HOW WELL DO WE ABSORB NUTRIENTS?
By Beatrice Trum Hunter
Consumers' Research, April 1994
Nutrients from food are not absorbed entirely. Actually, there is a wide range of absorption, determined by many factors. The amount absorbed by depend on the form of the nutrient or other nutrients present that either increase or decrease its absorption. Or nutrient absorption may depend on factors such as efficient functioning of the gastrointestinal system, lifestyle, medications, supplements, health problems, or age. The extent of nutrient absorption may differ if food is consumed raw or cooked, or if the nutrient is derived from food or from a nutritional supplement.
The official recommended intake of nutrients acknowledges the fact that a nutrient may not be released readily from food during digestion, or may not be absorbed efficiently.. The bioavailability of vitamins and minerals varies greatly from food to food. To establish official recommendations for intake from the limited information available about most nutrients, an estimate is made and then an average value is established for the bioavailability of a nutrient from the major food sources in the diet. To compensate for low bioavailability, the recommended intake is increased.
Differences in estimating bioavailability result in a lack of uniformity in official recommended intakes in different countries. Human requirements do not differ from place to place, but food availability and food habits differ. An example is dietary iron. In many Western countries, much of the iron intake is in the form of heme iron (for example, from meat) that is well absorbed. Heme iron absorption is estimated to be about 20% to 25%. In countries where most of the iron is from plant foods (non-heme iron), iron absorption may be as low as 2% to 8%. Because of this disparity, there may be a two-to-three fold difference in the recommended allowances for iron in different countries. For example, in Canada, where heme iron intake is high, 32 mg of iron daily are recommended for women. In India, where non-heme iron intake is high, 32 mg of iron daily are recommended for women.
Currently, there is much interest in carotene. Even with normal diets, its absorption from foods is limited. The carotene in green leafy vegetables is far better absorbed than from red or yellow vegetables. Yet even with good absorption more than half is excreted, unabsorbed.
Any condition that impairs fat absorption also impairs absorption of fat soluble vitamins, such as vitamin A and carotene. Adequate bile levels may be necessary for carotene absorption
Interactions Affect Absorption
Some Nutrients Increase Absorption. Non-heme iron from plant foods consists mainly of iron salts. Its absorption is influenced by its solubility in the upper part of the small intestine, which in turn depends on how the composition of the entire meal affects iron solubility. For example, non-heme iron absorption from a meal containing animal protein foods, such as meat, fish, or poultry, is about four times greater than if the major protein source is from equivalent portions of milk, cheese , or eggs. Iron absorption tends to be poor from meals in which whole grain cereals and legumes (beans) predominate. However, the addition of even relatively small amounts of a heme iron source of food substantially increases iron absorption from the total meal.
Another nutrient that increases iron absorption is vitamin C (ascorbic acid), which keeps the mineral soluble and available for absorption in the duodenum where the pH is normally alkaline.
For example, orange juice consumed with other foods increases non-heme iron absorption.
Neither human breast milk nor cow's milk is a good source of iron. Yet the low level of iron present in breast milk is relatively well absorbed by the infant, compared with iron absorption from an unfortified formula based on cow's milk. This relatively good absorption, even when the level is low, may explain why the breast-fed infant is less vulnerable to iron deficiency than an infant fed unfortified formula. Additional beneficial nutrient relationships follow:
* Magnesium helps to convert thiamine to its biologically active form.
* Phosphorus intake strongly enhances the metabolism of vitamin B
* Copper absorption averages only about 30% of the intake. Absorption is increased by acids. Copper is absorbed in the stomach and the duodenum.
* Selenium and vitamin E enhance each other. Their respective mechanisms of action are closely related. A deficiency of one may be relieved by the other.
* Absorption of vitamin A is improved when emulsified. Vitamin A is well absorbed in milk because the fat is emulsified in the liquid.
* Vitamin D promotes calcium absorption and facilitates magnesium absorption in the intestine.
* Calcium absorption requires the presence of bile salts, bile, and adequate but not excessive dietary fat. Calcium must be soluble to be absorbed. Acids, such as the stomach's hydrochloric acid, as well as ascorbic and citric acids, and some amino acids, such as glycine and lysine, can increase calcium's solubility and thus increase its absorption. Lactose, the milk sugar, also helps the absorption of calcium from milk.
Some Nutrients Decrease Absorption. Some nutrients, especially if taken in excess, can decrease the absorption of other nutrients. For example, excessive calcium decreases the absorption of iron, phosphorous, magnesium, zinc, and manganese. Excessive calcium may also interfere with vitamin K synthesis and/or its absorption.
* High intake of phosphorous-rich foods, such as meats, grains, potatoes, and soft drinks or other manufactured foods to which phosphorous compounds are added, decrease calcium absorption.
