How do we know how much food nutrients are actually absorbed by the human body?

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I know there are chemical processes to detect the quantity of nutrients in food but how do we know how much of these nutrients are actually absorbed by the human body? How much variance is there between different people?

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Food often has information on the packaging on how many calories/joules it contains. Calorie is the physical unit to express the chemical energy of a thing and is measured by fully burning a sample of said thing and then measuring the rise in temperature of the container this experiment was conducted in. Using this method, the calories of the substance per weight unit are calculated.

However, how much of that chemical energy is absorbed through digestion is another story. I imagine there could be done some measurements by monitoring the blood sugar after food intake.

We had people fast, then did bloodwork to see how many nutrients are in their body when they haven’t eaten for a long time.

Then we fed them the stuff we wanted to test out. We took more bloodwork several times over the day to see how the nutrients in their bloodstream changed with respect to the time since they had eaten.

We did that for hundreds and thousands of people and built up a pretty good model.

One possibility is to measure the nutrients that remain in fecal matter or urine, then compare that to the original nutrient levels in the food. [This video](https://www.youtube.com/watch?v=9wZ0wTqJIxY) by Applied Science is a good example of this.

Many nutrients are actively transported through the intestinal wall (the lumen) because as molecules they’re too large to readily diffuse. As a result, absorption of these nutrients is subject to regulation by the body.

Small molecules like sugars diffuse, so the blood concentration of them after you eat will depend on their concentration in the food mass.