Bioavailability is the degree to which food nutrients are available for absorption and utilization in the body. It is a critical issue for many nutritional concerns.
Why Do We Care About Measuring Bioavailability?
The role of bioavailability is important in establishing nutrient requirements and using those requirements in food labeling. The amount of a nutrient in a food that the body can actually use may vary depending on age and physiologic condition, such as pregnancy. Nutrient availability is also important in testing and marketing infant foods, nutritional supplements, and enteral formulas (for patients who can't digest solid foods).
An understanding of bioavailability is also important because consumers continually change their dietary patterns for reasons of health, economics, or personal preference, and knowledge of nutrient bioavailability may influence their choices. Furthermore, as the range of food products from which consumers may choose constantly increases (especially with production of new and unconventional convenience foods), the food processing industry has a critical interest in the effects of food processing and preparation on the bioavailability of nutrients.
Demographic changes also expand food choices, so that determining the nutrient availability and adequacy in ethnic foods is of greater concern. The use of vitamin and mineral supplements by as many as 50 percent of Americans suggests a need for accurate data on the availability of nutrients in these supplements. Finally, nutrient-drug interactions may alter nutrient bioavailability and thus affect nutritional status in individuals who are taking certain drugs.
Analyzing and Measuring Bioavailability
Bioavailability refers to the amount of a nutrient in a food that the body may ultimately use to perform specific physiological functions.
Several factors influence the bioavailability of a nutrient. These include:
Bioavailability may be quantified to some extent by measuring (1) the amounts of the nutrient in various body tissues and fluids or (2) the growth or enzyme activity that depends on the nutrient. A nutrient is rarely stored in a single body tissue, however, so that determining the nutrient levels in single tissues may not accurately reflect the true bioavailability. For example, levels of nutrients in blood, which is an accessible tissue for measurement purposes, may not reflect the levels in other tissues that are the major stores, such as liver.
Changes in response variables such as growth, immune competence, or enzyme activity must also be validated by comparison with other criteria, since individually they may not reflect true bioavailability. Growth, for example, does not reflect the degree to which nutrients are stored in tissues in an animal that is already fully replete with this nutrient. Selenium-dependent glutathione peroxidase activity in liver may not indicate the bioavailability of selenium for other proteins that require it. Moreover, none of these functional responses reveals much about the processing of a nutrient at the specific stages of digestion, absorption, and utilization.
Each of the steps involved in the process that makes nutrients bioavailable can be affected by a variety of factors in the food itself, and also by the nutritional status of the individual. It is particularly difficult to assess bioavailability when the nutrients are present in many different forms in foods and tissues.
As complicated as it appears to be, the assessment of nutrient bioavailability still remains critical to our understanding of how humans utilize essential nutrients from consumed foods and to our appreciation of how foods satisfy our nutritional requirements.
Researchers have found new analytic techniques that permit more accurate identification and measurement of nutrients in foods and tissues, and they have creatively applied these techniques to improve our understanding of observed variations in the bioavailability of a nutrient from different foods.
Techniques to measure vitamin and mineral levels include affinity and high-performance liquid chromatography for separation and isolation of individual nutrients; mass spectrometry for separation and identification with very high specificity; and the use of "tagged" nutrients (or isotopes, that can be chemically identified at various stages) as tracers that allow monitoring of the effects of nutrient handling at each step that may affect bioavailability. In some cases, foods can be intrinsically labeled with tagged nutrients by growing the plants or animals in the presence of tagged nutrients. This experimental approach provides a more valid or realistic model for examining nutrient bioavailability than does one that adds the tracer form of the nutrient to foods that are ingested.
Individual Nutrients and Food Factors That Affect Bioavailability
A variety of components in foods may reduce or enhance the bioavailability of the nutrients. Some components may form complexes with a nutrient and prevent its digestion or absorption or even degrade the nutrient, as is the case with foods that contain an enzyme that breaks down the B vitamin, thiamin. Protein inhibitors that often reduce nutrient bioavailability are generally destroyed by cooking. Other complexes can increase solubility and, thus, enhance absorption. Recent developments in the availability of selected nutrients are summarized below:
Efforts to understand the metabolic and dietary factors that lead to osteoporosis, or the loss of skeletal mass with aging, emphasize the importance of calcium bioavailability. Calcium in foods exists mainly as complexes with other factors (phytates, oxalates, fiber, lactate, fatty acids) from which the calcium must be released to be absorbed.
Plant constituents of the diet, in particular, may reduce calcium bioavailability so that people who do not use dairy products are less likely to obtain adequate amounts of calcium. Oxalates, present in some foods, normally bind with calcium in the gut, and the body excretes both of them together, thus limiting calcium absorption and availability. Researchers are using plants intrinsically labeled with tracer forms of calcium to evaluate the effects of plant food constituents on calcium bioavailability. Calcium supplements are also being evaluated by these techniques to determine their availability to humans.
