Humans are constantly obsessed with their weight. We watch our calories, our fats, our carbs. Why do we have to be so careful? One group of animals on the planet does not need to even give diets a second thought.
Animals in the wild are not concerned about gaining too much weight. Pretend you are a monkey who has to climb trees to find fruits and nuts to eat. You expend as much energy looking for and obtaining food as you eat.
Grazing animals have to eat all day to obtain enough nutrients. Eating grass and plants provides a very high fiber diet and inefficient to digest. Here again, they do not consume excess calories.
Predators such as lions have to hunt their prey. They may walk for days to find the right food. Then they chase it, kill it and consume it. The calories they take in may last for several days. Then they have to go back and do it again.
If humans were to expend more energy to obtain food, we may not need to worry about the weight coming from excess calories. Think about walking to the store every time you need something to eat. How about climbing up a tree to eat an apple? How about just eating the calories you will be burning off every day? If you are more active, you will need more calories. If you are inactive, you will need less calories.
It is not as hard as you think.