* Inorganic iron is antagonistic to vitamin E. The inorganic iron combines with vitamin E in the gut and renders the vitamin inactive. This form of iron is found in fortified cereals, enriched flours, and in mineral supplements.
* Zinc absorption can be decreased by excessive calcium, copper, folic acid, iron, and phytates.
* Copper absorption can be decreased by excessive zinc or calcium. Copper availability is inhibited by molybdenum in combination with sulfate, which blocks absorption and/or increases its excretion.
* Excessive dietary fiber, decreases the absorption of calcium and zinc.
* Oxalates and phytates, naturally occurring compounds present in many plant foods, decrease the absorption of minerals, such as calcium and zinc.
* Iron absorption is decreased by soy, coffee, or tea.
* Excessive sugar consumption decreases calcium absorption and increases the urinary loss of chromium.
* Excessive fat consumption decreases calcium and magnesium.
* A high intake of vitamin E may reduce the intestinal absorption of vitamin K, and decrease the effectiveness of this vitamin in its role as a blood coagulant.
A Special Case. Vitamin D has a unique method for absorption. By exposing the human skin to the sun, this vitamin can be absorbed, provided ample time is allowed for the fat on the skin to help absorb the vitamin. Swimming or showering directly after exposure to sunshine, for example, interferes with vitamin D's absorption from sun exposure.
In the animal kingdom, birds obtain vitamin D by preening themselves with oil from the preen glands of their beaks and spreading it over their feathers that have been exposed to the sun. Removal of the preen glands makes birds more susceptible to rickets, a vitamin D deficiency disease. Similarly, the fur of animals appears to be a place where vitamin D is formed. Rats prevented from licking their fur become deficient in vitamin D. The incessant washing of cats for cleanliness, or the practice of monkeys grooming each other to hunt for fleas. also may provide a means for obtaining enough vitamin D to meet their requirements.
We absorb vitamin D, too, from foods of animal origin, and from fortified foods. Vitamin D has been shown to be well absorbed from nutritional supplements.
Lifestyle Factors. Caffeine intake decreases absorption of calcium, magnesium, phosphorous, potassium, and sodium. Increased excretion of these minerals is in the urine.
Alcohol intake is especially damaging to the mucosal lining of the intestine and contributes to deficiencies of many nutrients. In the alcoholic, absorption, especially of thiamine and folic acid, is impaired due to decreased intestinal function. Other depleted nutrients include other B fractions, such as riboflavin, niacin, pyridoxine, and B12, as well as ascorbic acid, fatty acids, and electrolytes. Smoking reduces calcium absorption. Also, it reduces substantially the possibility of absorbing calcium from a nutritional supplement.
Undue physical and emotional stresses can reduce calcium absorption and result in unexplained dumping of calcium in the intestinal tract. In times of worry and tension, fecal calcium excretion may be twice that of the dietary intake. Ascorbic acid, too, is lost by undue stress.
Over-The-Counter Drugs. Mineral oil, used to relieve constipation, hinders the absorption of fat-soluble vitamins A, D, E , K, and beta-carotene. In tests, mineral oil, given in large doses along with vitamin A, interfered with the absorption of the vitamin. Much of the vitamin A was dissolved in the mineral oil, rendered unavailable, and excreted. Beta-carotene, too, dissolves in mineral oil and becomes unavailable for absorption. Other laxatives may cause calcium and vitamin D losses.
Antacids, from magnesium and aluminum hydroxide compounds, can impair calcium absorption. The pH of the small intestine is altered by antacids and, in turn, decreases absorption of folic acid and vitamin B12.
Prescription Drugs. Drugs can affect nutrient absorption in different ways, by absorbing the nutrient, impairing its absorption, or changing its characteristics. The nutrient may be rendered insoluble, or have its pH altered. These changes may result in nutrient malabsorption by producing maldigestion, or impair the functioning of the mucosa lining.
Health Problems. Some diseases may decrease or delay absorption of nutrients. For example, evidence of decreased absorption of vitamin A is based on its level found stored in the blood of a living patient, or measured in the liver after death.
Chronic constipation impairs absorption of vitamin A and carotene.
Ascorbic acid (vitamin C ) is absorbed from the small intestine. A number of food-borne pathogens, such as Salmonella and Escherichia coli, not only result in food poisoning, but also decompose ascorbic acid. In patients with gastrointestinal problems, such pathogens prevent ascorbic acid absorption.
Abnormal conditions in the intestinal tract, such as diarrhea, decrease arterial absorption of ascorbic acid, even when this vitamin is administered in high dosages. Also, ascorbic acid absorption may be impaired in patients with achlorhydria (an absence of a normal supply of hydrochloric acid from gastric secretions).
Any health condition that impairs fat absorption also impairs absorption of vitamin A and carotene.
