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If I eat a very large meal, how long does my body take to convert the excess calories into fat?

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    I find that this is a great question, exactly because it happens to be wrong - we don't really "convert food into fat" the way you describe it. But it would take somebody with a better understanding than me to describe this very interesting process. – rumtscho Feb 3 '16 at 20:15
  • I can't see how an average person, who is not deeply involved in metabolism research could have any benefit from an answer to this question. I can research and come up with an answer, for example: it takes 8 hours to turn food into fat. How can this info help you? – Jan Jan 3 '19 at 11:47
  • @Jan Well I record exactly what food I eat & weigh myself daily. I want to do a regression analysis to find out which foods affect my weight and rank them accordingly. So it's important to know how long to associate a given food with delta weight. – ottotts Jan 3 '19 at 15:10
  • @ottotts, I can guarantee you, that knowing the timing of metabolism will not help you answer this. The pretty much only thing you need to know is the calorie content of a certain food. – Jan Jan 3 '19 at 15:12
  • I would caution you about explaining why you asked this question. Personal medical advice is strictly off topic here, so making the question about you might lead to closure. The question has a lot of upvotes because it's a good question. I, for one, would like to see a good answer. – Carey Gregory Jan 3 '19 at 22:04
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There would be a lot of factors involved in this question; The rate of digestion, the rate absorption of GI tract, the rate of transfer from splanchnic circulation to the liver and other organs and the rate of metabolism of an individual in including different rates of different enzymes. But since OP is asking for "calorie to fat" we will limit it from glucose(since calorie is already a unit of energy)to fatty acids and Triglycerides.

During well-fed state or post-absorptive state, the body starts to either convert glucose into ATP/energy via glycolysis and citric acid cycle, into glycogen in liver or muscle or into fatty acids in the liver, kidney, adipose tissue etc.-excerpt from (NCBI)Food Intake and Starvation Induce Metabolic Changes


Let's start with the steps of conversion from glucose to fatty acids

metabolism

The first step is Glycolysis - red circle. The second step is the Citric acid cycle or Kreb's cycle - green circle. The third step is Lipogenesis - blue circle.

Glucose is converted to pyruvate in the cell cytosol1. Pyruvate is converted to several substrates including citrate which is essential in lipogenesis in the cell mitochondria2. Lipogenesis is the process by which acetyl-CoA is converted to triglycerides, lipogenesis encompasses both the process of fatty acid synthesis and triglyceride synthesis, where fatty acids are esterified to glycerol3.


Rates of conversion

...ratios and rates varies widely depending on the nutritional status...

-Citrate and the conversion of carbohydrate into fat. A comparison of citrate and acetate incorporation into fatty acids

 

...high fat diet abolishes lipogenesis... The rate of lipogenesis from available carbohydrates seems to be regulated not only by the carbohydrate content of the diet; glucose utilization increases as the carbohydrate content increases or the fat content decreases.

-DIETARY EFFECTS ON LIPOGENESIS IN ADIPOSE TISSUE

 

When the glycogen stores are saturated, massive intakes of carbohydrate are disposed of by high carbohydrate-oxidation rates and substantial de novo lipid synthesis (150 g lipid/day using approximately 475 g carbs/day) without postabsorptive hyperglycemia.

Glycogen storage capacity in man is approximately 15 g/kg body weight and can accommodate a gain of approximately 500 g before net lipid synthesis contributes to increasing body fat mass.

-Glycogen storage capacity and de novo lipogenesis during massive carbohydrate overfeeding in man


Summary

It depends on several variables, nutrition, rate of metabolism of an individual, lifestyle and activity etc, but mainly the short term storage' glycogen stores' saturation. So as long as the glycogen stores are saturated, the body will start lipogenesis.

150 grams of fat per day from 475 grams of glucose/carbs

or 3.17 grams of glucose/carbs to produce 1 gram of fat

*when glycogen stores are saturated

more details on -Glycogen storage capacity and de novo lipogenesis during massive carbohydrate overfeeding in man


P.S.

*Some biochemistry textbooks say that 1 molecule of glucose yields between 36-38 ATPs. However, the amount of energy as ATP revolves around these numbers. According to Guyton, 1 ATP has ~12,000 calories (12 kcals). Thus 38 ATPs would have 456,000 calories or 456 kcals.

*de novo synthesis, meaning "new", from glucose to fat.

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  • Thank you for the infos. I really wanted to know how long it takes for the input (food) to be fully converted to the output (weight gain/loss). I can't quite extract that from your post. Do you have anything on that? – ottotts Jan 18 '19 at 10:21
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    This answer actually does not answers the question, but shows that the question, as general as it is, is unanswerable. There are too many factors (calories consumed/burnt, what does the person eat, level of physical activity, status of the glycogen stores, etc.), which are mentioned in the answer, and from which you can only conclude: it depends. Also, it is not clear what is the starting point to measure the time: when someone puts the food in the mouth or when the nutrients are absorbed into the blood. – Jan Jan 18 '19 at 13:07
  • @Jan yes, it really depends. It is answerable with a number, but that range of number would be very wide and would be very difficult to measure in the first place. Transit of food and absorption is easy to measure but, we will need to measure individual enzymes involved e.g. i.imgur.com/GHVbCuA.jpg which is quite complex. During my research, I can't seem to find an article regarding the exact rate of conversion and only managed to find the closest average which I already have posted. I will revisit the matter as soon as more research material is available. – kit Jan 18 '19 at 15:02
  • For this concept to work (to get at least a range of numbers), you would need to have a very specific scenario: knowing your level of hydration, exact salt intake in last week, exact nutrient composition of every single food you eat, etc. – Jan Jan 18 '19 at 15:11
  • @Jan Not really. All these peripheral factors will balance out over time (many months) and leave the contribution of each kind of food as the main causal factor. – ottotts Jan 18 '19 at 19:18

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