Find out why it’s so important for vegetarians – and especially vegans – to know about the bioavailability of nutrients in plants.
What does bioavailability mean?
Nutrient bioavailability is basically the proportion of a nutrient that is absorbed from the diet and becomes available for the body to use. It is important to understand that this can be well below 100%, especially when it comes to trace minerals and vitamins.
It is also important to understand that even if a nutrient is available the system will not necessarily take all of it up. This part of the process is called bioaccessibility. Factors affecting the actual uptake of minerals and vitamins include:
- General level of health
- Levels of that particular component already stored in the body
- Stress levels
Heme iron (that found in meat, poultry and fish) is absorbed more easily than non-heme iron (that found in dairy products, eggs and plant foods). 15-35% of heme iron can be absorbed, compared to 2-20% of non-heme iron.
This does not necessarily mean that vegetarians and vegans are at risk of iron deficiency. The greater quantities of fruit, vegetables, nuts and seeds consumed in a plant-based diet means that it is quite possible to consume a higher level of iron than omnivores.
The amount of iron absorbed is also related to the iron stores in the body, so the percentage uptake is naturally variable. Studies have shown that in a well-balanced plant-based diet there is no significant risk of anaemia. (1)
Different factors can enhance or inhibit the absorption of iron, and it is important to be aware of these in order to maximise iron uptake. One major problem is the presence of phytates and other antinutrients in food. It is worth taking measures to reduce these substances. Things that can inhibit the absorption of non-heme iron are:
- High amounts of zinc in the diet
- Soy protein
- Tea and coffee
It is also useful to be aware of what can enhance iron absorption. About 75mg of ascorbic acid or vitamin C can increase the absorption of iron in a meal by 3-4 times, as long as it is taken with the meal. It has also been shown that vitamin A / beta carotene can have the same effect. Substances that help with non-heme iron absorption are (1):
- Ascorbic acid (vitamin C)
- Citric, malic, lactic and tartaric acids
- Fermented soy products
- Vitamin A / beta carotene
It can be seen that the absorption of iron can be increased with careful thought to a meal. Plenty of fruit and vegetables, as well as nuts, seeds, grains and legumes, will provide sufficient iron and the ability to utilise it. Good sources of beta carotene are carrots, sweet potato, spinach and collard greens.
Iron / zinc interactions
The widespread use of iron fortification and supplementation makes any interaction between iron and other micronutrients of special nutritional relevance. Studies have demonstrated that when given as an aqueous solution – a situation similar to intake of supplemental iron – the absorption of zinc was reduced by iron. However, when iron was added to solid foods, no effect on zinc absorption was observed. (2)
Similar to the iron / zinc interaction, zinc in high doses in aqueous solutions impairs iron absorption, while no effect is observed when zinc is added to meals.
These results suggest that in the presence of organic substances iron and zinc are absorbed by different mechanisms and that the risk for interactions is larger when the nutrients are provided as supplements. (2)
Iron / manganese interactions
In contrast, manganese affects iron absorption in a way that suggests that the intestine cannot differentiate between manganese and iron. A high manganese intake – as could be the case in tea-drinking populations – may therefore impose a risk of reduced iron uptake.
Iron / calcium interactions
Calcium can reduce the level of iron absorption. This effect is mainly relevant when calcium and/or iron supplements are taken without food. Therefore the best advice if you use those supplements is to take them at different times of day, so as to avoid this interference. Calcium does not appear to affect zinc uptake, unless calcium levels are really high. (3)
Zinc / copper / iron interactions
Copper is essential for iron transport between tissues. Zinc can interfere with copper intake. Conversely, copper can interfere with the uptake of zinc. Low levels of zinc can affect the utilisation of vitamin A.
It is therefore important to be careful with the use of supplements and try to keep a good, natural balance by carefully planned meals.
Calcium bioavailability from plant foods can be affected by their contents of oxalate and phytate.
Oxalate and its related compound, oxalic acid, can be found in many plant foods. High oxalate intake may increase your risk of getting kidney stones, as the oxalate in the blood can combine with calcium to form calcium oxalate. Citric acid can help to prevent this happening. (3)
Fat soluble vitamins
Vitamins A, D, E and K are fat soluble vitamins. This means they need a certain amount of fat/oil to be taken up. Cooking also helps carotenoid absorption. (4)
Where does all this leave us?
Firstly, do what you can to reduce antinutrients. This can be soaking, germinating, fermenting and/or cooking.
Secondly, look at the factors that increase the uptake of specific minerals. For example you can improve the uptake of iron from your spinach by adding broccoli or carrots to the meal, adding a splash of lemon juice or having some currants or berries for dessert. Taking a vitamin D supplement will help with the uptake of calcium. Balancing zinc and copper intake is important, as vegans tend to have a high-copper diet, because of the use of nuts, seeds and lentils.
1. Iron Status of Vegetarians, Winston J Craig
2. Micronutrient interactions: effects on absorption and bioavailabilty, Brittmarie Sandstrom
3. Choices for achieving adequate dietary calcium with a vegetarian diet, Connie M Weaver, William R Proulx and Robert Heaney.
American Society for Clinical Nutrition
4. Application of in vitro bioaccessibility and bioavailability methods for calcium, carotenoids, folate, iron, magnesium, polyphenols, zinc and vitamins B6, B12, D and E, Paz Etcheverry, Michael A Grusak and Lisa E Fleige.
Frontiers in Physiology.
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