Intestinal Absorption Physiology

The key to our metabolism !

Digestion is the set of metabolic mechanical and biochemical processes which transform nutrients into substances that can be assimilated by our bodies.

Once chewed and swallowed, food reaches our stomachs where its nutrients undergo modifications which allow our bodies to further absorb them.

From there, digestion continues in our small intestine where digestive enzymes turn glucids into glucose, lipids into fatty acids and monoglycerids, and proteins into amino acids. These nutrients are absorbed through the intestinal wall and passed on to the bloodstream.
Nonetheless, contrary to what is commonly believed, macro-nutrients absorption does not occur throughout the whole length (about 20 feet) of the small intestine. The fact is that 80% of the carbohydrates and lipids and 50% of the proteins consumed are absorbed in the first 28 inches of the small intestine.

People believe that all of the carbs, lipids and proteins consumed are absorbed as soon as they are transformed. This is why most nutritionists and other dietitians give their patients the impression that all of the calories they consume are available as soon as they have been digested. This is far from true.

Carbohydrate absorption

Carbs are digested thanks to the salivary and pancreatic alpha-amylase digestive enzymes. Sugar hydrolyzation —transformation into absorbable glucose— depends directly on a carb’s Glycemic Index.

Glycemic Indexes measure carbohydrates’ potential to raise blood sugar levels, that is, to generate glycemia. They also measure the corresponding carbs hydrolyzing potential as well its chances of being transformed into absorbable glucose.

In other words, Glycemic Indexes measure the portion of the carbohydrate transformed into glucose which will be absorbed and will thus pass into our bloodstream.

If the GI for glucose is ranked at 100, what this means is that it will be totally (100%) absorbed in the small intestine. Comparatively, white bread has a 70 GI which means that 70% of its pure carb content (starch) will be hydrolyzed and pass the intestinal wall as glucose.

Likewise, we can assume that for lentils, which have a 30 GI, only 30% of lentil starch will be absorbed as glucose. 
Thus, equal amounts of calories in different carbs will not necessarily pass the intestinal wall in the same proportion, the proportions that pass the intestinal wall as glucose might be half or twice as much, it depends on the carb’s GI.
This why we can say a carb’s GI measures the bioavailability of its glucidic content.

For further information on Glycemic Indexes

This phenomenon is currently expressed by traditional nutritionists in terms of "calories."


"Calories" in our plates for 100g of pure carb

Glycemic Index

Calories available as glucose in our organism after absorption

Glucose Syrup

400 Kcal


400 Kcal


400 Kcal


380 Kcal

White bread

400 Kcal


280 Kcal


400 Kcal


120 Kcal

As we can see, when we eat fries the number of calories available in our bodies after digestion will be three times higher than the number of calories available if we eat lentils, even if the proportion of pure carb is exactly the same. To word it from another angle, if we eat equal glucidic amounts of fries and lentils, we will be ‘consuming’ three times less calories when we eat lentils.

It is interesting to note that tests have shown that eating sugar at the end of a meal will have little, or practically no, incidence on the meal’s glycemic outcome. Considering the complexity of a meal, particularly the degree of fibers and proteins consumed, sugar (70 GI) absorption will be considerably reduced.  
Another thing is when we eat sugar on an empty stomach, like in soda pop or coke. In this case, the carb is almost totally absorbed by our intestine.

This is an important point!

It is one of the basic principles behind the Montignac Method. It helps us to understand how to eat the same amount of food and still lose weight simply by eating it differently.

This point is important because it makes us aware of the fact that, contrary to what is preached by traditional diets, not all of the calories we eat are available in our organisms immediately after we consume them.

A good number of nutritionists have begun using the Glycemic Index concept. However some have, unfortunately, still not understood that GIs just measure glycemia peaks. For them, the idea behind low GIs (as in lentils) is to avoid glycemia from rising by prolonging the time required to absorb glucose. This is their idea of slow absorption sugars, a totally mistaken notion as has been shown by researchers such as Professor G.Slama.

For further information on the mistaken notion of “slow/fast sugars”


As we have tried to explain by using Jenkin’s demonstration, GIs measure the glycemia triangle area which is provoked by eating glucidic foods (carbohydrates) and corresponds to the amount of glucose that crossed the intestinal wall. Naturally, the lower the GI, the smaller the amount of glucose freed when the carb is digested through the intestinal wall.

To conclude we can say that a carb’s GI (apart from glycemia) measure a carb’s absorption rate, that is to say its bioavailability and, accordingly, increased blood sugar levels simply signal the proportion of the carb which has been absorbed after having been turned into glucose.

Lipid (fats) absorption

Lipids or fats are the traditional nutritionists « boogey man ». The repulsion they feel for fats is tied to the fact that fats are rich in calories: 9 Kcalories per gram.

