exercise, heart rate, carbs, fat

Factual Questions
1

This could probably be two or three seperate questions, but they’re all inter-related, so I’ll throw them all here.

First, I’m trying to get a better understanding of what goes on in the body in regards to nutrition and exercise. Here’s my understanding:

Protein is the amino acids you’re made of. It’s used to build or rebuild muscle and organs.

Carbs are ingested, some slow, some fast (fast=bad, slow=good), and converted into glucose, which is used by cells for immediate energy. glucose that’s not used by cells is converted into glycogen, which is stored in the cells and liver. When these stores (in the cells and liver) are saturated, the remainder is turned into fat.

When needed, glycogen can quickly be converted back to glucose and used for quick energy. At the same time, fat is also somehow made available as an energy source (although I don’t believe it’s converted into glucose).

Fat eaten is stored as fat (and/or burned for energy as above).

a) Does fat become glucose? I assume it doesn’t. Is there anything noteworthy about the way that fat is used for energy? I.e. is it really a glucose alternative, or are there substantial differences?
b) I understand some organs (notably the brain and something else I can’t recall) can’t survive on fat, but require glucose. How does the brain survive these Adkins things? Is there another way it gets energy?
c) I’m presuming that after all the glycogen is gone (and assuming there’s not a fresh supply of glucose from food), that fat will be all that is around to burn. True? how much energy is stored in glycogen in a typical body? (in calories preferably)

Next, I understand that there’s an ideal range of heart rate when exercising (so-called “fat-burning range” and “cardio range”). Typically, I’ve done cardio exercise far exceeding this desired heart rate, with the intent of burning as many calories as possible. But I’m told that a slower heart rate is better, as it forces the burning of more fat and less glucose.

d) Is my assumption above correct, that the advantage of exercising at a lower heart rate is to burn less glucose and more fat? If not, what is the advantage?
e) What’s the advantage of burning fat anyway? If I burn say 700 calories, isn’t that just 700 calories, which will be deducted from whatever calories I eat? I.e. won’t the end result as far as fat loss be related only to calories burned, not “calories burned proerly”?

Thanks.

2

Once fat is available for use, it’s pretty much the same. The difference is that fat can’t be burned until it’s taken out of the fat cells, and that requires oxygen.

Protein is converted to glucose by a process called gluconeogenesis.

Protein (from muscles, organs, etc.) is an available energy source. That’s why starvation kills you.

Not sure about the figures, but you’d be hard-pressed to go through all your glycogen.

The only advantage to exercising more slowly is that you can keep it up longer. Heart rate is overrated as a measure of how much fat you’re burning.

If you only burn the calories you eat, you’ll never lose weight. You have to burn more than that, and those have to come from somewhere.

3

In terms of weight loss, calories burned equals calories lost. It doesn’t matter so much which type of calories they were.

Think of your body’s energy tank as having three “sections”. The first is available carbohydrates, the second is fat stores, and the third is glycogen. Depending on how hard you’re exerting yourself, your body will extract energy from those sections in varying amounts. At low exertion, the primary energy is from available carbohydrates. At moderate exertion, the primary energy is from fat stores. And at high exertion, the primary energy is from glycogen. But your body can get energy from more than one section at the same time.

For weight loss, it doesn’t matter which section your body is using because the only thing that matters is to remove calories from your energy tank. If you use available carbohydrates, you prevent those calories from being moved to the fat stores. If you use glycogen, your body will later convert fat into glycogen to replace what was used.

The advantage of excercising at a lower level is that you can do it more comfortably which can allow you to do it longer which can allow you to use more calories.

The advantage of exercising at a higher level is that you burn the calories more quickly so you don’t have to exercise as long. In addition, your body can have a Post-Exercise-Caloric-Expenditure of up to 50% additional calories after strenuous exercise. With low and moderate exercise, when you stop the activity, you don’t lose any additional calories. But with strenuous exercise, your body can burn almost 50% more calories in the 24 hours after the exercise is complete. This is from the additional energy required to replace the glycogen and repair your body from the exercise.

Just remember that for weight loss, the important thing is the total number of calories burned. If you walk for 500 calories or run for 500 calories, you’ve lost the same amount of weight.

4

Thanks, ultrafilter, filmore. So you’re both saying what I suspected, that there is zero advantage (and disadvantages in fact) to doing exercises at a lower heart rate. (except you can exercise longer and have a lower risk of injury, both of which aren’t important to me).

So, why is there such a prevasive amount of stuff out there (fitness sites, posters at the gym, charts on the gym equipment) talking about this 65% of Max Heart Rate (or whatever the percentages are)? They make it out to be a good thing that you’re burning mostly fat at those heart rates, but filmore, you’re saying (and it makes sense to me) that this is really a disadvantage, as your body burns extra calories later with this process.

5

Short answer: 90% of questions that begin with “why” can be answered with “People are stupid.” This is one of them.

Long answer: There is a history behind it, but I don’t have time to go into it right now.

6

Is it possible that one would exercize at lower heart rates to avoid strain on an out of shape heart? (this is a concern for me, as I’m thinking of upping my exercize routine.) also to avoid overstressing the body. (Again I have very fragile knees and some back problems)

Also the thought might be that getting people to exercize at 65% max heart rate is better than them not exercizing at all.

7

I would guess the fat burning zone is so promoted is because it’s a good compromise between exertion level and amount of time required. At that level, most people can exercise for a long time. Exercise lower than that and it may take too long to burn sufficient calories and people could get bored and quit. Exercise higher than that and it may be too physically tough to keep going and people woud quit. But at a moderate level, people can accomplish the workout and burn a significant amount of calories. Unfortunately, it leads to people thinking they can only burn fat in the fat-burning zone.

8

You’re not burning mostly fat at those percentages, but the reason for the target zones for HR is not weight reduction but fitness. You must get your HR up to a certain percentage to obtain any fitness benefit.

Fats (triglyceriedes) must first be converted into the free fatty acid constituents, which is the mode in which the body can use it.

Your blood always contains some glucose. Blood glucose levels represent a balance between the rate of glucose production by the liver and the rate of glucose utilization by muscle and other tissues. Muscle lacks glucose 6-phosphatase and is therefore unable to produce glucose from glycogen. Instead the glucose 6-phosphate enters another metabolic pathway known as glycolysis, and this can proceed without the provision of an adequate oxygen supply. Glycolysis becomes very active during high-intensity exercise even though there is an adequate oxygen supply to the muscles.

For fat to be used as an energy supply, the fatty acid components must first be freed from the glycerol molecule. This is achieved by hormone-sensitive lipase. Physical training increases the sensitivity of hormone-sensitive lipase. An important effect of training is to increase the amount of muscle triglyceride used for energy while sparing muscle glycogen.

The higher the intensity of exercise, the more rapid the rate of glycogen utilized. Since ffa (free fatty acids) has a slow rate of transport into the cell, as the intensity of the exercise increases, the more that glucose is used. At exercise intensities greater than 95% VO2 max, only carbohydrate is burned. However, as the duration of the exercise increases, the more the fat is used. This is due primarily to the slowing of the rate of muscle glycogen breakdown.

The major effect of athletic training (fitness) is that during exercise of any intensity or duration, more energy comes from fat oxidation rather than from carbohydrate oxidation. This allows the trained person to run farther before becoming exhausted due to carbohydrate depletion.
[source: Lore of Running, Tim Noakes, MD, 1991]