Recent research has shown that the bioavailability of calcium from calcium carbonate, a widely used supplement, is similar to that from milk. It has also been shown that vitamin B6 deficiency may reduce calcium availability.
Iron deficiency is widespread in the United States and is a major cause of anemia in susceptible populations, especially in those whose demand for iron is high, such as growing children or pregnant women. Many factors, including dietary components (phytates, tannins, phosphates, and high calcium intake), exercise, menstruation, and maturity may increase or reduce iron availability. Iron absorption and utilization increase as iron stores are depleted, but inhibiting factors in such foods and beverages as soybeans and tea can impair iron absorption. Conversely, including meat or foods containing vitamin C in a meal enhances iron absorption. It is not known how meat achieves this effect, but recent research suggests that some factors in meat form a complex with iron to increase its absorption. Meat also increases gastric acid secretion, which may increase iron availability and absorption.
The optimal criterion for measuring the bioavailability of iron is not clear. The most commonly used response criterion is hemoglobin concentration in blood. The most recent research suggests that regeneration of red-blood-cell hemoglobin (an oxygen-transporting protein) can be used to measure iron bioavailability, thereby providing an easily obtained index of iron availability. Protocols are being developed to predict the bioavailability of iron in humans based on animal models. Recent research also shows that interactions of other minerals, such as zinc and calcium, with iron may reduce iron bioavailability. Copper deficiency, cooked meat, and raw vegetables are thought to enhance iron absorption.
Copper deficiency can result in anemia, bone disease, and diminished immune competence. Excessive intake of copper can lead to toxic effects, especially vascular problems such as low blood pressure and high blood-cholesterol levels. The bioavailability of copper is affected by a variety of factors. Among those which decrease bioavailability are suboptimal levels of acid in the gastrointestinal tract; the boiling of foods, which may leach away copper; and the consumption of uncooked protein foods. Copper bioavailability may also be reduced by interaction with other minerals such as iron, zinc, lead, cadmium, and selenium.
Intake of lead has become a major public health concern. Lead toxicity is most widespread in children, in whom it may lead to impaired mental development. In poorly nourished populations, it commonly results in anemia by interfering with the availability of essential nutrients, such as iron and copper. Recent research indicates that increasing meat intake reduces lead absorption from drinking water or other sources of ingested lead. Additional copper intake is more effective than either iron or zinc in reducing lead absorption, although intake of all three minerals seems to protect against lead toxicity.
Vitamin B12 deficiency rarely occurs from inadequate dietary intake but can become a problem for the elderly, leading to serious hematologic, neurologic, or gastrointestinal consequences. With age, the stomach secretes less of a protein necessary for the absorption of B12. Research indicates that pectin and other soluble dietary fibers can interfere with absorption of vitamin B12 from foods, as well as with reuse of the vitamin made available from secretions into the intestine. Inadequate knowledge of the actions of such fibers in the digestive tract, along with dietary recommendations for increased fruit and fiber intake, indicates a need for additional research.
Folic acid (folate)
Studies implicating folic acid in birth defects from impaired development of the spinal column and brain suggest that the recommended dietary allowances need to be reexamined as more accurate data on folate bioavailability and utilization are obtained. This will be especially critical for pregnant women. The bioavailability of folate in a typical U.S. diet is about 50 percent. An examination of folatedepleted rats indicates that folate bioavailability varies from about 70 to 100 percent depending on the food source. Folic acid labeled with stable isotopes is now being used to better standardize assessments of foodfolate bioavailability in humans.
Vitamin B6 occurs in several forms in foods and is necessary for normal lipid and amino acid metabolism, red-blood-cell function, hormone production, and immune competence. The forms present in plant sources may include a complex with a glucose molecule, which appears to reduce the bioavailability of other forms of vitamin B6 present in foods. The vitamin B6 present in foods from animal sources exhibits very high availability—as much as 100 percent in tuna—while availability in foods from plant sources is low, 20 to 40 percent, due in part to the presence of the complex. Vegetarians are thus at particular risk for low vitamin B6 intake. Vitamin B6 status also appears to decline with age for reasons that may include reduced absorption. Research on the bioavailability of vitamin B6 is emphasizing the effects of the glucose complex in foods.
Improving Our Food Choices
Knowledge of nutrient bioavailability is key to our understanding of the role of nutrients in maintaining human health. Improved knowledge of nutrient bioavailability can help in providing definitive, quantitative dietary guidance, and it can help us translate what we know into optimal and desirable eating patterns and food choices.