We've known for 300 years how to fatten livestock, but we plead that we don't know what makes people fat. That's only possible if we somehow think that people are not mammals just like pigs. The same science applies to both species.
If your body has strong insulin sensitivity, and that's probably true when you are young, you can deal with a high carbohydrate diet without getting fat. Your body will simply run hotter, and you'll burn off the excess energy as heat.
However over time carbohydrate resistance is likely to develop, in conjunction with insulin resistance. Your ability to burn glucose will decline and your body will begin to store more fat.
Eventually, over 30 or 40 years, you develop a set of diseases that are common in our society. Together we call these diseases metabolic syndrome. Most adults over 40 have it, and we think it's "normal" but it's a clear sign of our poor diet. Our diet is not healthy, but it can be changed. While metabolic syndrome is common, it should rare. Metabolic Syndrome is not "normal."
Skim milk, low fat, corn and molasses
Keep in a crate.
Green top milk, low fat, and high carbohydrates.
Watch Television.
The recommended diet is a fattening diet, so we shouldn't be surprised if it makes us fat.
Exercising is an inadequate and ineffective solution to faulty dietary practice.