Whole grain products and legumes provide a rich variety of phytochemicals and antioxidants that reduce the risk of cardiovascular disease, hypertension, cancer and diabetes (1). Furthermore, these foods are often good sources of iron and zinc. These minerals, however, may be poorly available from certain plant foods (2-6). Therefore, vegetarians may need guidance on appropriate diet planning and food selection in order to achieve an adequate intake of bioavailable iron and zinc. This article will review the dietary components and food processing methods that enhance iron and zinc absorption. This information will assist dietetics professionals when counseling clients and producing client education materials.
Plant sources of iron include green leafy vegetables, legumes, soy foods, nuts, whole-grain and fortified breads, cereals, and pasta, while plant sources for zinc include cereals, nuts, legumes and soy products. There are two kinds of iron, heme and non-heme iron. In plant foods, iron exists in the less bioavailable non-heme form (2). Relative to heme iron, non-heme iron is far more sensitive to the enhancers and inhibitors of iron absorption, and also to physiological iron need (7, 8). Hence, non-heme iron will actually protect individuals against iron overload.
Bioavailability of non-heme iron and dietary zinc is greatly influenced by both dietary inhibitors and enhancers (7, 9). This may place certain segments of the vegetarian population at risk for iron and zinc deficiency. The balance between absorption facilitators and inhibitors, along with the existing iron/zinc status of the individual determines the bioavailability of iron and zinc from individual foods or from a meal (10-23). Amino acids (especially cysteine), ascorbic acid, citric acid, and fructose enhance iron absorption. Inhibitors of iron include phytate, polyphenols and fiber, oxalate and, to a certain extent, calcium.
Promoters of zinc absorption include amino acids such as histidine and cysteine (3). The relatively high absorption of zinc from breast milk compared to cow's milk or soy protein-based infant formula has been explained by the presence of zinc-binding proteins in human milk which are more easily digested than casein in cow's milk (24). Alternatively, the higher content in breast milk of the amino acids, histidine and cysteine, may also provide a partial answer (25). While phytate has been singled out as the most potent dietary inhibitor of zinc bioavailability (26, 27), other known inhibitors include oxalate, fiber, EDTA, and polyphenols (such as tannins) (24).
Ascorbic acid is the most potent enhancer of iron absorption (7, 28-32). By adding substantial quantities of ascorbic acid to a plant-based meal, iron absorption may be increased as much as 6-fold (28). This influence is most pronounced in meals that contain high levels of phytates and polyphenols (31). The solubilizing effect of ascorbic acid counterbalances the negative consequences of dietary fiber and phytic acid (7).
In iron deficient individuals, vitamin C greatly enhances iron absorption from foods when consumed in the same meal (33). In individuals with normal iron status, vitamin C has a lesser effect on improving iron absorption. The absorption-enhancing effect is much smaller when the vitamin C is taken 4 to 8 hours before the meal. Both synthetic and dietary ascorbic acid enhance iron absorption (7). For meals consumed in the morning, the iron-enhancing effect of vitamin C is high. With meals consumed later in the day, the effect may be somewhat dampened.
When food is held at warm temperatures, ascorbic acid is oxidized and the iron absorption enhancing effect is decreased (33, 34). Ascorbic acid also increases the iron bioavailability of those iron compounds used in fortification (2). Citric, malic or tartaric acids found in fruits and vegetables improve iron absorption two- to four-fold (35). Lactic acid, found in sauerkraut also enhances iron absorption (35). There is limited information about the influence of vitamin C on zinc bioavailability.
Polyphenols commonly present in many vegetables include phenolic acids, flavonoids and their polymerization products (35). There are numerous kinds of phenolic compounds in beverages such as tea and coffee, herbal teas, cocoa and red wine. These compounds form insoluble complexes with iron and may exist as an "iron-tannin" complex and thus inhibit iron absorption (5,15). The relative order of polyphenol content per cup of beverage follows the following order: black tea>coffee>cocoa>herbal teas. There is reason to believe that in iron deficient individuals, drinking polyphenol-containing cocoa, tea or coffee beverages along with a phytate-containing wheat-, rice- or maize-based meal may further compromise iron status. For clients with severe iron deficiency, use of herbal teas may be the preferred option. Similarly, phenolics present in vegetables such as butter beans, spinach and lentils may strongly inhibit iron absorption from a composite meal (35).
Phosphorus in unrefined cereals, legumes, nuts, seeds, and tubers is mostly present as phytic acid (phytate) (26). Since phytate is negatively charged it complexes with positively charged Fe or Zn ions, and in doing so it inhibits the uptake of these minerals (26, 36). Only the hexa- and penta- phosphate esters of inositol in foods appear to be potent inhibitors of both iron and zinc absorption (37). Through some food processing methods such as fermentation these phosphate esters can be hydrolyzed to substances containing fewer phosphate groups.
