T Nation

Biology of Fat Usage?


Short version: How does your body decide whether to draw energy from fat or from lean mass when in a caloric deficit from dieting?

Long version: I understand that if someone diets too severely, the body apparently interprets this as a period of 'starvation' and attempts to horde fat and use lean mass as its energy source. However, I'm very curious as to how the body determines what qualifies as too severe a diet.

It would seem to me that the body really has no awareness of a specific calorie threshold. I'd think it can't tell that it was in a 700 calorie deficit today as opposed to 500 yesterday; it only knows when the food energy runs out and it has to start munching on internal stores to make up the difference. It also knows when it's hungry.

With that in mind, is the 'starvation' effect truly due to just eating too few calories or eating so little food mass that the body really is starving? I've seen what some people attempt to get by on while dieting, and it doesn't surprise me that the body freaks out over how little sustanence is coming in. But how much of that is simply due to an exceptionally low volume of food?

I've got no idea about this, but it seems to me that as long as the body has a regular supply of food, keeping the digestive system occupied, it has no feeling that it is being 'starved'. It would seem to me that's why plans like the Velocity Diet are effective - calorie intake can be severely diminished, but you're chugging those shakes all day long and your body never gets a real sense of deprivation.

Assuming this is so, does that mean the 'starvation' effect is one more of satiation than actual calorie intake? To head off the obvious criticism, I'm not advocating dieters cut down to a thousand calories a day and gorge themselves on iceberg lettuce to 'successfully' crash diet; I understand the importance of a healthy diet with adequate nutrition. I'm simply curious as to how the body decides when to switch from burning fat to mucnhing on muscle. Long post, thanks for reading :slight_smile:


Try reading the Anabolic Diet thread. Lots of information there about it.


Well I think the body does it by a sneaky way - when you are low on calories, it switches on depression.

I think depression (not all its forms) is a survival tactic, the aim of which is to minimise enthusiasm for effort, shut down libido, shut down thinking (your brain uses 1/3rd of resting energy, our big human brains are the SUVs of the animal world) ...

If you starve yourself of calories you are still eating far, far more than the average, extremely hard working person in Africa who walks miles a day up hills doing the farmers walk with kilos of water ... and they can still be damned strong, lean and mean.

If however you cut calories and sit around all day, and don't have the energy to exercise, and go half assed with your intensity, well I think your body starts stealing away your muscles.

If you don't use them you lose them.

You body doesn't want to carry more muscles than it needs and will shred them first chance it gets.

If you cut calories you switch on depression (even in a mild form) and your body THINKS that there is not enough food - wipes out any muscle you don't need, saves as much fat as possible, shuts down your brain as much as possible, prepares for what it expects is going to be a horrendous, long haul famine.

The only way around it is to use willpower, exercise, make your body think it needs the muscles.

Personally I am not a big fan of reducing calories. Instead of cutting calories and doing aerobics for an hour, increase calories and do aerobics for SIXTEEN HOURS A DAY. Go take a few days off and do some serious hiking, with a heavy pack. (seriously you can do 8 hours a day not 16).


Length of time.

In the initial stage of starvation, your liver releases glycogen to supply glucose to your CNS.

Once glycogen stores are depleted, a process that generally takes a few hours, your body begins to convert tissue protein to glucose. Ninety percent of the glucose is provided by tissue protein, while the other 10% is provided by fat. However, since only the glycerol portion of fat can be used to make glucose and glycerol is a small part of a triglyceride's weight, fat breakdown still doubles during this phase.

If tissue catabolism continued, you'd die in about 10 days, so your body begins to break down more fat, making ketones to take the place of glucose derived from protein. Protein breakdown still continues, however, just at a much slower rate than in the beginning of starvation.