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There's a study based on data from the Minnesota Starvation experiment that says that the maximum rate of fat loss is about 31.4 calories of fat per body fat per day. So, If you are a 200lb man with 15% body fat, you have 30lbs of body fat and can afford a 900 calorie deficit per day which your fat stores can provide. But I don't understand how this fits in with low intensity exercise, which primarily uses energy from fat. For example, suppose that same person walks for 5 hours (suppose he's at an amusement park, mall, etc) and he burns 1200 calories, with 60% of it coming from fat. He burned 720 calories of fat, but that's not even considering a caloric deficit. What if he was at 10% bodyfat so he could burn at most 600 calories of fat per day?

Some problems with the study: -I read some articles that analyzed the study, and the subjects performed "moderate exercise" which doesn't tell me whether or not they did a weightlifting program to preserve muscle mass and urge more calories to be burned from fat.

-The subjects were fed food similar to what soldiers would eat in WWII, so bread, potatoes, other starchy foods that were low in protein. This would lead to muscle wasting, and potentially less fat burned.

Heres the link: http://www.ncbi.nlm.nih.gov/pubmed/15615615

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My general idea was that the more you walk and the more exhausted you get from it, the less you're actually using fat for energy. Maybe that's because you're nearing the limits of fat utilisation. At the very least, it should be possible to push that limit by walking more often (and longer). I'm just guessing though, looking forward to answers from other people. –  LarissaGodzilla May 6 at 16:55
    
I do know this much: don't consider the body to fit the second law of thermodynamics very well. The problem is that the body has a bunch of interconnected systems, so simple math only gets you in the ballpark to a certain degree. The confounding factors you sited can affect the rate of fat loss as well. –  Berin Loritsch May 6 at 18:35
    
The full study will have the details of the "moderate exercise" protocol. It will most likely some sort of cardio based exercise like a treadmill. By the reading of the excerpt, it appears that once you exceed the maximum fat transfer rate the body starts consuming fat free mass to compensate. I can't say for sure without paying $40 for the full text. –  Berin Loritsch May 6 at 18:40

1 Answer 1

As with all research, the aim is to isolate variables, which can then be analyzed and compared to other parameters, without the effect of extraneous variables. This study investigated energy contribution from fat while maintaining exercise intensity at a fixed level. So actually, the type of exercise and intensity, are unimportant.

What this research shows is that maximum speed at which fat can supply the body with energy in times of famine (hypophagia). Every process has its upper speed limit, and that speed limit was the aim of this study.

The problem with this study, and with all of science, is that the results need not apply to other situations, with altered variables. If the participants would instead have engaged in intense exercise, or have been sedentary, then the speed limit of fat energy supply could be different. So a discussion of the type of exercise is not needed, but further studies have to be done with altered variables. What is interesting, however, is what you mentioned about body fat percentage. They did not discuss the effect that fat percentage has on the fat energy supply. They only kind of titrated the caloric deficit to test the average speed of fat utilization in a population. To follow up on this, they should do the same thing, but for people of different fat percentages. Two possibilities arise: 1) either the maximum speed of fat breakdown is regulated extrinsically: through enzymes. Then this would be less dependent on body fat percentage. Or 2) it is regulated intrinsically: determined by the volume of fat. Or more specifically, the surface area of fat that is in contact with blood vessels. If this is the case, then body fat percentage would have a direct correlation to the maximal speed limit of energy utilization of fat.

Also, to answer your question about weight lifting and muscle sparing. You cannot measure the energy contribution by fat directly. You can only measure it indirectly. I'm assuming they quantitatively measured fat metabolites in the blood of the participants, and by that calculated the energy supply. What they say in the study is that if you eat slightly less, then you will have a caloric deficit that will be covered through you fat stores. If you eat much less, then your fat stores can't cover the energy demands, and your body starts breaking down muscle tissue. Im assuming they did not do weight lifting during the study, as that would have added an unneccesary variable (since it would have antagonized the muscle wasting in greater hypophagia); first you have to examine how the body functions in isolation, it is only later that you can alter variables and check how they affect results for for example athletes performing weight lifting, or sedentary people eating junk food.

I hope I cleared your confusion at least slightly; I'm sorry if I didn't answer exactly what you were wondering about, but from what I understand, you wanted an explanation of the design and results of the study. If there is something else or if something wasn't adequately explained, then comment and I will try a different approach.

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