Phytate may be of minor significance in affecting the zinc status of most vegetarians. Phytate/zinc molar ratio and Ca/phytate/Zn molar ratios are of importance. From high phytate meals, zinc absorption is typically less than 15% (38). Exact critical molar phytate to zinc ratios that compromise zinc bioavailability have yet to be quantified. The phytate:zinc molar ratio is calculated as the millimoles of phytate intake per day divided by the daily intake of zinc in millimoles. Zinc absorption impairment may occur at molar phytate:zinc ratios above 10:1, but whether this poses a threat to the zinc status is unknown (3). Typical ratios range from 8-12 for American vegetarian women and 9-23 for vegetarian men. Calcium potentiates this inhibitory effect. Millimolar ratios of phytate x calcium/zinc per MJ greater than 22 may occur in the diets of postmenopausal women who supplement their diet with calcium to decrease the risk of osteoporosis (38). This may induce marginal zinc deficiency (39).
Food processing, for example sprouting beans, can dephytinize food products, thus improving iron and zinc bioavailability (40-42). Additionally, vitamin C content is greatly increased with sprouting. Vitamin C level increases 17.5 times in the germinated lentil and 8.5 times in germinated mung bean. Also, 100g of germinated lentils and mung beans supply 90% and 96% of the RDA for iron for adult men and 41% and 43% of the allowance for women. Twenty percent of the daily zinc allowance can be met by consuming 100g of germinated lentils and mung beans (43). Soaking oats followed by sprouting the oats reduces phytate content and doubles the amount of absorbed zinc in comparison with untreated oats. In short, iron and zinc absorption is improved when leavened products are used rather than unleavened ones (44, 45). During sourdough fermentation the lactic acid which is formed may improve mineral absorption.
For many vegetarians, soy protein is a major component of their diets. Soy has a high quality protein and is an excellent source of the health-promoting phytochemical, genestein. However, most forms of soy (soy flour, soy protein isolate, and tofu processed with calcium sulfate) reduce the absorption of non-heme iron and zinc, due mainly to the presence in soy of phytic acid (12, 24, 39, 46-48). The inhibitory effect can be largely eliminated by using fermented soy-based products (for example, soy sauce, tempeh, natto, miso), silken tofu (made from using glucono-delta-lactone as the coagulant) or alternatively, iron- and zinc-fortified soy products (such as fortified soy infant formula) (46, 49). Fermentation reduces the phytate content by two mechanisms - it releases endogenous phytases and incorporates yeast during the process (43). Adding substantial amounts of vitamin C-rich foods to a soy-based meal will also greatly enhance iron absorption and counteract the inhibitory effects of soy.
Vegetarians may need specific guidelines for selecting a diet that would accommodate the recommended fiber intake without compromising their iron status. Dietary fiber per se does not markedly impact iron absorption (50). The effects seen with fiber-rich diets are compounded by the quantity of minerals and protein in the diet, the presence of phytate or oxalic acid, and also the type of fiber present (51).
High fiber foods are often associated with a diminished zinc absorption. However, refined foods that are low in fiber have substantially lower levels of zinc, so that while the relative zinc absorption from low fiber foods is improved, the overall total zinc absorption is greater from the high fiber foods. For example, almost 40 percent of the zinc in white bread is absorbed, while only 17 percent is absorbed from whole grain bread. However, the total amount of zinc absorbed from whole grain bread is almost 50 percent more than that absorbed from white bread because whole grain bread contains more than three times the level of zinc found in the white bread (52).
Another matter of concern, relates to the issue of consuming fiber supplements along with a low zinc diet. This may pose a risk to one's zinc nutritional status, and is a relevant issue for certain segments of the population, such as the elderly.
Fiber may have a greater effect on iron and zinc balance in infants and children compared with adults. In adults, up to 32 g per day of dietary fiber and 2 g per day of phytic acid may exert no adverse effects on iron and zinc bioavailability (53). Among children, up to 25 g per day of dietary fiber and 1 g per day of phytic acid is unlikely to have a deleterious effect on iron and zinc bioavailability (54).
The influence of calcium on iron and zinc bioavailability remains controversial (55, 56). The level of iron inhibition by calcium depends on the quantity of calcium consumed, the meal size and the meal composition (57). The inhibition may be more pronounced in small, single-food meals than in complex meals (58). Lacto-ovo-vegetarians, who consume diets which are generous in dairy products, should be advised to eat substantial amounts of iron-rich vegetables, grains, and legumes to minimize the inhibitory effect of calcium, and also to consume plenty of vitamin C-rich fruits to facilitate absorption of the non-heme iron. Vitamin C appears to enhance iron retention when calcium is consumed at the same meal. (59).
