Among the many gene interactions with nutrition, the most important worldwide is the adaptation to starvation in individuals who then consume a high fat/high sugar/high salt processed food diet and remain inactive. They are living in the modern world where their genes are no longer well-adapted with the result being overfatness or obesity. Whenever you eat more calories than you burn, the excess is stored as fat.
The most striking example of this is among certain Polynesian islanders where on some islands up to 80% of the inhabitants are obese. They have both more muscle and more fat, since only the strongest individuals were able to survive paddling from the mainland of Asia to the Pacific Islands. Today many college football teams recruit from Samoa as the size and strength of those young men is an advantage in American football.
However, in most of the world the trend for obesity is associated with chronic diseases such as diabetes. In China, there are hundreds of millions of people at risk of developing diabetes as the result of urban diets and lifestyles including the invasion of Western fast food establishments.