Many studies show that leanness is associated with better health, a lower risk of
developing serious diseases and that death rates are lower. However, a number of
epidemiologic studies have found that this link is not true for the lowest weight
categories—a situation sometimes called the “obesity paradox.”

The finding of lowest mortality in those with a BMI somewhat above the normal
range of 18.5-24.9 has been found by a number of studies, including a widely-publicized
meta-analysis of studies published in the Journal of the American Medical
Association (JAMA) in 2012.48 The study found that individuals who were
overweight (BMI, between 25 and 29.9) were actually 6% less likely to die from
any cause than people whose weight was considered to be normal weight according
to their BMI. And even people who were “mildly” obese (BMI 30 to 34.9) were no
more likely to die than their normal-weight counterparts. However, for people who
were moderately or severely obese (BMI over 35 or 40, respectively), the risk of
dying was increased by 29%.

An editorial commenting on the JAMA study noted that the body mass index is
an imperfect measure of adiposity. It accounts for only about two-thirds of the
between-individual variation in total adiposity. Nor does BMI account for the fact
that people with the same BMI can have different health risk factors present. For
example, one person with a BMI of 28 may be in perfect health while another with
the same BMI might be a smoker, have type 2 diabetes and hypertension. So perhaps
the higher risk of death among the normal-weight individuals in the JAMA
study may have been due in part to the inclusion of sick or frail underweight people
in the “normal” weight category.

Walter Willett and his colleagues have pointed out that basing guidelines for an optimal
healthy weight on mortality rates is misleading. Reverse causation, that is to
say illness is the cause of low BMI rather than low BMI causing illness and higher
mortality, is the most serious problem. People frequently lose weight as a result of
an illness that is ultimately fatal, a situation that creates the appearance of higher
mortality among those with lower weights. Confounding factors such as smoking,
alcoholism, and physical activity may distort the association between body weight
and mortality. Smokers tend to weigh less, so smoking will make leaner persons
appear to be at elevated risk. Willett and his colleagues note that “…in analyses
adjusted only for age, the relation between body weight and mortality is typically
U-shaped, with increased death rates among both the leanest and the heaviest persons.
However, because of the potential for bias, these results should not be accepted
as evidence that low weight is harmful.”

In many studies when reverse causation is accounted for, and the analysis is limited
to persons who have never smoked, mortality increased linearly with increasing
body-mass index from very lean to clearly obese. For example, the long-term
Nurses’ Health Study and Health Professionals Follow-up Study found that a combination
of at least three low-risk lifestyle factors and a BMI that is considered normal
weight, 18.5–22.4, was associated with the lowest risk of all-cause mortality.
Evaluating the importance of obesity on overall health is also complicated by the
role of cardiorespiratory fitness in attenuating the health risks of overweight or obesity. Studies have found that fit individuals with metabolically healthy obesity are not at significantly higher risk of CVDs than metabolically healthy normal weight
individuals. Maintaining cardiorespiratory fitness appears to be protective against
the risks of overweight and obesity.

Although being mildly over-fat may represent a small increment of increased health
risk, and if you are physically fit very little or no increase, the last word on this
might be found in a large meta-analysis of 230 cohort studies that tracked 3.74 million
deaths among 30.3 million participants. It found that the lowest mortality was
among never smokers with a normal BMI of 20‑22.

This blog presents opinions and ideas and is intended to provide helpful general information. I am not engaged in rendering advice or services to the individual reader. The ideas, procedures and suggestions in that are presented are not in any way a substitute for the advice and care of the reader’s own physician or other medical professional based on the reader’s own individual conditions, symptoms or concerns. If the reader needs personal medical, health, dietary, exercise or other assistance or advice the reader should consult a physician and/or other qualified health professionals. The author specifically disclaims all responsibility for any injury, damage or loss that the reader may incur as a direct or indirect consequence of following any directions or suggestions given in this blog or participating in any programs described in this blog or in the book, The Building Blocks of Health––How to Optimize Your Health with a Lifestyle Checklist (available in print or downloaded at Amazon, Apple, Barnes and Noble and elsewhere). Copyright 2021 by J. Joseph Speidel,