related unhealthy weight loss.
- Another large meta-analysis in The Lancet that looked at participant data across four continents found that for every five units higher of a BMI above 25, risk of premature death increased by about 31%. (20) The study also looked at specific causes of death, and found that for each 5-unit increase in BMI above 25, the corresponding increases in risk were 49% for cardiovascular mortality, 38% for respiratory disease mortality, and 19% for cancer mortality.
- This meta-analysis also excluded participants who were current or former smokers, those who had chronic diseases at the beginning of the study, and any who died in the first five years of follow-up (a combined 1.6 million deaths were recorded across these studies where participants were followed for an average of 14 years).
When examining the relationship between BMI and mortality, failure to adjust for these variables can lead to reverse causation (where a low body weight is the result of underlying illness, rather than the cause) or confounding by smoking (because smokers tend to weigh less than non smokers and have much higher mortality rates). Experts say these methodological flaws have led to paradoxical, misleading results that suggest a survival advantage to being overweight.
Also note that muscle and bone are denser than fat, so an extremely muscular athlete may have a high body mass index, but may not actually be overweight or obese. This is not a problem for most athletic people, however.
Read more about the BMI on the Obesity Prevention Source.
Not all fat is created equal. While overall body fat percentage is important, it’s especially important to monitor abdominal obesity (also called visceral fat), which may be more dangerous for long-term health than fat that accumulates around the hips and thighs (known as subcutaneous fat). (15)
- Some studies suggest that abdominal fat plays a role in the development of insulin resistance and inflammation, an immune system response which has been implicated in heart disease, diabetes, and even some cancers.
- The National Institutes of Health concluded that a waist larger than 40 inches for men and 35 inches for women increases the chances of developing heart disease, cancer, or other chronic diseases.
Waist size is a simple, useful measurement because abdominal muscle can be replaced by fat with age, even though weight may remain the same. So, an increasing waist size can be an important “warning sign,” and should prompt you to examine how much you are eating and exercising.
- Some believe that waist-to-hip ratio is a better indicator of risk, as waist size may vary based on body frame size, but one of the largest studies to date found that waist size and waist-to-hip ratio were equally effective at predicting risk of death from heart disease, cancer, or any cause.(16)
In people who are not overweight, waist size may be an even more telling warning sign of increased health risks than BMI. (16)
- The Nurses’ Health Study looked at the relationship between waist size and death from heart disease, cancer, or any cause in middle-aged women. At the start of the study, all 44,000 study volunteers were healthy, and all of them measured their waist size and hip size. After 16 years, women who had reported the highest waist sizes—35 inches or higher—had nearly double the risk of dying from heart disease, compared to women who had reported the lowest waist sizes (less than 28 inches). Women in the group with the largest waists had a similarly high risk of death from cancer or any cause, compared with women with the smallest waists. The risks increased steadily with every added inch around the waist.
Even women at a “normal weight”—BMI less than 25—were at a higher risk, if they were carrying more of that weight around their waist: Normal-weight women with a waist of 35 inches or higher had three times the risk of death from heart disease, compared to normal-weight women whose waists were smaller than 35 inches.
- The Shanghai Women’s Health study found a similar relationship between abdominal size and risk of death from any cause in normal-weight women. (17)
Know your waist size – how to measure and assess:
Wrap a flexible measuring tape around your midsection where the sides of your waist are the narrowest. This is usually even with your navel. Make sure you keep the tape parallel to the floor.
- An expert panel convened by the National Institutes of Health concluded that a waist larger than 40 inches for men and 35 inches for women increases the chances of developing heart disease, cancer, or other chronic diseases. (18) Although these are generous guidelines, (19) they are useful benchmarks.
14. Berrington de Gonzalez, A., et al., Body-mass index and mortality among 1.46 million white adults. N Engl J Med, 2010. 363(23): p. 2211-9.
15. Willett W, Nutritional epidemiology. 1998, New York: Oxford University Press.
16. Zhang, C., et al., Abdominal obesity and the risk of all-cause, cardiovascular, and cancer mortality: sixteen years of follow-up in US women. Circulation, 2008. 117(13): p. 1658-67.
17. Zhang, X., et al., Abdominal adiposity and mortality in Chinese women. Arch Intern Med, 2007. 167(9): p. 886-92.
18. Clinical Guidelines on the Identification, Evaluation, and Treatment of Overweight and Obesity in Adults–The Evidence Report. National Institutes of Health. Obes Res, 1998. 6 Suppl 2: p. 51S-209S.
19. Willett, W.C., W.H. Dietz, and G.A. Colditz, Guidelines for healthy weight. N Engl J Med, 1999. 341(6): p. 427-34.
20. Angelantonio, E. et al., Body-mass index and all-cause mortality: Individual-participant-data meta-analysis of 239 prospective studies in four continents. The Lancet. July 13, 2016.
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