As discussed earlier, consumption of fruits and vegetables has a strong protective association with various types of cancer. We still do not know which substances in these foods are responsible for the anticarcinogenic action. Although most attention has hitherto been directed to p-carotene, other substances likely play more important roles, especially phytochemicals. One class of possibly anticarcinogenic phytochemicals is soy isoflavones. The phytoestrogenic action of these substances and how they may be protective against breast cancer (and CHD) is explored by Wilson and Murphy. Similarities between these phytoestrogens and pharmacological estrogen agonists continue to blur the differences between foods and drugs.
With a better understanding of the anticarcinogenic potential of such substances as phytochemicals, folate, and selenium, it should be possible to produce a supplement that provides an effective, safe, and cheap way to prevent a great many cases of cancer. Vitamin C has a negative association with various conditions, including cataracts, asthma, and loss of pulmonary function. As in the case of p-carotene and cancer, it is critically important to bear in mind that what these data really reveal is not that vitamin C protects against these conditions, but rather that ascorbater-ich fruit and vegetables do.
As with cancer, therefore, once the responsible substances have been identified, we can better understand how to enrich our dietary intake, perhaps using supplements, so as to prevent these conditions. A nutrient that merits further investigation is vitamin E. There is evidence that it may have some prophylactic effectiveness against cancer and CHD. These benefits are generally seen only at intakes several times higher than the RDA. These levels cannot be achieved through food consumption alone. This suggests that vitamin E is acting as a nutraceutical rather than as a vitamin.
We have growing evidence that optimal dietary intake (especially vitamin E) may enhance immune function and thereby reduce the burden of infectious disease. Clarification of these relationships may be of enormous value, especially to the elderly and malnourished and to populations at high risk of infectious disease.
Optimal Intake of Nutrients
Information that has emerged in recent years challenges our concepts of nutrient requirements. Although the details are still hazy, it is becoming increasingly clear that many substances have disease-preventing actions in ways that do not fit into the classic model of nutrientJdeficiency disease. In some cases, these substances are nutrients, notably vitamin E and selenium, but are apparently most effective at intakes far above what is required for preventing deficiency diseases. In other cases, the phytochemicals, there is no requirement, as the substances are not nutrients in the classic sense; possible examples of this are particular carotenoids and flavonoids by Weinberges and Heaney. Indicate that this concept applies to potassium and calcium by virtue of their hypotensive action. In a previous work, one of us (NT) argued that it is short sighted to define recommended nutrient intakes narrowly in terms of preventing deficiency diseases. For various nutrients, we can more accurately speak of three levels of intake.
A low intake is clearly deficient by any definition and will produce clinical symptoms or, at least, subclinical deficiency. A somewhat higher intake will prevent deficiency symptoms but will not fully protect health; it is best described as a suboptimal intake. A yet greater intake is required for maximal health preservation: an optimal intake. In the case of the phytochemicals, of course, we cannot talk of deficiency, although there does appear to be an optimal intake. Taking vitamin E as an example, a daily intake of under 5 mg is deficient, 10 mg (the US RDA) is suboptimal, whereas an optimal intake (assuming the previously cited evidence is correct) is between 50 and 500 mg. The reason for these different levels of requirement stems from the relationship between the nutrient or phytochemical and the cause of the disease. With classic deficiency diseases, such as scurvy, the disease is solely due to lack of the nutrient.
As a result, one or more specific biochemical pathways fail to operate normally. But with diseases prevented by relatively high doses of nutrients and phytochemicals, the true cause . a factor, such as tobacco, which the protective substances can counter to some extent. However, they typically do this in ways where the required dose is much higher than that pertaining to the prevention of a deficiency disease. Just as the discovery of the relationship
tween nutrients and deficiency diseases necessitated the establishment of the concept of recommended nutrient requirement, so the foregoing arguments suggest the necessity of a new concept based on optimal intake. In this regard, it matters little whether the substance question is a nutrient or not. Gey's proposed term, recommended optimum intake, closely rresponds to the aforementioned concept.
Nutrition and Aging
Many features associated with aging-for instance, atherosclerosis, impaired immune unction, declining respiratory function, decreasing lens opacity (leading to cataract), and creasing blood pressure-are all associated with diet, and therefore preventable to a greater lesser extent. Recent evidence suggests that age-related cognitive decline and memory loss may also come into this category. Indeed, optimal nutrition may be acting not so much by a differerent mechanism in each of these conditions but by a common pathway. Oxidative stress he mechanism that first springs to mind, but the supporting evidence is far from rwhelming.
Diet and Fetal Programming
This has been a contentious issue for decades. Determining its importance with respect ·:"e development of obesity, CHD, diabetes, cancer, and other diseases in adults remains a enge. More work is clearly needed to clarify the ideal fetal and infant diet for the prevention of these diseases in adulthood.
Diet and CHD
The relationship between diet and CHD cannot help but appear as a confusing picture. -T"e are several areas of potential importance which require further investigation before formulate an overall strategy for the prevention of this ongoing pandemic. These areas include homocysteine, high doses of vitamin E (discussed previously), n-3 fatty acids, and determining the maximum safe intake of saturated fat and trans fatty acids.
Diet and Obesity
The chapter by Richards and colleagues authoritatively reviews our present position with regard to obesity. Four centuries ago, Wiiliam Shakespeare wrote: "They are as sick that surfeit with too much as they that starve with nothing" (Merchant of Venice). Despite decades of intensive research, ei:fective dietary therapy for the condition remains as out of reach as it was two-thirds of a century ago when Somerset Maugham wrote what is arguably the finest picture painted of the pain and frustration of weight loss in his story The Three Fat Women of Antibes. Here, in more ways than one, "An ounce of prevention is worth a pound of cure." The area of prevention and treatment of obesity will hopefully see ongoing progress.
The Ideal Diet
Central to any discussion on the subjects of CHD is the debate regarding the ideal level of fat, saturated fat, and dietary fiber. This also has major implications in regard to obesity, diabetes, and colon cancer. It has often been suggested that the ideal diet is one based largely on low-fat plant foods with little or no meat. Research may well prove that this is the best means to incqrporate into the diet the optimal quantities of fruit, vegetables, and perhaps soy foods. We should also keep our minds open to the possibility that there is no ideal diet for a population, but rather unique nutritional needs of the individual.
Herbal Medicine and Bioengineering
As discussed by Craig, traditional medicine continues to predominate in less developed regions of the world. In the next millenium, we can expect to see continued developments in these low-technology supplements and this will lead to promotion of health in the developed regions of the world. Our scientific understanding of herbal medicine and nutraceuticals remains in its infancy; clearly some provide reproducible health benefits and others are useless. Their nutritional significance remains hazy at present. Herbals that can be incorporated into our diet and do prevent diseases will become important given their generally low cost. Nutritional impacts of high-technology changes have been taking place since the 'days of Norman Borlaug and his "Green Revolution." The impact of biotechnology was discussed by Beitz. Genetically modified crops and livestock have become fairly common, although they are as yet poorly accepted elsewhere. The risks and benefits of the use of these technologies to increase yields comes with risks that are still poorly understood and risks that may yet to be appreciated. Until there is a much better understanding of the nutritional benefits/risks, the use of this technology is likely to remain controversial.
Dietary Assessment
An ongoing problem in nutritional epidemiology is the quality of dietary assessment. So many of the controversies in the area of nutrition and disease emanate from inaccuracies in measuring people's dietary intake. Well-known examples include sodiumlblood pressure, energy intake/obesity, and fat intakelbreast cancer.
