I know when dieting and exercising we are told that we burn fat and calories. I’d like to know when we burn that fat is it adipose tissue we are burning? If so where do the fat molecules or cells actually “burn” off to? Do we sweat it out? Does it get filtered into our urea or stool? Where does all this fat go when we burn it?
Fats get broken down into sugars and burned resulting, primarily, in water and carbon dioxide. We literally exhale the pounds away.
Thanks Q - that’s rather cool.
Not exactly. Your typical triglyceride is three fatty acids linked by a glycerol. The glycerol is broken down as sugar, but the fatty acids are not; they’re metabolized via a process called beta-oxidation. Only odd-chain fatty acids have metabolic byproducts that are convertible to sugars, but these fatty acids form a tiny minority of your fat reserves. Typical even-chain fatty acids are not glucogenic.
Why do you need aerobic exercise to burn fat? Because your skeletal muscles are preferentially fat-burners, not sugar-burners. Fatty acids cannot be metabolized anaerobically like sugars can, so your muscles need oxygen to burn them. Under heavy exercise, your muscles are contracting so strongly that they become temporarily ischaemic, because blood has a hard time getting into them. This makes them convert from aerobic mode to anaerobic (sugar-burning) mode, producing lactic acid, and giving you that characteristic burn.
But yeah, eventually, it all gets breathed out as CO2.