Why doesn't all exercise burn fat? Or does it?

There’s a lot of bad science and anecdotal evidence in fields of nutrition, fitness and weight loss, so I take most things I read on those subjects with a pinch of salt. But one thing I’ve consistently read is that certain kinds of exercise burn fat more effectively than others, and that you have to be exercising for a certain amount of time before fat starts to be burned.

I would think that even if fat isn’t being directly used at the time of exercise it will be used later to replace whatever other energy source was used. You can’t exercise without using energy. That energy must come from somewhere and it must be replaced.

So what’s going on?

Exercise burns calories. If you burn more than you take in, it’s a good thing in terms of losing weight. The body needs to get calories from somewhere, so it will dip into the calorie reserves: stored fat.

The body first and foremost relies on carbohydrates - namely, glycogen - for energy. After that comes protein. Then fat. So … exercising will not immediately “burn” fat … unless one has very little glycogen and protein.

So if you exercise enough to use the glycogen and the protein, but stop before fat is used, surely you still lose fat later as the glycogen has to be replaced by another energy source?

The body burns a mix of fat and glycogen, a higher percentage of glycogen at higher intensities of exercise. A small amount (~5%) of protein is burned.

You’re right, it burns fat indirectly. So long as you don’t “make up” the burned calories by eating more, you’re eventually going to lose weight, and that means less fat in the long term. Ultimately it’s a simple matter of in vs. out.

Not this again. :smack:


The myth that there is a “fat burning zone” of low intensity that one should stay in to maximize fat loss (as often labelled on various health club machines) is wrong for exactly that reason. Sure, the body can only mobilize fat into energy so fast, so during higher intensity exercise a larger percent will come from various glycogen stores, the cash on and so to speak. But exactly right, those glycogen stores will generally be replenished out of intake and if the result of exercise has been a modest deficit of calories in relative to calories burned over the course of the day (during the exercise and afterwards in reaction to it), then fat will be mobilized throughout the day.

There is some work that even suggests that high intensity interval training is superior at promoting overall fat loss per calorie burned (and many trendy fitness plans are built around that concept) … but that is more than what you were geting at I think.

There are no specific fat burning exercises. Extended aerobic exercise will burn some fat directly, but exercise is more about the total metabolic balance sheet. More calories burned than consumed, some fat will be lost.

Extreme starvation diets and actual starvation will cause loss of muscle mass. A minor calorie deficit causes fat loss. Regular exercise helps the body know that everything’s OK and that it’s not a good time to burn muscle.

One question though. Back before my ankles caved in I was quite a runner. I noticed on longer runs (1+ hrs) that my sweat started smelling kinda like combination of burnt sugar and paint thinner. Was this ketosis?

For kicks, a trip down memory lane, and a more recent one. Bozuit that reading you’ve been doing hasn’t been around these parts! :slight_smile:

Well thanks but the second link is about fasting and the first one is not the same question either, although now I see one of the replies does have a useful answer in it.

Most people who have responded here seem to have seen the words “exercise” and “fat” in my title and assumed I was asking “How do I lose weight?”. I’m just asking out of curiosity (and actually a disagreement with a friend who claimed her classes made more sense than my “conservation of energy” logic). The reason I turned to SDMB for an answer is that it’s not an easy question to “ask” Google. I tried but found nothing relevant.

Anyway thanks to all who replied.

The main operational problem with your hypothesis is that it assumes the body will have to start using fat to “replace the energy”.

Your body wants to hold onto the fat. So much so it will play all sorts of brain chemistry tricks to ramp up your appetite to push you to reach a level of caloric intake it deems acceptable. If you did not eat in the interim after exercising, then yes it would burn the fat, but it’s going to do everything it can in it’s power to avoid doing that. Burning fat is the last resort, and being in fat burning mode is not a pleasant psychological or physiological state.