Riboflavin is not absorbed readily by patients with gastrointestinal diseases. This vitamin is absorbed from the intestine and requires hydrochloric acid for its absorption. However, injected riboflavin is utilized by patients with gastrointestinal diseases. Given intravenously, most of the riboflavin goes into the small intestine, especially the duodenum, from where it can be reabsorbed. Most riboflavin is destroyed in the large intestine and in its passage through the kidney.
Individuals who lack bile in the intestine due to poor secretion of bile salts or obstructive jaundice (due to stones, cancer, or abnormal narrowing of a duct or passage) may have poor intestinal absorption of vitamin K.
Age. The age of an individual may play a role in nutrient absorption. For example, by examining fecal secretions, it has been found that vitamin A is poorly absorbed in infants, but well absorbed in adults. Also, infants and children absorb or convert carotene very poorly.
The gastrointestinal tract of children differs from adults in permeability, pH, transit time, and enzymatic activity. Children may absorb nutrients well or poorly, depending on the balance of these factors.
Calcium absorption is affected, in part, by the body's needs during different periods. If the need is low, calcium absorption from the intestine may be low. During periods of growth, pregnancy, and lactation, calcium needs are high and its absorption may increase greatly.
Iron absorption, like calcium, is affected, in part, by the body's needs during different periods. If iron stores are low, which is common for most women and children, the intestinal mucosa readily takes up iron and increases the proportion absorbed from the diet. Conversely, high iron stores, typical of men and most post-menopausal women, reduce the percentage of iron absorbed, which helps prevent iron overload.
The elderly may experience impaired B12 and calcium absorption in the intestine as well as other nutrients. As people age, many systems may function less efficiently than when they were younger.
Raw Versus Cooked Foods. It might be assumed that more nutrients are absorbed from raw than from cooked foods because the heat and pressure of cooking destroys certain nutrients. This assumption is true for some, but not for all foods. Anti-nutrients, present in some raw foods, are deactivated by cooking, and therefore nutrients are available for absorption. For example, tests showed that less vitamin C was obtained from raw than from cooked broccoli.
Years ago, experiments were conducted to determine carotene absorption from carrots. Results were mixed. One of the investigators found that 20% of the carotene in raw carrots was absorbed, compared to only 5% in cooked carrots. A second investigator had opposite results. Absorption in the raw carrots was only 1%, compared with 19% in the cooked carrots. Then, a third investigator showed that about 25% of the carotene absorbed was doubled if the carrots were pureed. These studies show the difficulty in establishing whether raw or cooked vegetables provide better nutrient absorption.
Iron supplements are absorbed best if taken on an empty stomach. Iron absorption is impaired if hydrochloric acid is deficient in the stomach. Iron supplements should not be taken at the same time as vitamin E supplements, for iron and vitamin E are antagonistic to each other.
Zinc supplements, if taken with egg, milk, or cereal, are not as well absorbed as when they are taken with other foods..
Inorganic forms of minerals may not be well absorbed. To improve absorption, manufacturers may chelate mineral supplements. This technique binds the mineral to an organic chelating substance that mimics the absorption process in the body. Examples of inorganic forms of minerals are carbonate, chloride, hydroxide, iodide, oxide, phosphate, selenate, selenite, and sulfate. Examples of organic chelated forms of minerals are ascorbate, aspartate, citrate, gluconate, glycinate, lactate, orotate, and any substances that end with the word "chelate." The chelated form of a mineral supplement may be a more expensive product than the inorganic form. However, it may be more cost-effective because of its improved absorption.
Calcium supplements cannot be absorbed unless they disintegrate and dissolve. According to standards established by the U.S Pharmacopeia, 75% of a drug tablet must disintegrate within a half hour. However, calcium tablets are regarded as nutritional supplements, not drugs. There has been no requirement that they meet this protocol. The Food and Drug Administration (FDA) has not required such testing for calcium or for other nutritional supplements.
In 1988, Dr. Ralph F. Shangraw, chairman of the Department of Pharmaceutics at the School of Pharmacy, University of Maryland, reported results of experiments he had conducted with calcium supplements. He found that many calcium tablets had poor disintegration and dissolution, which would indicate that the product might be absorbed poorly, if at all. As a result of Shangraw's findings, some manufacturers reformulated their calcium supplements to improve disintegration and dissolution of their products.
A simple tests roughly simulates how effectively a calcium supplement is apt to disintegrate and dissolve in the presence of hydrochloric acid in the normally functioning stomach. Place a calcium tablet in vinegar and stir occasionally. After a half hour, at least three fourths of the tablet should be dissolved. If this has not occurred, it suggests that the supplement will be absorbed poorly, if at all.
Foods Versus Supplements. Surprisingly, comparison data are largely lacking for nutrient absorption from foods with that from supplements.
