Your brain cannot burn fat directly, so the body turns part of it into sugar
A gram of fat holds about nine food calories, more than twice as much as carbohydrate, which is why animals store energy this way. But the brain cannot run on fatty acids. When it needs fuel, the body strips the glycerol backbone from stored fat and rebuilds it into glucose.
Chemically, most dietary and body fat is triglyceride: one glycerol molecule joined to three fatty acids by ester bonds, each link formed by releasing a molecule of water. Strictly, fat can mean only triglycerides solid at room temperature, excluding oils; loosely, it can stand for all lipids, including cholesterol, phospholipids and waxes. Fats are one of the three macronutrient groups, and they carry flavour, aroma and the fat-soluble vitamins A, D, E and K.
Digestion needs a workaround because triglycerides are too big to absorb. Bile and lipases split them into free fatty acids and monoglycerides, which intestinal cells take up and reassemble, packaging them with cholesterol and protein into particles called chylomicrons. These enter the lymph, reach the large vessels near the heart and join the bloodstream, where enzymes on vessel walls release the fatty acids for tissues. Fat cells store the surplus and release it under hormonal signals such as glucagon; adipose tissue also produces the hormone leptin.
Only two fatty acids are essential in the human diet: alpha-linolenic acid, an omega-3, and linoleic acid, an omega-6. The body can make the rest. Beef and full-fat dairy lean toward saturated fat, while fish, nuts, avocado and most plant oils lean unsaturated, with coconut and palm kernel oil as exceptions. Industry reshapes fats through pressing, rendering, hydrogenation and winterisation.
Health effects remain contested in detail. Many studies found that swapping saturated fat for cis unsaturated fat lowered risks of cardiovascular disease, diabetes and death, and the prevailing consensus accepts a causal link between saturated fat, blood cholesterol and heart disease. Critics warn that labelling fats simply good or bad oversimplifies, since most foods contain both, and that people cutting saturated fat may turn to refined carbohydrates instead. The Mediterranean diet is higher in total fat than northern European diets, mostly unsaturated, and reviews link it with lower cardiovascular risk.
Source: Fat
