BMI: Is Bigger Better?

By: Stacy Kennedy, MPH, RD, CSO, LDN; Reboot Nutritionist

A recent study showing that being overweight may actually be protective against death in older persons has renewed the heated debate about BMI.

The study looked at approximately 100 published studies that included about 3 million adults in order to estimate the chances of early or premature death at different weight categories (normal/healthy, overweight, various classes of obesity).

  • Persons in the obese groups combined (BMI > 30) had an 18% higher risk of premature death compared to those at a healthy or normal weight.
  • Persons in Grade 2 and 3 obesity (BMI >35) were 29% more likely to die early.
  • Persons in the overweight category were 6% less likely to die early compared to those in the normal weight category
  • People in the Class 1 obese group (BMI 30-34.9) were 5% less likely to die early compared to those in the normal weight category.

Does this mean that being at a higher weight is healthier?  Let’s look more closely at BMI and this study.

What is BMI?

Body Mass Index (BMI) is a number that’s calculated using a person’s height and weight.  BMI is considered to be a reliable way to estimate a person’s body fatness.  BMI is currently used as the “gold standard” to screen for a person’s risk of disease or death based on their weight category.


Weight Status

Below 18.5


18.5 – 24.9


25.0 – 29.9


30.0 and Above


Two places to calculate your BMI is The Centers for Disease Control and Prevention or the National Institutes of Health.  There are separate calculators for adults and children.

BMI is what we call a “J curve.”  This means that there are risks seen below and above the healthy range.  The “optimal” BMI range for health and disease prevention is thought to be between 20-25.

Pros & Cons of BMI

BMI replaced the former system of assessing Ideal Body Weight ranges, which were categories based on life insurance data of a primarily upper-middle class Caucasian population in the US in the 1950s.  While BMI is considered by most experts am important step in the right direction, critics find it is still seriously lacking.

Let’s weigh the good and bad about BMI



Noninvasive Doesn’t tell the whole story about a person’s health or risks
Inexpensive Isn’t specific for sex, race or age
Quick Overestimates for athletes with high muscle mass
Easy Is not a direct measure of body fatness
Anyone can calculate on their own  
Research correlates it to body fatness and disease risk  


BMI is just one measure in a big picture that can show a relationship between weight and health or disease risk.  Besides body weight, there are many other factors that should be considered when looking at overall health, including:

  • Waist circumference
  • Exercise/Physical activity level
  • Diet
  • Cholesterol
  • Blood Pressure
  • Blood sugar
  • Triglyceride levels
  • Cigarette Smoking

While BMI may be far from perfect, even the authors of the review study aren’t suggesting that people should aim to stay at a high weight or gain weight if you’re in the normal or healthy range.  This type of study doesn’t show cause and effect.  And people at the normal or lower BMI range in the study may have lost weight due to illnesses like cancer or from unhealthy behaviors like smoking.

There are serious risks involved with being obese including heart disease, Type 2 diabetes, certain cancers, gallstones, liver disease and respiratory diseases.  Being obese can increase suffering from illness or medications as well as increase the risk for death.  With more than one-third of US adults (35.7%) obese, working toward a healthy weight by eating a plant-based diet, being physically active and other wellness behaviors remains important. 

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Stacy Kennedy, MPH, RD, CSO, LDN; Reboot Nutritionist

Stacy is a Board Certified Specialist in Oncology Nutrition and an Integrative Nutritionist. She consults for various companies, focusing on health, wellness and innovative strategies to help increase individual’s fruit and vegetable intake. Stacy is an American College of Sports Medicine Certified Health Fitness Specialist; she holds a BS degree in Dietetics from Indiana University, completed her dietetic internship at Massachusetts General Hospital, and earned a Masters in Public Health from the University of North Carolina at Chapel Hill. She is a Senior Clinical Nutritionist at the Dana Farber Cancer Institute/Brigham & Women’s Hospital, Harvard Medical School teaching affiliates, in Boston, MA, with more than 20 years of experience. Stacy created and now serves as project manager and lead writer for nutrition services content on the Dana Farber website and the affiliated, nationally recognized nutrition app. Stacy is regularly featured on TV, radio, print and social media on behalf of Dana Farber and other organizations. Together with her husband, Dr. Russell Kennedy PsyD, they have a private practice, Wellness Guides, LLC. Stacy is an adjunct professor in Wellness and Health Coaching at William James College, currently teaching a graduate course in Health Coaching. Stacy is featured in the award winning documentary films, “Fat, Sick & Nearly Dead” and “Fat, Sick & Nearly Dead 2,” and serves on the Reboot with Joe Medical Advisory Board. Stacy lives in Wellesley with her husband, two sons and three dogs. She enjoys cooking, yoga, hiking and spending time with friends and family. Stacy is also one of the nutritionists who runs our Guided Reboot programs.

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