In general, sugars are absorbed quicker than proteins and fats because their digestion is simpler.
Sugar absorption through the mouth mucosa is negligible (Clinical Science and Molecular Medicine).
The time from the ingestion to absorption of macronutruients into the blood depends mainly on 2 factors:
- Gastric emptying
- Digestion and absorption in the small intestine
SUGARS
Not all sugars are absorbed at the same rate.
Glucose and galactose do not need to be digested and can be quickly absorbed in the small intestine via sodium–glucose linked transporters (SGLTs) - sodium acts as a cofactor that stimulates glucose and galactose absorption (Lumen Learning).
Fructose also does not need to be digested but is absorbed much slower than glucose via GLUT5 transporters without the help of sodium (Lumen Learning).
Sucrose needs to be digested in the small intestine with the help of the enzyme sucrase to glucose and fructose, which are then absorbed. Glucose stimulates the absorption of fructose (Food and Agriculture Organization). This means that glucose and fructose from sucrose will be absorbed slower than glucose alone, but faster than fructose alone.
Lactose needs to be digested to glucose and galactose, which prolongs the time in which they can be absorbed.
The absorption rate of various sugars is reflected in their glycemic index (the rise of blood glucose levels after ingestion): GI for glucose = 100, sucrose = 65, lactose = 46, fructose = 15.
STARCH
Starch is not digested in the stomach, so it can pass through it quickly, and is then, in the small intestine, quickly digested to glucose with the help of the enzyme amylase. The glucose from plain starch is absorbed almost as quickly as when ingested as glucose alone and faster than fructose, sucrose or lactose. This is evident from high glycemic index of foods made mainly of plain starch: cornflakes (81), instant oats (79), potatoes (78), rice porridge (78), white wheat bread (75).
PROTEINS
Proteins need to be digested in the stomach by the enzyim pepsin (which slows down gastric emptying) and then further in the small intestine by peptidases into amino acids, which can be absorbed. Slow gastric emptying is the main reason for slow absorption of proteins (Diabetes Care, Experimental Physiology).
FATS
Fats are not digested in the stomach but they slow down gastric emptying even more than proteins, so they are absorbed slower (Vivo Colostate, Virginia.edu).
LIQUID vs SOLID MEALS
Liquid meals pass through the stomach quicker than the solid ones, so glucose from sweetened beverages will be absorbed quicker than from whole fruits (Vivo Colostate, Virginia.edu).