In a word: “Insulin”. The body’s hormone that regulates your blood sugar levels is also a fat storage hormone.
Throughout most of human existence, sugars and grains were relatively quite rare and not a significant part of the diet. Grains have only been grown, hybridized, and stored for the last 10,000 years which is just a ‘blink’ in the evolutionary timeline.
In the last 50 years, the American/Western diet has shifted to a primarily grain and carbohydrate-based diet. In spite of advances in science and medicine, the health and fitness and appearance of the general population has declined to a state of being a public health disaster.
I think I have gotten ahead of my self. Let’s get back to some basic nutrition biology.
There are just three basic food energy groups and they are called “Macros”: Carbohydrate (sugars), Proteins, and Fats. Think of carbohydrate as a readily accessible energy and can circulate as Blood Sugar, as well as stored in tissues as a concentrated form called Glycogen. Proteins are made up of amino acids with a sugar ‘backbone’. Proteins are the building blocks for all the stuff that holds us together and helps us function (muscle, ligaments, tendons, skin, internal organs). Fats are for long term energy storage. There is twice as much energy in a gram of fat as there is in a similar amount of protein or carbs. And on that note, humans are fat storage machines. When food is scarce or only intermittently available, the body will be able to draw upon and use the stored fat. When food sources are plentiful, the body will try to store as much energy in the form of fat to be used for the proverbial ‘rainy day’ (or famine, long cold winter, etc.).
Now here is the real message. The body treats the different macros differently. The medical, nutritional, fitness, and weight loss industry has been repeating the same falsehood for decades. If someone is trying to lose weight, they will respond, in part, with: “A calorie is a calorie is a calorie”. The message they are trying to convey is that if you are trying to lose weight, you need to count calories and consume less calories (than usual) and expend more physical effort to burn calories. This is the “calories in-calories out” (CICO) model. The assumption is that the body respects the laws of thermodynamics in a simplistic way and everything can be understood in terms of an accounting spreadsheet. It doesn’t, and you can’t.
Why? Because of insulin. In medical school I was taught simply that insulin regulates the levels of Blood Sugars (BS) in the blood. And it does. If you eat a meal with a lot of sugars (including fruits) and carbohydrates (grains, pastas, breads, potatoes, rice), the blood sugars will increase which tells the pancreas to release more insulin which helps lower the blood sugar. Very simply, eating these food is a signal to the body that there is excess energy and so the other function of insulin is activated which is an energy storage signal and the excess will be stored as glycogen in the various tissues (like muscles and liver for immediate access for burst of activity, as well as stored long term as fat.