A 2016 review supported excess food as the primary factor.  Dietary energy supply per capita varies markedly between different regions and countries. It has also changed significantly over time.  From the early 1970s to the late 1990s the average food energy available per person per day (the amount of food bought) increased in all parts of the world except Eastern Europe. The United States had the highest availability with 3,654 calories (15,290 kJ) per person in 1996.  This increased further in 2003 to 3,754 calories (15,710 kJ).  During the late 1990s Europeans had 3,394 calories (14,200 kJ) per person, in the developing areas of Asia there were 2,648 calories (11,080 kJ) per person, and in sub-Saharan Africa people had 2,176 calories (9,100 kJ) per person.   Total food energy consumption has been found to be related to obesity. 
Weight gain can result from an increase in body fluid, muscle mass, or fat. An increase in body fluid can come from medications, fluid and salt retention, intravenous fluid infusion, kidney or heart failure . An increase in muscle mass is commonly seen with exercising. An increase in body fat is commonly seen as a result of diet or lack of exercise as the body converts muscle to fat. An excessive weight gain is referred to as obesity . Obesity is a function of environmental ( diet , exercise , lifestyle, etc.), hormonal, and inherited (genetic) factors in varying degrees. Weight gain is a normal part of pregnancy .