What is Bioavailability?
Bioavailability is a term used in nutritional science to describe the proportion of a nutrient from a food, supplement, or other source that is absorbed and utilized by the body for its metabolic functions. While macronutrients such as carbohydrates, proteins, and fats generally boast high bioavailability (typically over 90%), the bioavailability of micronutrients like vitamins and minerals can vary significantly. This variability is due to a complex interplay of internal and external factors that dictate how a nutrient is released from its food source and processed by the body.
Dietary and Food-Related Factors
The Food Matrix: The physical and chemical structure of food, known as the food matrix, plays a critical role in nutrient bioavailability. Within food, nutrients are not freely available; they are often bound within cellular structures or complex molecules. For example, carotenoids like beta-carotene are trapped within the cell walls of plants like carrots. These structures must be broken down during digestion before the nutrient can be absorbed. The integrity of the food matrix can either hinder or aid nutrient release. For instance, the tightly bound nature of lycopene in raw tomatoes makes it less bioavailable than in cooked tomato products, where heat processing breaks down cell walls.
Nutrient Inhibitors (Antinutrients): Many plant-based foods contain natural compounds, or antinutrients, that interfere with nutrient absorption. These inhibitors can reduce bioavailability by binding to nutrients, forming insoluble complexes that the body cannot absorb, or by competing for the same absorption pathways.
- Phytates (Phytic Acid): Found in the bran of whole grains, legumes, and nuts, phytates can bind to essential minerals like zinc, iron, and calcium, significantly reducing their absorption.
- Oxalates: Present in vegetables such as spinach, rhubarb, and beet greens, oxalates can bind to calcium to form insoluble calcium oxalate, making the mineral unavailable for absorption.
- Tannins: Found in tea, coffee, and some grains, tannins can interfere with the absorption of non-heme iron.
- Fiber: High intake of dietary fiber can decrease mineral absorption, though its effect is complex and depends on the fiber's properties.
Nutrient Enhancers: Conversely, certain food components can enhance the bioavailability of other nutrients. These promoters can work by keeping nutrients soluble during digestion or by modifying absorption mechanisms.
- Vitamin C: This potent enhancer is known to significantly boost the absorption of non-heme iron from plant sources by keeping it in a more soluble, absorbable form.
- Dietary Fat: The presence of a small amount of dietary fat is crucial for the absorption of fat-soluble vitamins (A, D, E, K) and carotenoids, as it facilitates their incorporation into mixed micelles.
- The 'Meat Factor': Unidentified compounds in meat, fish, and poultry enhance the absorption of non-heme iron from other foods consumed in the same meal.
- Fermentation: This process can increase the bioavailability of minerals and vitamins by breaking down antinutrients like phytates and increasing acidity.
Food Processing and Preparation: How food is prepared can alter its nutritional profile and affect bioavailability. Heat treatment, soaking, sprouting, and fermentation are common methods used to improve or sometimes decrease nutrient availability. While heat can destroy some heat-sensitive nutrients like Vitamin C, it can also increase the bioavailability of others by breaking down the food matrix.
Host-Related and Individual Factors
Physiological Status and Health: An individual's physiological state and overall health have a major impact on nutrient absorption. Factors include:
- Age and Gender: Nutrient needs and absorption efficiency can change throughout life due to growth, development, or hormonal changes. For example, calcium absorption increases during pregnancy. Older adults may experience reduced stomach acid, which can impair absorption of nutrients like vitamin B12.
- Gastrointestinal Health: The integrity and function of the digestive tract are paramount. Conditions such as Celiac disease, Crohn's disease, or chronic infections can damage the intestinal lining and significantly reduce nutrient uptake. Gastric acid is needed for the absorption of some minerals, and low levels can be problematic.
Genetic Makeup: Individual genetic variations can influence the efficiency of nutrient transporters and metabolic enzymes, leading to differences in bioavailability. For example, some individuals may have genetic factors that affect vitamin D or carotenoid absorption.
Nutritional Status: The body can regulate nutrient absorption based on its current stores. When a person is deficient in a particular mineral, their body often becomes more efficient at absorbing it from food. For example, iron absorption is inversely related to an individual's iron status. This homeostatic mechanism helps maintain nutrient balance but also means that absorption rates are not static.
Comparison of Factors for Different Nutrients
| Factor | Heme Iron (from meat, fish, poultry) | Non-Heme Iron (from plants) | Beta-Carotene (provitamin A) |
|---|---|---|---|
| Chemical Form | Found as part of the heme molecule; readily absorbed. | Found in ferric (Fe3+) or ferrous (Fe2+) forms; absorption depends on solubility. | Lipophilic molecule, requires fat for absorption. |
| Food Matrix | Found in muscle protein; generally high bioavailability and unaffected by most inhibitors. | Bound by phytates and other inhibitors in plant cell walls. | Trapped in plant cell walls; heat processing enhances release. |
| Enhancers | Not significantly affected by enhancers or inhibitors. | Vitamin C and the 'meat factor' significantly increase absorption. | Dietary fat is crucial for absorption. |
| Inhibitors | Not significantly affected. | Phytates, polyphenols, and some fibers can reduce absorption. | Excess dietary fiber and other carotenoids can reduce absorption. |
| Food Processing | Cooking can potentially alter protein structure but has minimal effect on heme iron. | Soaking, sprouting, or fermentation can reduce phytates. | Heating, pureeing, or homogenizing with fat significantly improves bioavailability. |
How to Enhance Bioavailability
- Combine Nutrients Strategically: Pair iron-rich beans (non-heme iron) with vitamin C sources like bell peppers or citrus fruits. Consume fat-soluble vitamins (A, D, E, K) with a meal containing healthy fats.
- Prepare Food Thoughtfully: Techniques like soaking grains and legumes or fermenting foods can reduce the level of antinutrients such as phytates. Cooking vegetables can increase the bioavailability of heat-stable compounds like lycopene and beta-carotene.
- Support Gut Health: A healthy digestive system is foundational for optimal absorption. Ensuring adequate fiber intake, a balanced gut microbiome, and addressing any underlying health conditions can significantly improve nutrient uptake.
Conclusion
Bioavailability is a dynamic and intricate process that determines the true nutritional value of our food. It goes far beyond simply knowing the nutrient content of a meal. A combination of food-related factors—like the chemical form of the nutrient and the presence of enhancers and inhibitors—and individual-specific host factors—such as age, health, and genetics—all play a role. By understanding these key factors affecting bioavailability of nutrients, we can make more informed dietary choices and utilize simple food preparation techniques to optimize our nutritional intake and improve overall health.
For more in-depth research on how food and host factors influence nutrient uptake, consult the report The Role of Diet- and Host-Related Factors in Nutrient Bioavailability and Thus in Nutrient-Based Dietary Requirement Estimates.