Maximum iron absorption occurs when iron and calcium intakes are separated. This may not seem practical when consuming iron-fortified cereals with milk. However, preliminary studies show that iron absorption may not be greatly affected in this case. Generally, iron absorption from a meal is not greatly affected by the time of day at which the meal is consumed. Also, in clients with borderline iron deficiency, prolonged and excessive use of antacids, such as calcium carbonate, should be discouraged (60).
Food treatments have a major influence on food composition and subsequently on mineral availability (61). Interactions with dietary components during food processing, and the thermal effects induced by heating and cooking foods may positively or negatively impact on iron and zinc availability (57). Maillard browning reactions take place commonly in heat-processed foods, such as milk and milk-based food products, breads, roasted coffee and breakfast cereals (62). Maillard reaction products can chelate zinc and decrease zinc retention. These products have little influence on the overall zinc nutrition of individuals consuming zinc adequate diets. Individuals with borderline zinc intakes need to be cognizant about these factors.
Another matter of real concern, is the consumption of large amounts of non-heme iron (in the form of a supplement). Non-heme iron has an adverse effect upon zinc absorption (63, 64). Conversely, large zinc intakes can also effect iron absorption.
Vegetarian diets generally contain adequate amounts of iron and zinc. A well-planned vegetarian diet can readily meet physiological needs for both iron and zinc. Studies have shown that the incidence of iron-deficiency anemia among long-term vegetarians is similar to that of nonvegetarians. Furthermore, the serum zinc levels and the zinc levels in hair and saliva of vegetarians (measures of the zinc status of an individual) were found to be within the normal range for each of these parameters (65 -67).
Suboptimal iron and zinc status may be of some concern in pregnant and lactating women, young children, adolescents and the elderly (68, 69). Consuming balanced meals that comprise a variety of foods (including whole or fortified grains, and legumes) is a very important element for meeting iron and zinc requirements of all those age groups.
The potential for iron and zinc deficiency can be diminished by paying careful attention to the dietary factors that enhance and inhibit the bioavailability of iron and zinc. Dietitians should help parents of vegetarian children identify a variety of zinc-rich foods to include in their children's diets, such as fortified cereals, cheese, legumes, and peanut butter.
It is important to emphasize that the health benefits and advantages associated with consuming a vegetarian diet outweigh the concerns mentioned above. Nevertheless, dietetics professionals should assist the client in making appropriate food choices so as to minimize the influence of inhibitors and to maximize the use of promoters of iron and zinc absorption. In addition, the diet may be supplemented with fortified foods and beverages.
Vegetarians may also be advised on the appropriate use of iron supplements (Table 1). Clients may be encouraged to use iron cookware, such as an iron skillet (70). Further recommendations for optimizing iron and zinc availability from vegetarian diets appears in Table 2. Additionally, meal scores are available to rank foods in terms of non-heme iron availability. Published lists of iron- and zinc-rich foods for vegetarians are available (65).
During counseling, steps must be taken to determine whether the client is at risk for iron and/or zinc deficiency since the bioavailability of iron and zinc varies depending upon the dietary source. The frequency of use of iron and zinc supplements and of foods fortified with these minerals may significantly influence the iron/zinc nutritional status of a person. It is important to increase awareness about the daily requirements for iron and zinc, and when necessary discuss the problems associated with iron and zinc deficiency (71). It would be prudent to pay attention to meal composition as well as to utilize food preparation methods to achieve optimal iron and zinc status.
Take between meals
Use a ferrous supplement, rather than ferric iron
Avoid taking with dairy products, whole grain products, a calcium or magnesium supplement, and tea or coffee.
| Initial Menu | Initial Menu Modified to Increase Iron And Zinc Bioavailability |
| Breakfast | Breakfast |
| Wheat toast | Fortified breakfast cereal with raisins |
| English muffin | English muffin |
| Tea with milk | Herbal tea |
| Apple | Orange juice |
| Lunch | Lunch |
| Vegetable salad | Sprouted bean with vegetable salad |
| Baked potato | Baked potato with low-fat cheese |
| Whole wheat bread and jelly | Whole wheat bread and peanut butter |
| Black coffee | Tomato juice |
| Dinner | Dinner |
| Tomato soup | Black bean soup |
| Noodles | Brown rice |
| Stir fry vegetables | Tempeh with stir fry vegetables |
| Strawberry yogurt | Orange-banana juice |
Srimathi Kannan, PhD is Visiting Assistant Professor of Nutrition, School of Public Health, University of Michigan, Ann Arbor.
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