I'm sure this has been asked before so I apologize if I missed it (used the search feature and everything).
I'm just curious about this. I understand that your body prioritizes using carbohydrates for energy, and that when your blood sugar falls your liver tells your body to start breaking down muscle for glucose. But at what point does your body begin to break down fat over muscle? Maybe there are a few key elements that need to be present for your body to burn fat, I don't really know, but I'm curious. Please explain
10-01-2010, 05:37 PM #1
When does the body choose to burn fat over protein?
10-01-2010, 07:57 PM #2
10-01-2010, 08:22 PM #3
I am not credited or any of that, but I do read a little bit about this stuff. my limited understanding;
The body simply has a "pecking order" of sources to draw energy from. Available glucose & glycogen from carbohydrates are obviously first, and following that, energy that is stored in bodyfat tissue. barring that, muscle tissue. What makes the body actively choose the fat over the muscle is some combination of available protein and I guess simple logic. Given adequate protein intake, I assume it's only natural for the body to put it to its primary use; repairing and maintaining muscle tissue. Therefore, it'd be counterproductive to interfere and take from the muscle what it is already using.
And because fat is not metabolically active, it is there as preservational/protectional tissue to be metabolized in the event of an energy deficiency. When you cut weight properly, the body draws from these sources the majority of the time.if I was a dinosaur I would be a Flexasaurus.
10-01-2010, 08:30 PM #4
It's more complicated than this...... you only burn glucose/glycogen if you NEED to ..... IE you're near VO2 max and totally doing serious cardio.
Then if you have low/steady requirements for energy you'll burn fat.
Then if you can't burn enough calories from your current fat stores, you'll break down proteins and burn the amino acids.
I'm not an expert but this is what I've been able to gather.
10-01-2010, 09:59 PM #5
You can measure if you are able to access VO2 max or metalyser equipment, which analyses your RER (respiratory exchange ratio) AT rest your RER will usually be approx 0.75: 1 that being the carbon dioxide to oxygen ration. carbon dioxide output will begin increasing with increasing intensities meaning more carbohydrate is being broken down for energy. Higher intensities= more lactate accumulation (lactate is a by product of glucose breakdown. you can have lactate from fat burning.) To clear lactate your body begins exhaling CO2 and O2 also aids in clearing.
The ideal fatburning RER is around 0.85 as fat is the primary fuel source because your body can comfortably get enough O2 for fat metabolism. At an RER of 1 your body will be burning purely carbs. If you are carb depleted then your body will turn to protein as it is easier to breakdown with less O2 than fat.
Hence why bodybuilders only do low intensity steady state cardio as is utilises fat for energy with minor muscle breakdown.
10-01-2010, 10:18 PM #6
My understanding is that there is no choice to burn fat over muscle. After ingested and stored energy supplies are exhausted the body releases catabolic hormones into the bloodstream. These hormones are not tissue specific. They grab whatever they can get. This is why there is always some degree of muscle loss when on a cut and why it is important to get as much protein and do as much resistance training as you can while cutting. You aren't preventing muscle loss so much as you are rebuilding muscle that has already been lost by stimulating new growth, anabolism, during the times of day that you aren't in catabolism.
This also explains why the lower your body fat percentage gets the harder the harder it gets to lose fat. There is simply less of it available so the catabolic hormones catabolise more muscle than they do when you have a higher body fat percentage.04/2010 - 295 Fattest
11/11/11 - 171.8
10-01-2010, 11:01 PM #7
So the fuel the body chooses for consumption is Fat > Glucose/Glycogen > Protein and these are related to the intensity of the workout and the time when your body runs out of glucose/glycogen.
Are there other thoughts or things to know about this process?
10-01-2010, 11:41 PM #8
It's real hard to tell exactly when does the body begin to use fat as energy, but for that you have to follow a diet low in carbohydrates, in order for the body to have low levels of insulin and high levels of glucagon. Conventional traditional diets high in carbohydrates didn't help people lose weight in the 90s, that's why low carb diets are so in fashion today. Thanx
10-02-2010, 07:07 PM #9
If you wanna lose fat 40-60 minutes fast walking first thing in the morning before breakfast. There's a study thats shown within 6 weeks of empty stomach low intensity carido your body begins to switch to fat metabolism. Even if you are carb depleted your body will choose fat over carbs as is learns to preserve carbohydrate for when it's needed. Which is why endurance athletes train their body for fat metabolism and glycogen sparing in order to offset early fatigue. Trust early morning cardio with caffeine will switch you body to fat metabolism quickly. Email me if anything else. firstname.lastname@example.org
10-02-2010, 07:11 PM #10
10-02-2010, 09:57 PM #11
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02-02-2012, 03:36 AM #12
Now the question, I believe, is "what factors cause/control the rate of fat metabolism?". Well, fat metabolism is regulated by lipolysis. Triglycerides that are stored in the body are broken down into FFA's and glycerol by lipases. Lipases are usually inactive until they're stimulated by certain hormones (but that's outside the scope of my response). But during low intensity exercise, those hormones begin to build up, thus creating a better atmosphere for lipolysis. Lipolysis then increases the amounts of FFA's in the blood and the muscles, and that promotes fat metabolism. IN GENERAL, lipolysis is slow and fat metabolism occurs only after several minutes of exercise.
Now, the main factor that determines the role of fat as a substrate during exercise is its availability to the muscle. To even be available, triglycerides (fat) must be broken down to FFA's and glycerol (again). And when triglycerides are split, FFA's can be converted into acetyl-CoA.....which allows it to enter a bioenergetic cycle called the Krebs Cycle. (And off on a completely different tangent, it has to be aerobic exercise, meaning oxygen is present to facilitate the electron transport chain, etc. If you're exercising anaerobically, then there is no oxygen present and the Krebs cycle is rendered useless since you can't move on to the electron transport chain to produce ATP).
Believe it or not, the times you use the most plasma FFA (adipocytes, "fat") is when you are exercising at about 25% of your VO2 max. When you exercise at that level, about 75% of your energy is coming from plasma FFA's (fat).
There is no pecking order really. It just depends on the intensity of the exercise. When you're working out at a high intensity, your body is using a lot more "fast twitch fibers", and these fast twitch fibers contain a bunch of enzymes that promote the breakdown of carbohydrates/sugars, so that's when you're going to use more carbs.
Like I said, it's all really dependent on the availability of the fuel source. Once you transport the "fat", your body loves to use it because it's sustainable energy.
Hope this helped a little.
02-02-2012, 08:47 AM #13
02-02-2012, 10:14 AM #14
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Here is a nice little summary: Excess meaning excess above and beyond maintenance calories.
Excess dietary fat is directly stored as fat
Excess dietary carbs increases carb oxidation, impairing fat oxidation; more of your daily fat intake is stored as fat
Excess dietary protein increases protein oxidation, impairing fat oxidation; more of your daily fat intake is stored as fat
02-02-2012, 11:43 AM #15
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