Essential Nutrients: Food or Supplements?: Where Should the Emphasis Be?. (PART 2)

Bioavailability and Bioactivity of Nutrients From Food vs Supplements

In general, nutrients provided as isolated compounds are highly bioavailable. The bioavailability of folate is significantly greater from folate supplements than folate in cooked spinach or yeast, and beta carotene bioavailability is significantly higher from a supplement than from a wide range of vegetables. Also, there is a higher conversion efficiency rate for synthetic beta carotene to vitamin A than for beta carotene found in food (sweet potato or spinach).

However, there are important factors that can influence the bioavailability of nutrients from foods. For example, the coingestion of fat has been repeatedly shown to increase the bioavailability of lycopene and other carotenoids from foods. Food processing also affects the bioavailability of nutrients. For example, although chopping and dissolving the cell matrix of spinach does not affect the bioavailability of lutein, it increases the bioavailability of beta carotene; cooking and pureeing carrots increases the bioavailability of beta carotene; heat processing of tomatoes increases the absorption of lycopene; and the degradation of phytate increases the bioavailability of iron and zinc from legumes. Other classic examples are of flour fermentation increasing the bioavailability of zinc and alkaline treatment of corn increasing niacin bioavailability.

The bioavailability of certain nutrients from some foods may be equal to or greater than that from supplements. The bioavailability of lutein is higher from eggs than from either lutein or lutein ester supplements, the presence of soluble fiber (but not insoluble fiber) increases the bioavailability of some minerals, and the bioavailability of iron is higher from meat than from vitamin-mineral supplements, as assessed by serum ferritin levels.

Collectively, these data suggest that nutrient and nutrient-food interactions are complex and have many facets. Factors such as food matrix, amount and type of food processing, competitive interaction among structurally similar compounds in the gut, and the presence of other compounds (eg, fat, fiber, alcohol) in the diet affect nutrient bioavailability and bioactivity. Far too little is known about nutrient bioavailability as a function of plant variety and maturity.

Dietary Patterns and Health Outcomes

A number of observational studies have reported that certain dietary patterns are associated with positive health outcomes. For example, diets high in fruits and vegetables, low-fat dairy products, or whole grains have been associated with decreased risk of heart disease, blood pressure, and cancer.

A few high-quality large-scale interventions studies have confirmed these observations. For example, the Dietary Approaches to Stop Hypertension (DASH) diet, a dietary pattern rich in fruits, vegetables, and low-fat and nonfat dairy products, with or without restrictions in sodium intake, resulted in significantly reduced blood pressure. Yet, as already discussed, single nutrients or a combination of nutrients such as beta carotene, vitamins C and E, folate, and fiber, which are contained in the foods associated with beneficial effects in both observation and intervention studies, have shown disappointing results. These findings suggest that individual nutrients may simply be markers for other beneficial substances in food or other lifestyle behaviors or act in concert with other nutrient or nonnutrient substances in food to have a beneficial effect on disease rates when used in lower, nonsupplemental doses. Support for this hypothesis comes from recent work suggesting that phylloquinone, the plant form of vitamin K, can be used as a marker for a heart-healthy diet. Data suggest that individuals with high phylloquinone intakes are at lower risk of developing coronary heart disease. However, after controlling for standard coronary heart disease risk factors, this association was no longer significant, suggesting a more casual than causal association.

In a similar vein, tomato powder was more effective than lycopene alone in reducing the development of prostate cancer in a rat model, and fat-soluble extracts from vegetable powder were more efficacious than beta carotene in inhibiting cell proliferation and inducing morphologic changes consistent with apoptosis (cellular shrinkage, chromatin condensation, and nuclear fragmentation) in a cancer cell line. Individuals who were given fruits and vegetables exhibited a greater increase in erythrocyte glutathione peroxidase activity and resistance of plasma lipoproteins to oxidation than those who received a nutrient supplement formulated to be equivalent to the amount of vitamins and minerals found in the fruits and vegetables.

Impact of Shifting the Emphasis From Food to Nutrient Supplements

There are good data to suggest that certain dietary and lifestyle patterns are associated with decreased risk of chronic disease. However, providing nutrient supplementation to mimic these effects has failed to result in the efficacy that was initially anticipated. These findings suggest that science is not at a point at which researchers can identify with relative certainty the putative compounds that are driving the food-disease relationship or the compounds that are modulating these outcomes. A number of factors need to be considered.

First, researchers are far from certain that all the beneficial nutrients or biologically active factors in food have been identified. Although there have been no new essential nutrients discovered in more than 5 decades, there is no assurance that all chemical substances in foods have been identified that could promote positive health outcomes or all nutrient interactions have been identified that might prove crucial in providing a health benefit. Lack of outright deficiency states of these substances may be due to their passive introduction into the diet from various foodstuffs. These compounds may have unrecognized functions or have functions that are currently misattributed to other nutrients with which they covary. Some biologically active compounds may be conditionally essential, that is, become limiting only during chronically high demand, such as during disease states, overweight, or high exposure to physical or environmental stresses.

