Body mass index estimates your body fatness, using a ratio of your height to your weight. Although not a perfect measure for all adults, BMI is simple to compute and noninvasive. A BMI that's too high alerts your doctor that you may be at a weight that's unhealthy, putting you at risk for a number of health complications.
A BMI of 30 is classified as obese -- a condition that's a serious threat to your well-being. Examples of people who have a BMI of 30 are a man or woman who is 5 feet 4 inches and 174 pounds; 5 feet 10 inches and 209 pounds; or 6 feet 1 inch tall and 227 pounds.
Chronic Disease Risk Increases With Obesity
Being obese makes it more likely that you'll develop a chronic disease, such as heart disease, type-2 diabetes or cancer. According to the Obesity Society, about 90 percent of people who currently have type-2 diabetes are overweight or obese. This makes obesity the No. 1 predictor of the condition.
Obesity can also increase your blood pressure, blood sugar, triglycerides, low-density lipoprotein cholesterol levels and inflammation. When elevated, these raise your risk of cardiovascular disease, stroke and premature death.
Obesity doesn't put you at increased risk for all types of cancer. But a BMI of 30 directly associates with increased incidences of breast, endometrial, ovarian, kidney, esophageal, pancreatic, colon and rectal cancers.
Diminished Reproductive and Musculoskeletal Health
A BMI above 30 can compromise a woman's fertility. The Harvard School of Public Health reports that about 25 percent of ovulatory infertility in the United States is likely due to obesity. Men may also experience lower sperm production and less motile sperm due to a high BMI.
Your musculoskeletal system suffers due to the excess weight associated with a high BMI as well. Your bones, muscles and joints may not be able to healthfully carry your size, resulting in arthritis and the need for joint-replacement surgery.
A BMI Above 30 and Overall Quality of Life
The heavier you are, the harder it is for your breathing organs to do their job. Asthma and obstructive sleep apnea can both result when you have a high BMI. Asthma can make daily tasks difficult, and sleep apnea can prevent you from getting quality rest.
Both of these factors can impede your daily function. Along with the other side effects of being obese -- such as decreased mobility; chronic disease, which requires specific medication and care; increased risk of gallstones; chronic kidney disease; fatty liver disease; and possible negative effects on mood -- you experience a decline in your overall quality of life. As BMI increases above the healthy range, such as a BMI over 30, multiple aspects of your health can be negatively affected, putting you at risk of premature death, according to Harvard School of Public Health.
Possible Complications With BMI
In the case of a particularly muscular person, you may register a BMI of 30 but have a perfectly healthy physique with a low level of body fat. In most cases, a visual examination reveals that you're not obese, and your doctor will rely on other measures of health, including waist size and blood tests, to rule out any significant health concerns.
If you register a BMI below 30, don't necessarily assume you're in the clear either. BMI can miss people who are of normal weight but carry too much body fat -- 20 percent or more for a man and 30 percent or more for a woman. In these cases of normal-weight obesity, you may have many of the same health risks as clinically obese people; it's just not easily determined by your BMI. A measure of your waist circumference can help you ensure you're healthy if you're in doubt. A waist that's more than 40 inches around for a man or 35 inches around for a woman is a cause for concern.
Note that a different equation is used to determine BMI in children and adolescents. The gender and age of the child is taken into account in this equation.