Look around you at the bodies of the extremely old - when was the last time you recall seeing an obese centenarian? Excess fat held over the years is a killer, and the oldest people are very rarely overweight. I noticed a paper today that works backwards from medical and mortality data to further support the same conclusion:

There has been ongoing debate about the health risks associated with increased body weight among the elderly population. One issue has not been investigated thoroughly is that body weight changes over time, as both the reasons and results of, the development of chronic diseases and functional disabilities. Structural models have the ability to unravel the complicated simultaneous relationship between body weight, disability, and mortality along the aging process. Using longitudinal data from the Medicare Current Beneficiary Survey from 1992 to 2001, we constructed a structural model to estimate the longitudinal dynamic relationship between weight, chronic diseases, functional status, and mortality among the aging population. A simulation of an age cohort from 65 to 100 was conducted to show the changes in weight and health outcomes among the cohorts with different baseline weight based on the parameters estimated by the model. The elderly with normal weight at age 65 experience higher life expectancy and lower disability rates than the same age cohorts in other weight categories. The interesting prediction of our model is that the average body size of an elderly cohort will converge to the normal weight range through a process of survival, senescence, and behavioral adjustment.

Become fat and stay fat, in other words, and you'll remove yourself from the picture much sooner than would otherwise be the case. In addition, your health in the years ahead will be much the worse for it. Do as you will with your life, but don't say you weren't warned.