WHO has suggested that the prevalance of obesity is the greatest challange for the modern world. Scientist has proved that many disease (especially chronicle disease) can attribute to (at lease at certain degree) obesity.

Cardiovascular disease risk increases due to overweight elevating blood pressure, cholesterol, triglycerides, and increasing insulin resistance. The location of excess body fat can further increase CVD risk. Central obesity for example, is directly associated with an increased risk of heart disease. A 20% reduction in body weight can reduce CVD risk by 40%. This can be achieved by keeping BMI in the normal range. Over 50% of all cases of hypertension are simply due to being overweight.

The incidence of diabetes increases with increasing weight. Diabetes is three times more likely in obese individuals with a BMI of 28 or greater.

Osteoarthritis symptoms are worse in men and women with a BMI over 25. Excess body weight stresses joint cartilage.

Cancer risk can increase due to elevated hormones associated with obesity, influencing cancer development. Excess estrogen is linked with reproductive system cancers. Adipose tissue (fat tissue) is a major site of estrogen synthesis in women. Scientists link a BMI of 28 to 30 with an increase in cancer risk.

Other diseases associated with obesity include sleep apnea, abdominal hernias, varicose veins, gout, gall bladder disease, respiratory problems including pickwickian syndrome (a breathing blockage linked with sudden death), and liver malfunction.

Massive obesity, indicated by a BMI over 40, is so closely associated with health problems that it is regarded as a disease in its own right.