This news obviously seems to send some mixed messages. Learning more about what the research says can help you better understand your health.
Body Mass Index (BMI)
The BMI score is meant to be a quick way of determining whether a person has too much body fat. BMI scores of 20 to 24.9 are considered normal, scores of 25 to 29.9 are overweight, scores of 30 to 34.9 are obese, and scores above 35 are extremely obese. Scores under 20 are considered underweight.
You can easily calculate your score using a calculator.
Virtually all studies using BMI scores agree on a couple of points:
People who are obese or extremely obese have a greatly increased risk of all-cause mortality. People who are underweight also have an increased risk of death. This is thought to be mainly because of underlying disease processes—such as heart disease, lung disease, cancer, or infection—which, on their own, often produce weight loss with disease progression.
If there is a controversy, it centers around individuals who are classed as overweight, but not obese—that is, whose BMI scores are a bit over 25. Most studies have shown increased medical risk even for this mild state of overweight, but a few studies show a slightly lower risk for these individuals.
Several explanations for this apparent discrepancy have been suggested. The one that has the most traction is the idea that the BMI measure itself—which simply takes into account one’s weight and height—often gives a false measure of “overweight” if a person is simply in good shape and has good muscle mass.
That is, for healthy individuals with BMIs of 25 or 26, the “excess” weight may actually not be fat.
The Obesity Paradox in Heart Disease
Since the early 2000s, research into mortality for people with heart disease found that survival statistics favored those who were in the overweight BMI range. Further large systematic reviews and meta-analyses of studies have supported this finding.
The idea that people whose BMI measures are above the normal range may have reduced cardiovascular mortality has been called the “obesity paradox.”
A 2015 study published in the journal Heart compiled data from 89 studies, including more than 1.3 million people who had coronary artery disease. Those who were underweight had the highest risk of both short-term and long-term mortality (over three years).
Those who were overweight or obese had a lower risk of short-term and long-term mortality compared with those who had a BMI in the normal-weight range. However, those who were in the obese categories had a higher risk of mortality after five years of follow-up.
A 2018 study analyzed 65 prior studies involving 865,774 people who had coronary artery bypass graft surgery or coronary revascularization with percutaneous coronary intervention.
The study confirmed that compared to normal-weight individuals, all-cause mortality was increased for underweight people and lower for those who were overweight, obese, or severely obese. Being in the overweight BMI category was associated with the lowest risk of major adverse cardiovascular events.
Why does the obesity paradox exist? Current thinking is that BMI is an insufficient measure of a person’s cardiovascular risk since it fails to take into account a person’s muscle mass and overall cardiorespiratory fitness. For instance, very fit athletes often have elevated BMIs.
On the other hand, people who may formerly have been overweight, and then go on to develop heart disease, often develop muscle wasting, and their BMIs may drop back into the normal range. So the BMI by itself may give a misleading picture of a person’s cardiovascular health.
Many experts now say that, instead of relying on BMI to determine whether weight is contributing to cardiovascular risk, we should think more about abdominal fat.
Abdominal Fat and BMI
Having too much fat—specifically, too much fat in the abdominal area—places significant metabolic stress on the cardiovascular system and increases the risk of cardiovascular disease.
The BMI index is very accurate for individuals who are very underweight or very overweight (e.g., it is difficult to put on enough muscle mass to get your BMI above 30 without abusing steroids), but BMI is less accurate for detecting individuals who are merely overweight.
There are, indeed, some individuals who have BMI scores in the 25 to 29 range just because they are in great shape. But those individuals likely know who they are.
So, if you have a BMI score in the “overweight” category, answer this one question: Is your waist size less than your hip size?
If so, then you are probably one of those people in excellent physical shape, and the “excess” weight contributing to your BMI score is muscle and not fat. But if the answer is “no” and you have centrally deposited fat, there is reason for concern.
While the BMI score is sometimes useful and easy to measure, the waist-to-hip ratio is probably the more important index of cardiovascular risk.
An underweight woman (BMI less than 18. 5) pregnant with one baby should gain 28-40 pounds; if she is having twins, she should gain 50-62 pounds. A woman with a normal-weight BMI (20 to 24. 9) should gain 25-35 pounds with one baby or 37-54 pounds with twins. An overweight woman (BMI 25 to 29. 9) should gain 15-25 pounds with one baby or 31-50 with twins. A woman with a baseline BMI that is considered obese (BMI more than 30) should gain 11-20 pounds with one baby or 25-42 with twins.
Lowest risk: less than 37 inches for men and 31. 5 inches for womenModerate risk: Between 37. 1–39. 9 inches for men and 31. 6–34. 9 inches for womenHigher risk: Above 40 inches for men and 35 inches for women