My belief is that preparing your body to metabilicaly disapate fat is an effective strategy. Simply adding muscle mass helps to burn fat throughout the day. Muscle atrophy from lack of activity is a huge contributor to storing fat. Walking, bicycling, hiking are good for building up the legs which are a large muscle group. Involving all your muscles in some kind of regular activity will help to keep their mass up and keep your fat burner going all the time.

So in theory it burns the same amount of fat, but in practice, unless you’re very disciplined, it won’t give the same results? If you lose fat “directly” (as energy for exercise, not to replace depleted glycogen stores) does your brain not still demand food to replace that fat?

That’s one of the reasons I walk fast and run up the stairs at work when I think no one is looking.

You keep focusing on the body “burning fat” during exercise. My point is that a person is almost never going to get to that point unless your exercise is unusually intense or prolonged. Most people will burn only minuscule amounts of fat in daily exercise routines. Fat is the absolute last resort fuel before eating your own muscles, and most people have more than enough free carbs in their system to fuel a typical exercise session without the body needing to touch the fat reserves. I do not believe your model where fat is constantly being hit and needs to be restored accurately represents what is going on in most cases.

Where fat is mainly burned is in long term caloric restriction where there are not enough calories being consumed to support the metabolic requirements of the body so fat is burned as the last resort fuel. If your exercise is really intensive and prolonged, and you are limiting calories to the point you quickly deplete your ready energy reserves, then yes you will burn fat like crazy during exercise. The problem is that if you are in this mode it takes a lot to stay focused as burning fat as exercise fuel after glycogen depletion is (in my experience) exhausting and a decidedly unpleasant sensation.

Fat and carbs are burned together in varying amounts. Fat is not a fuel of last resort.
Cite-look at the charts, read the article.

Actually the subjects are discussed in some detail in both threads.

Sorry but you have it all wrong.

Let’s go with runner pat’s cite’s decent enough of a way to visualize it - the different fuel tanks. Their mission is to provide the energy to meet various demands and to keep blood sugar within a fairly narrow range during the day during both routine demands, periods of extremely high demand as muscles pull lots of sugar out of the blood stream, and all points in between.

The glycogen tank has a pretty big spigot that can be dialed up high to supply energy directly (in the case of muscle glycogen) and to pour glucose into the bloodstream (from liver glycogen) when needed and then filled back pretty fast too. Filling it back up is a pretty high priority as well.

The fat tank has a smaller spigot that can only be dialed up a very little bit. But it is a very big tank.

Lower levels of exercise dial up the fat spigot all the way pretty fast … and that is enough to supply a fair portion of low level exercise needs. Higher exercise intensity requires more energy per minute than fat can pour out through that spigot, so glycogen is called upon and converted into glucose, providing energy and keeping blood sugar stable. It can send out a lot in a short time but only for so long … then it’s pretty much tapped out.

Exercise ends. Muscles and liver start gradually replenishing their glycogen stores pulling glucose out of the bloodstream. Carbs eaten are used for that. Muscles recover, repair, and build. Dietary proteins are used for that so long as there is enough around when they are needed. Fat keeps getting mobilized to provide for other energy needs keeping its spigot open rather than rebuilding itself - so long as there is a total calorie deficit.

Of course reality is a bit more complicated. The body responds to fat loss by altering appetite and various metabolic measures, and exercise can have psychological effects that result in some feeling like they earned the right to splurge on emotional eating and conversely physiological effects on brain centers allowing the body to accept a lower set point. Regular exercise can increase, slightly, how wide that fat spigot can open. Some exercise results in more calories needed for repair and rebuild after the fact. Glycogen stores can be driven down to lower baselines or tanked up extra high. So on.

But of all the simple ways to understand it “fat burning zones” is the most wrong and the visualization that if a total calorie deficit over a full day is produced by exercise (with intake unchanged), it will be fueled by fat to similar degrees whether it is burned during the event or after to fuel the total calorie deficit of the day, is much closer to reality.