Toxicants in foods and their effects on nutrition

Potential sources of toxicants in food include nutrients, natural food toxicants, contaminants, and chemicals or substances intentionally added to food (food additives).

Nutrients

One usually does not relate the ingestion of a specific nutrient with concerns about the toxicity of that nutrient. However, intakes of essential dietary chemicals from zero to excessive produce responses, from lethal because of nutrient deficiency to an optimal health response and back to lethal because of intolerably high concentrations. Thus, as the solid line in Figure 1.2 illustrates, an organism cannot tolerate either of the two extremes over an extended period. The figure illustrates that there will be intakes, both low and high, associated with lethality. Also, there will be minimum low and maximum high intakes associated with good health and a valley associated with optimal health. The valley of the curve for optimal health will vary, depending on a number of physical, biochemical, or physiological effects of the nutrient. For example, the intake level of vitamin E for optimal health has a rather wide valley compared with that for intake levels of vitamin D, vitamin A, or various essential metals for optimal health.

Concentration or dose

FIGURE 1.2 Concentration (dose) effect of nutrients (solid line) compared with a typical dose-response curve (dashed line).

Concentration or dose

FIGURE 1.2 Concentration (dose) effect of nutrients (solid line) compared with a typical dose-response curve (dashed line).

With the exception of vitamin D, vitamin A, and some minerals, the intake of nutrients from natural food sources will not pose any significant health problems. However, one can argue that the health problems associated with high intakes of protein, fats, or energy are really manifestations of nutrient toxicity, i.e., cardiovascular diseases, cancers, and eye diseases such as macular degeneration and other chronic diseases. The other potential means whereby nutrient intakes can present health problems is the abuse of nutrient supplementation. A nonfood source of a nutrient can produce pharmacological actions at concentrations well above normal dietary amounts.

Over the last few years, dietary reference intakes (DRIs) have been developed by the Food and Nutrition Board of the National Academy of Sciences. The premise for developing DRIs is that such values reflect the current knowledge of nutrients, particularly with respect to their role in diet and chronic diseases. Similar to recommended dietary allowances (RDAs), DRIs are reference values for nutrient intakes to be used in assessing and planning diets for healthy people. A vital component involved in the development of DRIs is the value for tolerable upper level (UL). UL may be defined as the point beyond which a higher intake of a nutrient could be harmful. UL is the highest level of daily nutrient intake that is likely to pose no risk of adverse health effects in almost all individuals in a specified life stage group. The interest in developing ULs is partly in response to the growing interest in dietary supplements that contain large amounts of essential nutrients; the other concern is the increased fortification of foods with nutrients. For example, for vitamin C and selenium, the UL refers to total intake from food, fortified food, and nutrient supplements, whereas for vitamin E it might refer only to intakes from supplements, pharmacological agents, or their combination. Often, ULs apply to nutrient intake from supplements because it would be extremely unusual to obtain such large quantities of a specific nutrient in food form.

A risk assessment model was used to derive specific ULs, which included a systematic series of scientific considerations and judgements. The ULs were not intended to be recommended levels of intake because there are little established benefits for healthy individuals if they consume a nutrient in amounts greater than the RDA. Also, the safety of routine long-term intakes above the UL is not well established. The objective of ULs is to indicate the need to exercise caution in consuming amounts greater than the recommended intakes. It does not mean that high intakes pose no risk of adverse effects.

Naturally Occurring Toxicants

The notion that potentially toxic substances can be commonly found in conventional foods is difficult for the layperson and some well-educated people to accept. On an emotional level, food is regarded as that which sustains life, should be pure, unadulterated, and sometimes has a spiritual aura. Thus, many individuals are astonished to find that plants and some animals that are sources of food can produce an array of chemicals that can be harmful. There are some notable examples. A well-acknowledged naturally occurring toxicant is the toxin produced by the puffer fish, Fugu rubripes, which is popular in Japanese cuisine. Another example is the poisonous mushroom Amanita muscaria. The production of toxicants is more common than one might first realize. Plants produce both primary and secondary metabolic products. In the plant kingdom, many phytochemicals are produced as secondary metabolites, e.g., metabolic by-products of metabolism, excretion, and elimination. Through evolution, some of these secondary metabolites have become important defense chemicals used by the plant against insects and other organisms. The plant's weapons are not as technological as the one shown in the cartoon in Figure 1.3, but many are quite sophisticated biochemically. Primary metabolites are chemicals that have key roles in important physiological plant processes such as photosynthesis, lipid-energy and nucleic acid metabolism, and synthesis. It is likely that secondary metabolites evolved in response to and interaction with organisms of the animal and plant kingdoms or certain herbivores and pathogens. Recent advances in genetically modified foods have used such knowledge for developing plants with the ability to better defend themselves against disease and predators.

Food Additives and Contaminants

A wide variety of chemicals enter foods during processing either because they are intentionally added or the food becomes contaminated with various substances. Food

FIGURE 1.3 Plants have natural weapons.

additives include chemical preservatives such as butylated hydroxytoluene (BHT) and nitrite and microbial retardants such as calcium propionate. The food industry adds chemicals as texturing agents and flavors. Various chemicals may enter the food chain at different stages of processing, such as residues from fertilizers, pesticides, veterinary pharmaceuticals and drugs, and environmental chemicals such as lead or polychlorinated biphenyl (PCB). Some additives are generally recognized as safe (GRAS) items and require no testing for safety. Others require a battery of tests to ensure their safety for use in consumer foods.

Food additives can provide many benefits for the consumer and the food producer. Longer shelf life is advantageous not only to the producer but also to the consumer, for whom a longer shelf life means lower prices, reduced spoilage and waste, and fewer trips to the grocery store to stock up. However, some may argue whether such convenience is a benefit or a ploy by the industry to use more of their products. There are a multitude of reasons for using additives, some less meritorious than others (green catsup, anyone?). The bottom line is whether the product is safer with the additive present. Does the product have nutritional negatives, i.e., is it less nutrient dense or higher in saturated fats?

impact of diet on the effects of toxicants

For several decades, nutritional research was concentrated on establishing a better understanding of macronutrients and micronutrients. The lack or deficiency of any specific nutrient will have devastating health ramifications. The lack of any specific nutrient in the diet may affect protein synthesis. It may produce membrane alteration , resulting in the loss of cellular structural integrity and changes in membrane permeability or various functional abilities of various macromolecules, which can subsequently affect the ability of the organism to metabolize various toxicants.

Several nutrients have been recognized for their roles in protecting against the toxic effects of noxious chemicals such as alcohol and free radicals. Recent research has directed our attention to studying other chemicals in the diets, studying phytochem-icals, and reexamining how macro- and micronutrients may modulate our response to various toxicants. Specific phytochemicals have been found to act as anticancer agents and antioxidants, and to have other potential health benefits.

However, with these exciting advances in nutrition and health will arise concerns about safety and efficacy that must be addressed. Thus, with such advances, we can expect to see the field of food and nutritional toxicology at the forefront, addressing issues of mechanisms of action, risk, and safety and what is appropriate for optimal health.

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Responses

  • MAY
    What are nutritional toxicants?
    7 months ago
  • conall
    What are the effect of toxicant on nutrition?
    3 months ago

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