Body mass index or BMI is a statistical index using a person's weight and height to provide an estimate of body fat in males and females of any age. It is calculated by taking a person's weight, in kilograms, divided by their height, in meters squared, or BMI = weight (in kg)/ height^2 (in m^2). The number generated from this equation is then the individual's BMI number. The National Institute of Health (NIH) now uses BMI to define a person as underweight, normal weight, overweight, or obese instead of traditional height vs. weight charts. These classifications for BMI are in use by the NIH and the World Health Organization (WHO) for White, Hispanic, and Black individuals. The cutoffs underestimate the obesity risk in the Asian and South Asian populations, so their classification has slight alterations. The BMI number and classifications are listed below.[1][2] However, individual variations do exist, and BMI is insufficient as the sole means of classifying a person as obese or malnourished. In certain populations, like elite athletes and body-builders, an elevated BMI does not directly correlate to their health status due to their increased muscle mass and weight falsely increasing their BMI. Moreover, in the pediatric population, BMI allows comparison between children of the same sex and age. For children, a BMI that is less than the fifth percentile is underweight and above the 95th percentile is considered obese.[3]

• Severely underweight - BMI less than 16.5kg/m^2
• Underweight - BMI under 18.5 kg/m^2
• Normal weight - BMI greater than or equal to 18.5 to 24.9 kg/m^2
• Overweight – BMI greater than or equal to 25 to 29.9 kg/m^2
• Obesity – BMI greater than or equal to 30 kg/m^2
• Obesity class I – BMI 30 to 34.9 kg/m^2
• Obesity class II – BMI 35 to 39.9 kg/m^2
• Obesity class III – BMI greater than or equal to 40 kg/m^2 (also referred to as severe, extreme, or massive obesity)
• Asian and South Asian population
• overweight - BMI between 23 and 24.9 kg/m^2
• obesity - BMI greater than 25 kg/m^2

The main issue of concern in regards to BMI involves the growing obesity epidemic and the increasing population with high BMI numbers. The U.S. National Health and Nutrition Examination Survey (NHANES) of 2007 indicated that 63% of Americans are now in the overweight category and 26% are in the obese category.[4] A new current report from the Center for Disease Control provides updated data on trends in BMI, height, weight, and waist circumference, from 1999-2000 through 2015–2016. It shows that there was an increase of over 8 pounds in American men and 7 pounds in American women over this period. BMI in American adults has increased over the past 18 years. The average BMI for American men over age 20 for the year 2015-2016 is now 29.1. The average BMI for American women over age 20 for the year 2015-2016 is now 29.6. Since 1991 in addition to BMI, mean weight and waist circumference increased for all age groups. No change in height was seen over time except for a decrease in crude estimates among all women.[5]

The data collected for NHANES from 1988 to 1994, 1999 to 2000, and 2015 to 2016, demonstrates the age-adjusted prevalence of obesity in the United States has been increasing progressively: from 22.9 to 30.5 to 39.6 percent. In 2016, the incidence of obesity in American men rose to 37.9 percent, and the prevalence of obesity in American women increased to 41.1 percent. The age-adjusted prevalence of class III obesity (BMI greater than or equal to 40 kg/m^2) has risen from 5.7 percent to 7.7 percent between 2007 and 2016.[3] Obesity now qualifies as a chronic disease that is increasing in prevalence around the world. It is a significant contributor to rising healthcare expenses and overall poorer health in most countries. Data from 2015 demonstrates that approximately 108 million children and 604 million adults globally had a BMI of 30 to classify them as obese; signifies an increase in the prevalence of obesity in almost all countries since 1980 and a doubling in prevalence in 70 countries during that period.[6]

