A barefoot layman’s view of the biochemistry of fat storage
The dynamics of fat storage are pretty complex; here’s a boiled down version:
It all starts with the regulation of blood sugar (glucose). Glucose is transported by the bloodstream as a source of fuel for our cells, however too much sugar in the blood is a problem, so the body regulates it by pancreatic insulin secretion. The quickest way to get sugar out of the blood is to shuttle if off for storage as fat in fat cells, and insulin in the blood triggers just this process.
As carbohydrates are digested, they turn into glucose. In reaction, insulin in the blood directs glucose into fat cells (and away from muscle cells). In the process, fatty acids in the blood, say from eating bacon, will be also be turned into fat. So, the carbs indirectly cause temporary building up fat from the carbs and other fat sources. If everything is working properly, glucose levels come down, insulin follows, and everything is hunky dory. Over time built up fat gets burned off into energy for the rest of the bod.
The foods we think of as “carbs” (and sugar) are hyper-easily digested, turning very rapidly into glucose and thus cause the body’s red lights to start flashing and insulin secretion to spike. This control system evolved over millions of years of pre-agrarian history in the days before French fries and even rice or bread. So it was not designed to handle the spikes caused by these foods. Human diets evolved much more rapidly in the last 10,000 years than bodies have. As a consequence, the sugar control system operates imperfectly in the face of conventional modern diets. Insulin levels stay high longer than they are needed, keeping energy from going where it’s needed causing another familar control system takes over…we get hungry, the thing that causes us to eat. It’s no surprise that eating one potato chip causes us to crave another.
Carbs are not just potatoes and bread; you’ll find them in all kinds of foods including very healthy vegetables like spinach (known sometimes as “good carbs”). So if you eat a whole lot of spinach you could ingest a lot of carbs. However the carbs in spinach are bound up in fiber, which is very slow to digest, thus glucose is released more slowly into the bloodstream. Spinach contains almost as much fiber as carbs; a potato has 10X carbs over fiber. So eat a potato and some bacon, and all the glucose and carbs get stored as fat; put a bunch of bacon on your spinach salad and the fat in the bacon is burned as fuel.
Fructose, the sugar in fruit and dominant processed food sweetener, strains the system in a particularly complex way. Fructose is only processed in the liver and when there’s insulin in the blood, it causes fat build up in the liver. Through a not very well understood mechanism, over time this causes a condition called insulin resistance. Cells throughout the body become less sensitive to insulin, thus requiring more of it to be secreted and for longer. More insulin in the blood means more fat buildup. Wrap the fructose in a bunch of fiber to slow it down, as in fresh fruit, and it’s not so bad. Liberate it in juice, or worse, refine it as syrup, and it’s pretty bad. (“Poison” according to Dr. Robert Lustig in this 90 minute, worth-watching, video.)
I’m not sure I’ve got it all right, but there is little argument about the fact that carbs turn to glucose, glucose triggers insulin, and insulin facilitates fat build up.