Nutrient absorption
The passage of nutrients from digestion into the bloodstream across the gut wall, the stage that determines how much of what was eaten is actually usable.
Absorption is the stage that follows digestion: once broken down into assimilable units, nutrients cross the wall of the small intestine to reach the blood or lymph. It is this, and not the composition shown for a food, that determines what the body actually has available.
It takes place mainly in the small intestine, whose surface is multiplied by the villi and microvilli. Each family of nutrients takes its own route: simple sugars and amino acids pass directly into the blood, whereas most fats first travel through the lymphatic system.
The absorption rate varies enormously with the nutrient. Carbohydrate and protein are absorbed at over 90%, whereas non-haem iron from plants is often capped at around 5 to 10%, and calcium at around 30%. Comparing stated contents without allowing for these gaps leads to wrong conclusions.
Several factors modulate the result: food combinations (vitamin C improves absorption of plant-based iron, tea and coffee reduce it), the presence of fat for fat-soluble vitamins, the cooking method, the state of the gut barrier and the balance of the microbiome. This is the notion of bioavailability, which is more useful than that of content.