According to experts from the University of Alabama, humans aren't the only ones who are packing on the pounds. Many animals - even those on tightly controlled diets - have also experienced an upward spiral of weight gain during the last few decades, they claim.
After studying a group of marmoset monkeys at a primate centre in Wisconsin, the researchers were puzzled over why the monkeys had all gained weight, since there was no obvious reason for it. So they started examining data on other animals - including dogs, cats, chimpanzees and even wild rats. And as they expected, each group of animals had inexplicably gained weight over time.
The experts admit they cannot explain why humans and animals are getting fatter. But they have a few theories. Artificial light may be one explanation, they say, since exposure to light while you're asleep may disrupt the hormones that control your metabolism.
Viruses may also have an effect on obesity rates, the researchers suggest. One recent study, for instance, found that nearly 80% of children exposed to a virus called adnovirus-36 were obese.
Even chemicals in our environment could play a part in why we're getting fatter, the experts believe.
What are your thoughts on why both humans and animals are gaining weight?