Recently, a study was conducted by researchers at the USDA's Human Nutrition Research Center in Beltsville, Maryland, comparing the absorption of vitmain C from foods with that from vitamin C supplements. Sixty-eight men were placed on a diet designed to be very low in vitamin C to deplete them of this nutrient. After one month, the diet was supplemented with vitamin C-containing foods or supplements. Both foods and supplements were equally effective in restoring plasma vitamin C levels, with the exception of raw broccoli. In a raw state , broccoli was at least 20% less effective in raising plasma vitamin C levels than other foods that contain significant amounts of vitamin C.
Unlike vitamin C, it is far more difficult to obtain extra vitamin E from foods. According to Orville A. Levander, from the Beltsville center, it is virtually impossible to obtain more than 25 International Units (IUs) of vitamin E per day solely from the diet. The U.S. RDA is 15 IU of vitamin E for men and 12 IU for women. Levander reported that the average intake is about 10 IU to 15 IU. In recent years, higher levels have been suggested for health maintenance or treatment of diseases.
Levander and his colleagues studied 65 men for a comparison of how diets and supplements contribute to plasma levels of vitamin E. The group of men depending on vitamin E absorption solely from food showed no significant differences in their blood levels. The average daily intake was less than 20 IU of vitamin E from dietary sources. However, the group of men who added a multivitamin supplement at least every other day were able to obtain an extra 15 IU to 60 IU of vitamin E daily. In the third group, the men who took daily vitamin E capsules were able to obtain at least 100 IU of vitamin E above their dietary intake.
Compared with the group that did not take supplements regularly, plasma vitamin E levels averaged 14% higher in the group taking multivitamin supplements.. However, the levels were more than twice as high in the group taking vitamin E supplements daily.
The researchers suggested that persons who desire to increase their vitamin E plasma levels substantially need to take supplements.
At times, a nutrient may be absorbed more effectively from a supplement than from a food source. In a recent study of the Human Nutrition Research Center on Aging at Tuft's University, as well as from other studies, it was found that 20% to 40% of the elderly lose their ability to absorb the protein-bound nutrient, vitamin B12 from food. However, they were able to absorb the crystalline form from a supplement.
The answer to the question "how well are nutrients absorbed?" is far from simple. The basics of good nutrition offer the best possibilities. These basics have been stated frequently but bear repetition. Eat a wide variety of basic foods, avoid excesses to minimize imbalances, and improve lifestyle factors related to health. All these factors contribute to better nutrition.
Selected Drug Interactions
PREDNISONE, and other gluco-corticoids, used with allergies and collagen disease, can impair calcium absorption. Other drugs that can impair calcium absorption include anticonvulsants, such as phenobarbital, diphenylhydantoin, and primidone; sedatives from glutethimide; and diphosphonates, used with Paget's disease.
CHOLESTYRAMINE, a cholesterol-lowering drug, may induce steaorrhea (excessive fat in the feces) and malabsorption of fat-soluble vitamins, as well as interfere with the absorption of folic acid, iron, and vitamin B12.
CIMETIDINE, used with duodenal ulcers, as well as slow-released potassium drugs,used with high blood pressure, reduce vitamin B12 absorption.
BIGUANIDES, used as hypoglycemic agents with diabetes, may result in vitamin B12 malabsorption.
COLCICHINE, used with gout, may interfere with the absorption of vitamins K and B12, carotene, sodium, lactose, and fat.
NEOMYCIN, an antibiotic, may decrease absorption of vitamin B12, calcium, iron, potassium, sodium, nitrogen, fat, lactose, and sucrose.
PARA_AMINO SALICYLIC ACID, used with tuberculosis, may interfere with the absorption of vitamin B12, folic acid, and fat by the mucosa.
POTASSIUM CHLORIDE, used as a salt substitute, may interfere with vitamin B12 absorption.
SALICYLAZO-SULFPYRINE, used as an anti-inflammatory agent with ulcerative colitis and regional enteritis, interferes with absorption of folic acid by the mucosa.
ON THE OTHER HAND, certain foods can speed up or slow down the body's absorption of the drug and influence the efficacy of the drug. Follow directions given by your doctor or pharmacist.
FOODS HIGH IN VITAMIN K (such as liver and dark green leafy vegetables) interfere with the action of anticoagulants that prevent blood clots.
CALCIUM-RICH FOODS (such as dairy products) reduce the effectiveness of tetracyclines used to fight infection.
CAFFEINE-CONTAINING DRINKS (such as coffee, tea, and cola) as well as acidic fruit and vegetable juices (such as orange, grapefruit, and tomato juices) may destroy some drugs in the stomach before they can perform their work.
HIGH FIBER FOODS (such as whole-grain products) may reduce drug absorption in the body.