BMI and Nutrition

BMI, or body mass index, is a simple and relatively accurate screening tool for body fatness. Defined as weight in pounds divided by height in inches squared, multiplied by 703, BMI provides valuable information that can be part of a nutrition assessment, contributing to the overall picture of health and susceptibility to chronic disease.

Significance

After your BMI is calculated, your doctor notes where it falls in the range, then uses this information, combined with other data, to get an overall picture of your health.

Types

Classification of body weight by BMI falls into ranges. Someone who is underweight has a BMI less than 18.5. A normal weight individual would fall into a range of 18.5 to 24.9. Overweight people would have a range between 25.0 and 29.9. With obesity, the classifications become more specific. Obesity class I is between 30.0 and 34.9. Obesity class II is between 35.0 and 39.9. Obesity class III is above 40. This specificity within the obesity classification is due to the increasingly high medical risk factors associated with very high levels of body fat.

Low BMI Scores

A BMI score below 18.5 is considered underweight. This is an especially significant value if there has been weight loss. If appropriate, a dietitian would consider encouraging this person to gain weight. It is generally recommended to gain approximately 1/2 to 1 pound a week by adding between 250 to 500 calories to the diet every day. Because it takes about 3,500 additional calories to equal one pound of body weight gained, adding these caloric increments weekly generally does not cause the patient discomfort or anxiety. Suggestions may include eating six small frequent meals a day and perhaps more calorie-dense foods.

Overweight and Obesity

According to the Obesity Education Initiative Expert Panel on the Identification Evaluation and Treatment of Overweight and Obesity in Adults, body weight, including overweight and obesity, is officially classified via the use of BMI. Someone who would classify as overweight would likely trigger a referral for nutrition counseling for weight loss by reducing dietary fat and calorie intake and increasing physical activity. This information would also be an important indicator for the doctor to look into additional risk factors such as diabetes, cholesterol and heart disease risk. Being in the class III obesity level would also trigger conversations about gastric bypass surgery and very low calorie diets.

Limitations of BMI

BMI is only one tool of many in determining nutritional status. Someone genetically thin that makes poor food choices would potentially have a deceptively healthy BMI. A body builder would have a higher proportion of muscle to fat and a disproportionately high BMI because muscle is heavier than fat. BMI is also inaccurate for pregnant women and people with fluid balance problems.

Expert Insight on BMI and Nutrition

BMI is a valuable tool for the evaluation of nutritional status. It is important to note that no one tool can provide all the clues a physician or dietitian needs to guide patients to improved health and nutrition.

References

• The A.S.P.E.N. Nutrition Support Core Curriculum: A Case-Based Approach-The Adult Patient"; Michele Gottschlich, editor; 2007
• Obesity (journal); Clinical Guidelines on the Identification, Evaluation and Treatment of Overweight and Obesity in Adults; 1998