DIETARY SUPPLEMENTS:BENEFITS AND LIMITATIONS

OBJECTIF NUTRITION 81 (OCTOBER 2006)
by Dr. Jean-Michel LECERF Service de Nutrition (Institut Pasteur, Lille)

Neither food nor medicine, do dietary supplements rectify real or supposed nutritional deficiencies, or can they be used as prevention?  Relevant studies are in their initial stages answers remain ambiguous. What are dietary supplements? What are their nutritional benefits? What are the limitations of their use? Practitioners must draw their patients´ attention to the risks that could be associated with these products and their inappropriate use, and instead recommend improving the daily diet, on which all efforts must be focused.

DEFINITIONS : BETWEEN FOOD AND MEDICINE

In France, an initial definition of dietary supplements was formulated by the Decree of 15 April 1996: “dietary supplements are products intended for intake as a supplement to the daily diet in order to remedy a real or supposed daily intake insufficiency”. This definition provides valuable elements by considering dietary supplements as a supplement to the daily diet. Food supplements are hence expected to comply with general food regulations and their composition must respect the provisions of the Decree of 15 April 1912! Dietary supplements are not medicines and therefore have no therapeutic effects. Compensating for food insufficiencies, their benefits are strictly nutritional.

European Directive 2002/46/EC provided another precise definition of dietary supplements under Article 2, which was reproduced in Decree 2006-352, published in the Journal Officiel de la République Française on 20 March 2006: “dietary supplements refer to foods intended to complement a regular diet and constituting a concentrated source of nutrients or other substances, which either alone or in association, have a nutritional or physiological effect, and are sold in the form of measured single-dose units containing small amounts of product”. This definition confirms that supplements are food. It also points to other elements, particularly the fact that they may contain certain substances with a nutritional or physiological effect, such as some phytoconstituents or plant-derived substances. With plants, the situation becomes more complex because some medicinal plants are governed by drug regulations (pharmaceutical use), while others, traditionally used in food, are part of dietary supplements.

Although considered as foodstuffs, dietary supplements are not food in the sense of the brilliant definition formulated by Jean Trémolières: “food is an edible, nourishing, appetizing and customary foodstuff”. Presentation in the form of capsules (or otherwise) is naturally incompatible with appetizing qualities (appealing to appetite and hence associated with pleasure).

Neither food nor medicine, dietary supplements have a status of their own, which is at times ambiguous.  This has been even truer since the advent of so-called "functional" foods, which are believed to be different through properties linked to their intrinsic natural composition or their added or modified constituents.
A list of substances authorized for use in dietary supplements has been drawn up by the competent regulatory authorities (table 1).

WHY USE THEM?

The first reason for using dietary supplements lies in their definition: supplements to food intended to compensate for possible insufficiency in the daily intake of nutrients, for which specific recommendations have been formulated (Recommended Daily Intake or RDI). Nowadays, nutritional deficiencies in developed countries are rare, except among the poorest, where dietary supplements are not a solution to the problem because of their high cost.  The other at-risk populations are prone to deficiencies because of specific behavior (vitamin B12 deficiency among vegetarians), diseases (malabsorption, cancer, anorexia, etc.) or physiological conditions (pregnant women lacking iron, vitamin B9). They also include the elderly and growing children.

Outside of these cases, food surveys show that observed intake levels are on average similar to recommended intake, even if food ratios are not perfectly in compliance with recommendations. However, public health nutritionists in France have made a new point in recent years: the fact that some deficits, outside of cases of deficiency (box 2), may encourage the occurrence of degenerative diseases.  It is normally acknowledged that a well-balanced diet is enough to cover daily intake recommendations. But we must also admit that a number of individuals are not willing or able to balance their diets. In addition, some claim that food manufacturing processes (processing, storage, refining, and preparation) alters nutritional density in a good number of foods, particularly vegetables.
The second reason for using dietary supplements, linked to their specific effects, also depends on the definition and notion of physiological effect. We are not referring to therapeutic effects: supra nutritional amounts of certain vitamins used for such purposes (e.g. vitamin B9 for correcting hyperhomocysteinemia) are not part of this picture. Physiological effects are documented with great variability. The effects of isoflavones depend in part on their metabolization by the intestinal flora as well as on hormonal environment.  As a result, their effects cannot be identically reproduced in all subjects. The same applies to the specific effects of certain carotenoids (luteine, lycopene), phytosterols, omega 3 fatty acids, functional peptides and prebiotics, and so on, in which research has taken an acute interest.