In regards to clinical practice, increasing BMI numbers are correlating with a larger number of patients found to be overweight. This determination of overweight is (BMI greater than or equal to 25 kg/m2) or to have abdominal obesity (waist circumference greater than or equal to 35 in [88 cm] in women or greater than or equal to 40 in [102 cm] in men) are at increased overall risk status for obesity-related comorbidities and increased morbidity and mortality. Assessment includes determining the degree of overweight by calculating the BMI and the presence of abdominal obesity (waist circumference), cardiovascular risk factors, sleep apnea, nonalcoholic fatty liver disease, symptomatic osteoarthritis, and other obesity-related comorbidities. The coexistence of several diseases, such as established coronary artery disease, peripheral atherosclerotic disease, type 2 diabetes mellitus, and sleep apnea, places obese patients in a very high-risk category for subsequent health-related issues and increased mortality.[7] Also, patients with increased BMI numbers are at a higher risk for metabolic syndrome. It represents a constellation of metabolic abnormalities that co-occur in an individual, which will confer an increased risk of cardiovascular disease (CVD) and diabetes mellitus type II (T2DM).

To be diagnosed with metabolic syndrome, an individual must meet three or more of the following criteria: 1. Central Obesity: Waist Circumference greater than or equal to 102 cm (40.16 inches) in men and greater than 88 cm (34.65 inches) in women. 2. Hypertriglyceridemia: triglyceride levels that are greater than or equal to 150 mg/dL or drug treatment for elevated triglycerides. 3. Low HDL Cholesterol: blood level under 40 mg/dL in men and under 50 mg/dL in women or drug treatment for low HDL cholesterol. 4. Hypertension: blood pressure greater than or equal to 130 mmHg systolic or greater than or equal to 85 mmHg diastolic (greater than or equal to 130/85); or drug treatment for elevated blood pressure 5. Fasting Glucose: Fasting plasma glucose (FPG) level greater than or equal to 100 mg/dL; or prior diagnosis of T2DM or drug treatment for elevated blood glucose levels.[8][9]

The clinician should assess the etiology of the weight gain and its associated health risk. Many factors can contribute to the development of obesity and include but are not limited to:

• Iatrogenic causes from prescription drugs causing weight gain
• Dietary-induced obesity, hypothalamic disorders
• Hypothyroidism
• Seasonal affective disorder
• Cushing syndrome
• Polycystic ovary syndrome
• Hypogonadism
• Growth hormone deficiency
• Pseudohypoparathyroidism
• Sedentary lifestyle
• Genetic induced disorders
• Social and behavioral factors
  • Socioeconomic status
  • Ethnicity
  • Psychological factors
  • Restrained eaters
  • Night eating syndrome
  • Binge eating disorders

However, most cases of obesity are simply related to modifiable behaviors such as a sedentary lifestyle and increased caloric intake. Secondary causes of obesity are uncommon; they should be considered and ruled out by clinicians. Prescription medications can be a common cause of weight gain, in particular glucocorticoids, antipsychotics, insulin, and sulfonylureas.[10]

The evaluation of the overweight/obese patient must include a detailed history, physical examination, blood pressure measurement, fasting glucose or glycated hemoglobin (A1C) or serum triglycerides level, thyroid-stimulating hormone (TSH), liver enzymes, fasting lipids panel, and an investigation of the presence of sleep apnea. Investigation into the etiology and plan future management strategies, the medical history should include: age at onset of weight gain, life events associated with weight gain, previous weight loss attempts/efforts, any change in dietary patterns, history of exercise and functional status, any current medications, and history of current tobacco usage. Clinicians should note that due to physiological events and genetic composition, women have a higher percent body fat of their body weight compared to men from puberty onward. Women also tend to gain more fat during adult life than men. Additionally, women may experience persistent increases in body weight and fat distribution after pregnancy or gain weight due to menopause. Findings from a physical examination that might point to a secondary cause of obesity include but are not limited to: neck goiter, proximal muscle weakness, moon facies, buffalo hump, and/or purple striae (Cushing syndrome), and acne and/or hirsutism in adult females (polycystic ovary syndrome). Additional testing may be necessary depending upon the clinician's findings on history, physical examination, and initial blood tests.[10][11]

Assessment and management of the overweight or obese patient and related comorbidities is an important part of the clinical health setting in the United States of America.  Further, the proven relationship between BMI and increased health risk allows for the identification of BMI categories that can be used to help guide the selection of weight loss therapy for the patient moving forward.[10][11]