Second, increasing reliance on supplements to meet nutrient needs presupposes a relatively high level of compliance. This assumption cannot be made casually. According to the limited data available on dietary supplement use, individuals who use supplements tend to be older, white, well-educated, and more affluent and more likely to consume a “healthy” diet, engage in regular physical activity, and have lower rates of smoking. Also, despite a prodigious public education effort promoting folic acid supplementation to avoid neural tube defects in the fetus of women who are capable of becoming pregnant, according the to Centers for Disease Control and Prevention, 60% of women in the United States remain noncompliant with this recommendation.

Third, knowledge is limited with regard to the issues surrounding the determinants of food intake and lifestyle behaviors. Little is known about how a population-wide shift in the message “rely on food to get your nutrients” to “rely on supplements to get your nutrients” would be interpreted. If the message perceived is that nutrient supplements provide an “insurance policy” against an imperfect diet, we must consider what impact this message would have on the balance of food choices and, hence, overall nutritional status, which is of particular concern if a combination of nutrients, rather than an individual nutrient, is responsible for the health outcome or if a nutrient not included in the supplement covaries with the target nutrient in food but is in fact the agent responsible for the positive health effect. Would the message to rely on supplements be interpreted to mean that it does not matter what food choices are made because a supplement will cover all nutrient needs? For example, would the message to take a nutrient supplement containing beta carotene or vitamin C be interpreted to mean “in addition to fruits and vegetables normally consumed” or “in place of fruits and vegetables in the diet”? If the latter, the impact on the intake of nutrients not supplied by the supplement but present in foods would be great.

Fourth, the issue of nutrient interaction and excess intake becomes more important because mainstream foods are heavily nutrient enriched or fortified. For example, many breakfast cereals are fortified with multiple nutrients, and calcium is added to a wide range of products not normally a source of this mineral. These foods are either passively or actively consumed as a result of a perceived health benefit or an assumption that if a little is good, more is better. Absent from the general population's consciousness is a consideration for the cumulative effect of multiple fortified foods on daily nutrient intake or the combination of these fortified foods with a multivitamin supplement. A recent report relating vitamin A intakes to increased risk of hip fracture in postmenopausal women highlights this point.

Last, would a shift in emphasis from food to nutrient supplements diminish other lifestyle messages by implying that nutrient supplements will “cover” all health needs? This point goes back to the recognition that lifestyle behaviors, physical activity, weight control, smoking, and perhaps even sleep and stress reduction affect health outcomes as much as diet. Carrying the nutrient supplement insurance policy analogy further, would such a message deemphasize, rather than emphasize, the importance of the whole package? The latest 5-year revision of the US Dietary Guidelines for Americans issued by the Departments of Health and Human Services and Agriculture has increased emphasis on diet and lifestyle behaviors rather than diet alone.

Targeted Supplementation

There are some strong reasons to make targeted recommendations for use of specific dietary supplements by certain segments of the population. Supplements are relatively inexpensive and can be reliably used to administer nutrients in precise doses. If used consistently, supplements can ensure adequate intakes of specific nutrients in targeted groups that have increased needs for those nutrients because of physiologic limitations or changes. As indicated above, folate supplementation significantly decreases the risk of children being born with neural tube defects.

Some elderly individuals have diminished ability to absorb vitamin B12 because of atrophic gastritis and a decreased capacity to synthesize vitamin D and can benefit from these supplements. Calcium and vitamin D supplements are the most practical way for older individuals to meet current RDAs for these nutrients. Fluoride supplementation is important for children who do not have access to a reliable source of fluoride. The AHA recommends omega-3 supplements for individuals with established coronary heart disease.

Additionally, our understanding of nutrient-gene interactions is in its infancy. It has been suggested that individuals with polymorphisms at specific gene loci, for example, for apolipoproteins such as apolipoprotein A-I, A-4, and E, and for enzymes involved in folate metabolism such as cystathionine beta synthase and methylenetetrahydrofolate reductase, may be candidates for personalized nutritional recommendations. These examples, however, are in contrast to broad-based recommendations for the general public.

Conclusions

There are insufficient data to justify an alteration in public health policy from one that emphasizes a food-based diet to fulfill nutrient requirements and promote optimal health outcomes to one that emphasizes dietary supplementation. Our conclusion is based on the lack of a complete understanding of nutrient requirements and interactions, disappointing results of intervention studies with single nutrients or nutrient cocktails, and limited understanding of how the message would be interpreted with respect to dietary and lifestyle behaviors. It is critically important to actively conduct rigorous research in these areas and to reevaluate this conclusion regularly as new data are published.