Whether we are destined to be fat or not really depends on your definition of destiny. From an evolutionary perspective, our bodies were not designed to live the way we do in modern times. We were not built to sit at desks or on sofas watching TV. Our bodies were structured by evolution to be active, but we longer live the active lifestyle of our ancestors. It is easy to see how we become overweight if we look at our sedentary lifestyles and our fast food/microwave dinner diets.
There is a genetic component to obsesity as well. We know that children of obsesse parents are much more likely to be obsesse themselves. Some of this is due to nurture but it can also be linked to nature. We do know there are genes which help determine our body's average weight, but science cannot yet explain the influence of these genes.