In all of the studied populations, the average weight had increased over the last decades. The chance of developing obesity had also increased significantly in all groups. Researchers assert that these results point to other explanations for weight gain besides too much food intake and / or too little exercise. Other reasons named are environmental exposure to substances that affect metabolism; hereditary tendency to gain weight; and the effects of certain viruses.
Overweight can lead to illnesses such as diabetes, respiratory difficulties, tendon and joint strains, and digestive system problems. Animals with overweight are more sensitive to heat and have a shorter lifespan. Laboratory animals with overweight can skew test results; furthermore, a shortened lifespan also means that more lab animals are needed for testing.