We will see that contrary to preconceived ideas, not all of the fats we eat are necessarily available for our bodies. Several factors modulate fatty acid absorption.

The nature of fatty acids

  • Saturated fats (butter, beef fat, lamb, pork, palm oil…) as well as trans fats (hydrogenated margarine …) have a greater tendency to be stored than burned, that is to be used as an immediate source of energy.
  • Monounsaturated fatty acids (olive oil, duck and goose fat) will normally be used after being absorbed. More so considering that they contribute to lowering blood sugar levels which reduces insulin secretion and limits fat storage.
  • Polyunsaturated fatty acids, and mainly « omega 3 » (fish fat, rapeseed oil, linum…) are systematically  used, (chiefly by increased thermogenesis) after having been absorbed (circulating fatty acids.) They furthermore stimulate lipolysis which contributes to eliminating stored fat and thus to losing weight.

As a result, the same amount of calories does not necessarily mean that the different fatty acids have similar metabolic effects. The effects can even be totally different.

Lipid absorption is conditioned by position of fatty acids on the glycerol molecule:

95 to 98% of dietary fats ingested are consumed as triglycerides. Daily diets supply an average of 100 to 150g.

Lipid absorption is conditioned by position of fatty acids on the glycerol molecule:

95 to 98% of dietary fats ingested are consumed as triglycerides. Daily diets supply an average of 100 to 150g.

It is important to keep in mind that triglycerides are made up of a glycerol molecule  (sugar-alcohol) to which three fatty acids are fixed in positions 1, 2 et 3 (see diagram).

A fatty acid’s degree of absorption depends on where it is situated  on the glycerol molecule. Only those fatty acids placed in position P2 are well absorbed since the digestive enzymes which act on lipids (lipases) work better on some positions than others.

All of the fatty acids ingested and counted as calories (in our plates) are not necessarily absorbed (and available by our organisms, contrary to what traditional nutritionists say). They might not be digested in the small intestine and might be partially or wholly eliminated by bowel movements.   .

  • In butter, for example, 80% of fatty acids (saturated) are in P2 and so completely absorbable. This also applies to milk fats as well as all non-fermented milk products.
  • In fermented cheese (and matured) fatty acids (even if saturated) are generally placed in P1 and P3 and therefore less absorbable.

Additionally, in cheese which is rich en calcium, as is generally the case (notably gruyere cheese), a non-absorbable soapy substance (fatty acids +calcium) is formed and it is also eliminated with bowel movements.

We can conclude that the degree of intestinal absorption of fatty acids in milk products is conditioned by their chemical environment (fermentation, calcium…). This also conditions the energy which will actually be available for our bodies as well as potential cardiovascular risk factors.

This physical-pathological mechanism is validated by epidemiological studies which show a correlation between the consumption of non-fermented milk products (milk, butter, cream) and rates of heart illnesses. Comparatively, studies show that countries with similar consumption levels of fermented milk products (cheeses) do not exhibit the same rates of cardiovascular risks.  
An interesting comparison between the Finnish and the Swiss shows that death rates due to heart failure in Switzerland are half of what they are in Finland for almost identical consumption rates per person. One of the main reasons is that the Swiss, as opposed to the Finnish people, essentially consume milk products in the form of fermented cheese.

The comparison between Finland and France is even more edifying. Although the French eat twice as many food products as the Finns, coronary death rates are 2.5 lower in France than in Finland.
Diverse factors explain this situation but one of the reasons is that the French basically eat fermented cheeses which are also matured.
Cheese maturing expands fatty acids’ position in P1 and P3 to the loss of P2, thus their low absorption rate.

Lipid absorption is  conditioned by their fiber content as well

Food fibers, basically those that are soluble, condition absorption of ingested fatty acids. This is why eating apples (pectin) and pulses can reduce cholesterol levels. It can also help to prevent weight gain by reducing the number of calories available with regards to the calories ingested.

Protein absorption

Protein absorption is also conditioned by diverse factors:

  • Their source
    Practically 100% of all animal proteins are absorbed by our intestine. They are thus available in our bodies. Vegetable proteins, apart from soy beans, have a lower degree of absorption:

- 52% for lentils
- 70% for chick peas
- 36% for wheat

  • Their  composition
    Proteins are made up of several amino acids. The absence of one or another amino acid can become a restraining factor for adequate use of the others.
    We can count the calories we consume from a nutritional angle but what we have to consider is that even if they are absorbed, calories might not be operational or they might be available but to diverse degrees.
  • Conclusion: Energetic nutrients once ingested are not, as many people believe, totally absorbed by our intestine.  Their degree of absorption depends on the food’s physical-chemical make up as well as on its dietary environment.
    These are important differences which should be kept in mind when attempting to lose weight and more so when trying to reduce cardiovascular risk factors.
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