Fat and obesity was considered to only be a storage depo for surplus calories. It is now known to produce and send hundreds of chemical signals (cytokines) to other parts of the body. Many of these signals stimulate inflammation.

Obesity is associated with low grade chronic inflammation within the fat tissue. Excessive fat storage leads to reactions within fat cells which in turn lead to the release of pro inflammatory chemicals form the fat cells and also from the immune cells (macrophages) within the fat tissue

Chronic inflammation is known to be associated with certain cancers and may contribute to worsening of joint disease (arthritis)

Obesity also leads to fat cells changing the chemical signals they produce leading to: 

- an inability to detect satiety despite high energy stores leading to further overeating. 

- changes in glucose regulation increasing the risk of diabetes mellitus.

- increased production of hormones which stimulate new blood vessel growth and cell division which are reinforced in cancer.

What is eaten plays 100% role in the development in obesity. Exercise will only reduce obesity by about 5%. Exercise alone will not lead to significant weight loss.