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What is BMI and why is it important?

Caitlin Guess, MPH, RDN, CSR, LD

It’s no secret that weight and waistlines can change during the holiday season. With holiday celebrations, family gatherings, and other festivities involving food and alcohol, it is only natural that your body may store the extra calories you consume, leading to changes in body shape and size as well as the number on the scale.

If you go to the doctor in January, one topic of discussion related to any weight changes may be your BMI, which is short for body mass index. After these conversations, you may find yourself wondering what is body mass index, and why is it important for your health?

Simply put, BMI is a measure of your body size based on your height and weight that classifies the result as normal, underweight, overweight, or obese based on pre-set categories established in 1998 by a panel of nine medical experts:

A BMI less than 18.5 is considered underweight

A BMI between 26 and 29.9 is overweight

A BMI greater than or equal to 30 is obese

A normal BMI falls between 18.5 and 25

The calculation can be done at home using a mathematical equation, or you can go to any number of calculators online that will do it for you. I personally like:

Does this mean that if your BMI is above or below normal you’re not healthy? Not exactly. In general, the higher your BMI, the higher your risk of developing a range of chronic health conditions linked with excess weight, including diabetes, high blood pressure, high cholesterol, arthritis, liver disease, and sleep apnea. However, with all of these diseases, BMI alone is not the only factor to consider when determining health or illness.

Important things to remember about BMI classifications are that they do not take into consideration weight from muscle versus weight from fat. They were also established based on population measures, not individual measures. For these reasons, BMI may be unreliable during pregnancy, for athletes, and for children or the elderly. Additionally, body composition, including percent body fat or amount of muscle mass, can vary by race and ethnic group. The current BMI definitions of overweight or obesity were based largely on white populations. So, while BMI may help predict health status among people who are white, it may be less accurate for people in other racial and ethnic groups.

What I like to remind my clients is that BMI alone does not define your health, but it absolutely needs to be considered in the overall assessment of your health. Independent of any particular disease, people with high BMIs often report feeling better, both physically and psychologically, after losing even 5-10 percent excess body weight. In general, the behaviors that affect your weight, and therefore BMI, like healthy eating, consistent movement or exercise, stress management, adequate sleep, and self-care are the most important things to focus on for your health in the new year and will lead to you feeling your best regardless of which BMI category you fall in.

Caitlin Guess is a registered dietitian/nutritionist at the Diabetes and Nutrition Institute at the HopeHealth Medical Plaza in Florence. She is a certified specialist in renal nutrition and is a member of the National Kidney Foundation. She is passionate about teaching others how to use nutrition for disease prevention and health promotion.



HopeHealth educates its patients on the importance of having a health care home. As a primary care facility, HopeHealth’s medical team works to prevent and detect illness and the early onset of disease, provide routine physical examinations and promote overall healthy lifestyles.

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