Absorption errors: how to improve nutrient absorption in the intestine
Some ingredients in foods can hinder the absorption of nutrients, while others improve their bioavailability. Many people eat a healthy diet without thinking about whether the nutrients contained in the food can be absorbed by the body at all. However, there are certain substances that can hinder or even block the absorption of important nutrients. The result is that a nutrient deficiency occurs despite a supposedly healthy diet. Today’s article explains why oatmeal should not be combined with coffee and which mistakes can be avoided.
How does the intestine absorb nutrients?
The uptake of substances (absorption, resorption) from the intestinal lumen occurs depending on the size and type of molecules or ions present in the food mush.
For example, apolar (fat-soluble = lipophilic) substances can easily dissolve in the membrane of the intestinal mucosal cells and penetrate them; small (up to ~4 nm in diameter) polar (water-soluble = hydrophilic) substances pass through the smallest “pores” of the cell membrane; larger polar molecules (>4 nm) depend for transmembrane passage on proteins that form special “channels” for the substance that only allow this substance to pass through.
The maximum capacity for substance absorption via the gastrointestinal system is usually about a power of ten higher than the daily actual absorption (functional reserve), for example: the absorption capacity for water in the intestine of an adult person is about 1 liter per hour — the daily requirement (beverages + water in “solid” food) is about 1–2 liters, with increased loss (sweating!) this number can multiply.
In addition, the intestine could reabsorb about 3000 grams of glucose daily (120 g/h, daily requirement ~300 g/d). For amino acids, the value is ~600 g (requirement ~60 g/d), for cholesterol, up to 4000 mg per day (a diet rich in meat, eggs and offal provides up to ~1000 mg/d, requirement is ~200 mg/d or less). The absorption capacity for iron is about 12 mg/d (requirement 1–2 mg/d), and absorption is multiply regulated and limited.
What does bioavailability mean?
We often stumble over the word bioavailability, but what exactly does bioavailability mean? The term indicates how quickly and how much our body can absorb of a particular food or active ingredient. Bioavailability is therefore a measure of how well food can be utilized and what added value can be offered in terms of the nutrients it contains. The higher the bioavailability of a food, the more we get out of it. Bioavailability is influenced by factors such as intestinal blood flow, intestinal motility, pH in the stomach, and enzymatic breakdown or conversion in the intestine and liver. These factors vary from person to person, so the bioavailability of a particular substance also differs from person to person.
Why bioavailability can be disturbed
There are a variety of “culprits” that can significantly affect bioavailability, resulting in little or no utilization of the nutrients contained in a food. These “culprits” can interfere with the digestion and absorption of micro- and macronutrients by
- competing for the same transport system
- altering the chemical form of the nutrients
- Bind the nutrients and render them unusable for absorption.
What substances inhibit bioavailability?
There are certain substances in foods that can significantly reduce the bioavailability of many nutrients. These substances primarily include polyphenols, phytates and oxalic acid, which are found in plant foods. So-called oxalates — also known as oxalic acid — can block the absorption of iron, zinc, magnesium and calcium. Oxalates are abundant in spinach, chard, rhubarb, almonds, sweet potatoes and sorrel, for example. Heating can reduce the amount of oxalic acid, and oxalic acid can be flushed out of almonds by soaking them in water for several hours. Vegetables low in oxalic acid would be broccoli, pick and iceberg lettuce, pak choi, white and red cabbage. Soy products, cassava, cabbage and millet reduce the bioavailability of iodine and can impair thyroid function or even worsen hypothyroidism.
Phytic acid, such as that found in whole wheat grains and legumes, worsens the bioavailability of iron from plant foods, zinc, and calcium. Iron with black tea or wine (tannins) is also an unfavorable combination. The polyphenols contained therein form so-called “complexes” with iron in the intestine, which can then no longer be absorbed.
Zinc against calcium, calcium against iron and iron against manganese?
Certain nutrients influence each other. For example, the bioavailability of zinc is reduced by calcium, iron, copper and magnesium; especially when these nutrients are taken on an empty stomach. Similarly, calcium can also reduce the bioavailability of iron from plant foods. Iron and manganese interfere with each other’s absorption because they use the same transporter in the intestinal wall. The bioavailability of beta-carotene, the precursor of vitamin A, decreases with dietary fiber, and that of vitamin D decreases with iron deficiency. Iron absorption improves with the simultaneous presence of vitamin C, citric acid, and protein.
Many medications also affect nutrient bioavailability, such as proton pump inhibitors (stomach-protecting tablets) that are quickly popped after a night of drinking. Because proton pump inhibitors inhibit the production of stomach acid, iron absorption is worsened, since an “acid stomach” is important for iron absorption. The same is true for certain antibiotics and zinc, for inhibitors of cholesterol absorption and vitamin E, and for histamine H2 receptor blockers (including gastric protection tablets) and vitamin B12.
How can nutrient absorption and bioavailability be improved?
To fully realize the nutrient potential inherent in a healthy and varied diet, a few basic things should be considered when selecting, preparing and combining foods to improve bioavailability and nutrient absorption. To boost the absorption of iron, zinc and calcium, foods rich in iron, zinc and calcium, such as oatmeal, salads, seeds and nuts, should be combined with vitamin C. Vitamin C, found for example in (freshly squeezed) fruit juices, citrus fruits, fruits and herbs (parsley), promotes the absorption of the nutrients just mentioned.
The fat-soluble vitamins E, D, K and A always require fat to be better absorbed by the intestines. Vitamin E is present in foods that are high in fat to begin with, such as nuts and avocados, so deficiencies are rare. Vitamin K, on the other hand, is present in spinach, chives, sauerkraut, flower, rose, red and green cabbage, and meat and grain products. A low-fat or even fat-free diet makes it difficult for the intestines to absorb vitamin K and A.
Text Sources:
(3) https://www.frauenarzt-frauenaerztin.at/userfiles/file/fachwissen_lifestyle.pdf
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