The adipose (fat) cells under the skin are where individuals store excess fat. The more excess fat they have, the more "strain" they put on these cells which then get bigger and don't work as well as they should. Having some fat under the skin is important. People who have a genetic defect which results in them having almost no fat under their skin have a very high risk of a condition called insulin resistance (where the body does not respond as well to insulin and blood sugar levels rise). This can lead to diabetes and heart disease despite them not being overweight.