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I understanding there are some complicated processes, but I'm interested in generally how fast the body converts certain chemicals into energy and the period of time this conversion occurs. I'm specifically interested in carbohydrates, protein and fat (perhaps distinguishing saturated from unsaturated).

I have the following notes where glucose is energy, do they appear correct?

  1. Carbohydrates convert to glucose over a 2 hour period after a meal. The peak glucose/blood levels are 30 - 45 minutes after a meal. Conversion starts about 15 minutes after a meal. 1 g of carbohydrate / 1 g of glucose conversion.
  2. Protein converts to glucose over 3 - 5 hours after a meal with a 70% conversion of protein into glucose. I also read that protein converts to glucose at a rate of 3 - 12 g protein / hour. 1.3 g protein / 1 g of glucose conversion.
  3. Fat converts to glucose over 4 - 6 hours after a meal. Only about 5-6% of the fat triglycerides are converted into glucose. I also read that 2.5 g of fat / 1 g of glucose conversion, which seems to differ then the 5-6% number.
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    $\begingroup$ This question seems to be based on the false assumption that proteins and fats are necessarily converted to glucose. It is possible for certain amino acids in protein to be converted to glucose by gluconeogenesis, but not others. However they may also be used for protein biosynthesis, metabolized oxidatively as different intermediates or even converted to fat. As regards fat itself, only the glycerol moiety can be converted to glucose, but whether this happens or the glycerol is metabolized differently depends on the nutritional state of the individual. $\endgroup$
    – David
    Commented Nov 19, 2022 at 16:34

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It is quite complicated, the speed of conversion of carbohydrates, protein and fat in to energy is definitely not linear and dependent on what the body is doing.

You should be close to right in carbohydrates. Since digestion starts in the mouth with the release of amylases to chop down starch and since the absorption of glucose is quite fast in the early stages of digestion in the intestines, your numbers should not be far off if you consider a non-branch form of starch. That will change depending on the type of carbohydrate, and some form of carbohydrates are even impossible for the body to digest and are either digested by bacteria in the gut or simply excreted in the feces.

Regarding protein, this is where your assumptions start to be off. The primary purpose of protein is not for energy production, but to supplement the cells with aminoacids to build other proteins. So, although it is possible to convert protein into glucose, it is not done in digestive system. Also, each aminoacid is converted into a precursor that will generate some energy eventually, not glucose. And depending on the aminoacid, the amount of energy differs, which makes it more complicated to give an estimation.

As for fat, they cannot be converted into glucose by the human body. As you mentioned, this is the slowest macromolecule to be absorbed by the body but it is also the most energy dense. Although 1 g glucose generates about 3 cal, 1 g of fat generates around 9 cal (protein is around the same as glucose).

Regarding on how fast macromolecules can be converted into actually energy, some cyclists can burn up to 120 g glucose per hour during a race.

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    $\begingroup$ The poster mentioned triglycerides in relation to fat, rather than fatty acids. The glycerol portion of triglycerides can be converted to glucose in the liver. I imagine that is where the 5–6% figure comes from. $\endgroup$
    – David
    Commented Nov 19, 2022 at 20:36
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    $\begingroup$ @David yes, the 5-6% of the triglyceride conversion to glucose figure is confusing. I read that body fat, which I understand is made of of triglycerides, is only 85% usable energy per unit weight. Much different than the former number. Any idea if these numbers are correct? $\endgroup$
    – Nick
    Commented Nov 20, 2022 at 4:01
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    $\begingroup$ @Vilut you mention that fat cannot be converted into glucose by the human body. You mention that 1 g of fat generations ~9 kcal of energy. Therefore ingestion of fat resulting in energy availability to the body. Its now my understanding that fats are converted into fatty acids and glycerol. These fatty acid chains are converted into energy via a process called beta-oxidation (electron transfer), which generates acetyl-CoA entering the citric acid cycle, yielding ATP. Is this correct? How long does the process take vs others like carbs? $\endgroup$
    – Nick
    Commented Nov 20, 2022 at 4:08
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    $\begingroup$ @Nick, yes, fatty acids are converted into Acetyl-CoA through beta-oxidation, which then enters the citric acid cycle to result in ATP that the cells can use. As for the length of the processes, I don't know. What I know is that, in athletes, glucose is used primarily in big and speedy efforts (100 m), while fatty acids are primarily used in long efforts (marathon and ultra marathon). Which means that glucose can be used to produce ATP fairly quickly relatively to fatty acids. Of course, other factors should be taken into account for more precise values. $\endgroup$
    – Vilut
    Commented Nov 20, 2022 at 21:16

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