What is a physical condition characterized by a high body mass index?

Obesity is a condition in which a person has too much body fat. To measure whether someone is obese, researchers typically use the Body Mass Index (BMI). Body mass index is calculated by dividing a person’s weight (in kilograms) by their height (in meters) squared. BMI is a more accurate measure of obesity than weight alone, and for most people it is a fairly good (although indirect) indicator of body fatness. Other measurements that reflect the distribution of body fat, such as whether more fat is carried around the hips or the abdomen, are increasingly being used along with BMI as indicators of obesity and disease risks.

These measurements include waist circumference and the waist-to-hip ratio. The default weight categories based on BMI are for adults aged 20 and over:

BMI in kg/m2 Weight Category
Below 18.5 Underweight
18.5 to 24.9 Normal
25.0 to 29.9 Overweight
30.0 to 39.9 Obese
40.0 or higher Severely obese

For children and adolescents (younger than 20 years of age):

BMIWeight Category
BMI-for-age at or above sex-specific 85th percentile, but less than 95th percentileOverweight
BMI-for-age at or above sex-specific 95th percentileObese

There is a relationship between obesity and cancer, but the extent of this relationship is not known.

The majority of evidence linking obesity to cancer risk comes from large cohort studies, a type of observational study. However, data from observational studies can be difficult to interpret and cannot definitively establish that obesity causes cancer. It is possible that these other differences—rather than their body fat—explain their different cancer risk.

There is consistent evidence that higher amounts of body fat are associated with increased risks of a number of cancers.

Obese and overweight women are two to about four times as likely as normal-weight women to develop endometrial cancer, and extremely obese women are about seven times as likely to develop the more common of the two main types of this cancer. The risk of endometrial cancer increases with increasing weight gain in adulthood, particularly among women who have not used menopausal hormone therapy.

Obesity Linked to Several Cancers

• Esophageal adenocarcinoma: People who are overweight or obese are about twice as likely to develop esophageal cancer, called esophageal adenocarcinoma, than people of normal weight, and those who are extremely obese are more than four times more likely to develop esophageal adenocarcinoma. People who are obese are nearly twice as likely as normal-weight people to develop cancer in the upper part of the stomach, that is, the part that is closest to the esophagus.

People who are obese are twice as likely as normal-weight people to develop cancer in the upper part of the stomach. People who are overweight or obese are up to twice as likely as normal-weight people to develop liver cancer. The association between overweight/obesity and liver cancer is especially strong in men. People who are overweight or obese are more likely to develop renal cell cancer, the most common form of kidney cancer, than people who are not overweight or obese. Obesity is not associated with renal cell cancer in individuals with or without high blood pressure.

Overweight people are 10% to 20% more likely to develop multiple myeloma. A slow-growing brain tumor that arises in the membranes surrounding the brain and the spinal cord is made 50% more likely in people who are obese and about 20% more likely in people who are overweight. People who are overweight or obese are 1.5 times as likely to develop pancreatic cancer as people of normal weight. Colon cancer is more likely in people who are obese compared to those who are normal weight.

A higher body mass index (BMI) is associated with increased risks of colon and rectal cancers in both men and in women, but the increases are higher in men than in women. A small number of people who are overweight or obese will develop gallbladder cancer. The increase in risk is greater for women than for men. Many studies have shown that, in postmenopausal women, a higher BMI is associated with a modest increase in risk of developing breast cancer.

A 5-unit increase in BMI is associated with an increased risk of 12%. among postmenopausal women, those who are obese have a greater risk of developing breast cancer compared with those who are of normal weight. The higher risks are seen mainly in women who have never used menopausal hormone therapy and for tumors that are expressed by hormone receptors.

Obesity is a risk factor for breast cancer in men.

In premenopausal women, overweight and obesity have been found to be associated with a 20% decreased risk of breast tumors that express hormone receptors. A study conducted by the National Cancer Institute showed that being overweight or obese is associated with a slight increase in the risk of ovarian cancer, particularly for women who have never used menopausal hormone therapy. For example, a 5-unit increase in BMI is associated with a 10% increase in risk among women who have never used menopausal hormone therapy.

• Thyroid cancer: A higher BMI (specifically, a 5-point increase in BMI) is associated with a small (10%) increased risk of thyroid cancer.

Obesity can lead to increased risk of cancer by increasing the chance for malignant tumors to grow in the body. Several possible mechanisms for how obesity might increase the risk of some cancers have been suggested.

• Obese people often have low levels of chronic inflammation, which can cause DNA damage and cancer over time. Overweight and obese people are more likely than normal weight people to have conditions or disorders that involve or cause chronic local inflammation and are risk factors for certain types of cancer. Chronic inflammation in the esophagus as a result of gastroesophageal reflux disease or Barrett esophagus is a likely cause of esophageal adenocarcinoma.

Obesity is a risk factor for gallstones and a history of gallstones is a strong risk factor for gallbladder cancer.

Liver cancer is a condition that is caused by chronic inflammation in the liver. This inflammation can be caused by ulcerative colitis or hepatitis, two conditions that tend to occur together. FAT tissue (also called adipose tissue) produces excess amounts of estrogen, high levels of which have been associated with increased risks of breast, endometrial, ovarian, and some other cancers. People who eat a lot of food and are obese often have higher levels of insulin and IGF-1 in their blood. The condition of having high blood sugar (hyperinsulinemia) or low sensitivity of the insulin (insulin resistance) is a risk factor for the development of type 2 diabetes.

High levels of insulin and IGF-1 may promote the development of colon, kidney, prostate, and endometrial cancers. Fat cells produce hormones that may stimulate or inhibit cell growth. The level of a hormone called leptin, which seems to promote the cells’ proliferation, increases as the individual gains weight.

Another adipokine, adiponectin—which is less abundant in obese people than in those of normal weight—may have anticancer effects.

• Fat cells can also directly and indirectly affect other regulators of cell growth, including the target of rapamycin (mTOR) and AMP-activated protein kinase in mammals. Other possible mechanisms by which obesity could affect cancer risk include changes in the mechanical properties of the scaffolding that surrounds breast cells and altered immune responses, effects on the nuclear factor kappa beta system, and oxidative stress.

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