WHAT ABOUT EVIDENCE?

Studies examining the benefits of dietary supplements are in their early stages: we are still far from EBM ("Evidence-Based Medicine"). Experimental data document the physiological and functional effects of all constituents. However, going from in vitro to in vivo, from animal to man, sometimes requires more than a single step, turning into an abyss. The antioxidant effects of polyphenols fail to clear all barriers of bioavailability. And how can we easily refer to polyphenols since there are more than four thousand molecules, with some that can be partly substituted for others? So taking some resveratrol or hydroxytyrosol in powder form, under the pretext that both are found in wine and olive oil is nothing but a risky extrapolation¡K  Better go to the source!

Epidemiological data prove to be invaluable. Observation studies (case-control and prospective) have suggested that lycopene may be linked to the prevention of cardiovascular disease and prostate cancer. But are tomatoes the cause or simply a marker of higher consumption of certain fruits and vegetables (similarly to ƒÒ-carotene)? Because of poor interpretation, supra nutritional levels of ƒÒ-carotene had been administered, leading to an increased risk of lung cancer in smokers who were also alcohol drinkers¡K The one-of-its-kind observational study SHEEP took a specific interest in the risk of myocardial infarction (MI) among consumers of multi-vitamin dietary supplements. The relative risk of undergoing MI was reduced by 21% in men and 34% in women, after adjustment for major cardiovascular risk factors. This association was not modified by smoking, physical activity or eating habits considered as healthy. However, only an intervention study could deliver the required evidence.

This was one of the goals of the SU-VI-MAX study. Designed to analyze supplementation with moderately supra nutritional amounts of 5 micronutrients (vitamin E, vitamin C, ƒÒ-carotene, zinc and selenium) among 13,000 healthy subjects for a period of 7.5 years, the study did not show any cardiovascular effects. By contrast, it demonstrated a reduction in the risk of cancer and a decline in mortality among men. The absence of benefits among women could be attributed to their initially better nutritional status, which did not justify the use of dietary supplements in this case. The absence of cardiovascular benefits may be linked to the fact that this population¡¦s risk was too low for any effects to be measured under the conditions of the study. This is, however, only a theory.

RISKS AND INDICATIONS

Primum non nocere remains the ultimate adage. The risks inherent to dietary supplements may be due to the product itself or its inappropriate use.
    •Toxicity is carefully evaluated in micronutrients. It is, of course, of crucial importance for supplements made with plants, which we know may be toxic, often because of their own activity (Ephedra, Kawa) or interactions with other substances (St. John´s Wort) or accidental substitution. Safety limits have enabled maximum tolerance levels based on NOAEL or LOAEL to be defined. Joint intake of supplements or intake in excess of recommended levels must be formally discouraged.
    •Inappropriate use may be harmful when prescription or self-mediation deprives the patient from adequate diagnosis and therapy.

The first logical indication for the use of food supplements seems to be to add micronutrients to the daily diet in order to compensate for its deficiency. Consumers of dietary supplements, however, are not necessarily those who need supplementation the most. A German study has clearly shown that recommended nutritional intake levels are substantially more often complied with by consumers of food supplements (with RNI of up to 36-43% for vitamin E). However, some subjects remain below RNI levels in spite of supplementation, while others reach 100% of RNI without it.
Other indications for physiological functions or specific goals of prevention, which will become more important and increasingly more documented, will gradually come to light.
Whatever the case, dietary supplements must remain within the limits of their purpose, i.e. completing so-called "everyday" nutrition, which remains the absolute priority and foundation on which all efforts are to rest.
 

Conclusion

Dietary supplements constitute a new field of investigation and research in nutrition through studies relative to the concerned substances. Their purpose in and benefits for human nutrition have not yet been established, but their diversity justifies analysis per type of product. Insofar as their components are naturally found in food, they at least have the merit of promoting a sense of “returning to the source” and thus also food.

References