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Healthy eating: Unravelling the disease/diet relationship

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It’s the era of food labels,1 Heart Foundation ‘ticks’2 and nutritional guidelines.3 Knowledge of the importance of a healthy diet is increasing. After years of government nutrition campaigns, consumers are more aware of associations between diet and disease, for example that too much fat and salt increases cardiovascular risk. They are also aware that certain foods (such as fruit and vegetables) protect against cancer and other diseases.4 Consumers now demand healthy food choices.5

As evidence of the importance of healthy eating mounts, the complexity of nutritional science is also increasing. Interactions between different micronutrients are better understood. The mechanisms of previously obscure vitamins (e.g. selenium) are receiving increasing attention. Scientists have demonstrated that there’s more to nutritional value than just vitamins; many bioactive substance in food (e.g. polyphenols) have been identified as potentially disease-protective, and ‘oils ain’t oils’ when it comes to heart disease.5

But advances in nutrition science don’t mean eating choices have to be more difficult. Natural whole foods like grains, fruits and vegetables remain the first choice for preventing chronic disease,6 and armed with scientific data, health professionals can be more confident when they recommend the use of supplements.

Eating to prevent disease

Nutritional intake is increasingly understood to influence the risk of chronic disease. Associations between diet and disease became apparent when lower rates of certain diseases were noted in geographic populations with distinct dietary practices. For example, in the 1950s, the cardiovascular-protective effects of a Mediterranean diet were investigated after the dietary practices and low rates of cardiovascular disease in Mediterranean populations were noted.7 

Five of the ten leading causes of death in developed countries, including coronary heart disease, certain cancers and type 2 diabetes, are now associated with nutrition. Unhealthy eating habits increase the risk of these diseases. Conversely, good nutrition protects against them.6 It’s clear that healthy eating in general is important, but surely we can be more specific?

There is general consensus in the professional community that certain vitamins have specific actions and are required by the human body in specific quantities. But increasing understanding of food and its component parts has also led to increasing recognition of the complexity of food, and the many ways in which the various substances of a whole food (e.g. a piece of fruit or fish) influence health and disease.5 

Substances in plant foods

It is difficult to separate out the effects of a single component of a food, as whole foods contain numerous vitamins, minerals and other components which interact and function in different ways. However, advances in nutrition science are enabling a better understanding of food’s component parts.5 For example, bioactive components of plants, which may protect against certain diseases, have been investigated in recent years. Amongst them, polyphenols – bioactive substances found in plant foods with anti-oxidant, anti-inflammatory and anti-carcinogenic actions – are receiving increasing attention in relation to disease prevention.8

Eating to protect against menopausal symptoms

There is significant evidence that cultural dietary differences are entwined with disease incidence and severity. For instance, amongst women in Asian countries, 10–20% experience hot flushes (the most common symptom of menopause) compared to > 70% in North America.9 In general, Asian women experience later onset and less severe menopausal symptoms.10 In comparison to North American women, Asian women consume relatively large amounts of soy products (which contain polyphenols called phyto-oestrogens), and this led to the hypothesis that dietary phyto-oestrogen consumption reduces the symptoms of menopause.10

Phyto-oestrogens are a type of polyphenol that have a similar structure to the oestrogen produced by the human body. They play a role in plant growth by protecting the plant in times of stress.10

There are many types of phyto-oestrogens, and an individual plant may contain several different types.12 The most common types include:10

  • Isoflavones: Food sources include red clover, soy and legumes;
  • Coumestans: Food sources include alfalfa and clover sprouts; and
  • Lignans: Food sources include linseed, grains and vegetables.

Isoflavones are metabolised in the gut to compounds with a similar structure to oestrogen12 and exert mild oestrogenic and anti-oestrogenic effects (depending on in vivo oestrogen levels).13 Metabolism of phyto-oestrogens is largely controlled by gastrointestinal flora and thus differs between individuals and over time. The types of foods consumed and duration of exposure to soy (e.g. lifetime or less) further influences an individual’s ability to metabolise phyto-oestrogens to biologically active forms.12

Numerous studies of the effects of dietary and supplemental forms of phyto-oestrogens in the management of menopause symptoms have been conducted. Evidence is currently insufficient to recommend phyto-oestrogens as a direct substitute for the current first-line treatment (hormone replacement therapy).14

However, long-term hormone replacement therapy, while effective in reducing menopausal symptoms such as hot flushes, is associated with adverse health effects, including venothrombotic disease, breast cancer, stroke and coronary heart disease.9 Similarly, health professionals have raised concerns about the long-term safety of supplemental forms of phyto-oestrogens. However, there is currently no evidence of safety risks14 and dietary polyphenols are considered safe.13 Although not recommended as routine therapy, natural therapies like phyto-oestrogens are preferred by some patients.9 The increasing scientific understanding of phyto-oestrogens has created a wider range of choices for menopausal patients, allowing them to make informed decisions about the type of therapy they use and specific changes to their diets which may be beneficial.


Polyphenol consumption has been associated with reduced cancer risk. For example, the soy-bean isoflavone genistein has been associated with reduced rates of breast cancer in ecological studies. These studies show that Asian populations, which have relatively high soy consumption, have lower rates of breast cancer than European and North American populations, which have relatively low soy intake. The high soy intake that Asian women are exposed to early in life is thought to be crucial in producing the observed cancer-protective effect.8  

The mechanisms through which polyphenols exert a cancer-protective effect relate to their mediation of DNA methylation. DNA methylation patterns are cell-specific and based on historic methylation and demethylation events. Abnormal DNA methylation is linked to cancer development through the inactivation of certain cancer-dependent genes, including those involved in tumour-suppression. Many possible cancer protective mechanisms related to DNA methylation have been identified for polyphenols. For example, in vivo and in vitro studies have shown that some polyphenols regulate gene expression in the cell cycle by mediating the enzymatic processes that underpin cell division and replication.8

Epigallocatechin 3-gallate (a type of a polyphenol found in green tea) and genistein (soy bean isoflavone) have been identified as candidate polyphenols that may exert a cancer-protective effect. Different polyphenols act on different body cells, and thus provide protection against different types of cancers. A reduced risk of some gastrointestinal, reproductive, prostate and skin cancers has been associated with green tea intake, and it has been proposed that genistein has a protective effect in relation to some cancers.8

Cholorogenic acid, a dietary phenol in coffee, is thought to be the component of this popular caffeinated beverage responsible for its disease-protective effect. A recent study showed that coffee consumption decreased the risk of pancreatic cancer in men and women by 4% with each cup per day increase in coffee consumption.15 The risk or benefit of increasing daily intake of coffee was not studied beyond 6 cups per day15 and caution should be exercised above this level.

There is evidence of associations between coffee and/or caffeine consumption and a reduced risk of other health conditions, including stroke,16 type 2 diabetes mellitus17 and Parkinson’s disease in post-menopausal women not using hormone therapy.18 Consumption of chocolate, the cocoa in which contains high concentrations of polyphenols, is also associated with a reduced risk of cancer19 and other diseases including cardiovascular disease.20

Cardiovascular disease and omega fatty acids

Cardiovascular disease is a leading cause of mortality in developed countries and has received much research attention.6 Numerous foods (in particular fat and salt) have been associated with increased cardiovascular risk.3 More recently, different types of fat (such as omega-3 and omega-6 fatty acids) have been shown to influence cardiovascular risk in different ways.21

Omega-3 fatty acids are a type of fat found most commonly in seafood, but also in some plants. They have been associated with cardiovascular protective effects.21 A key mechanism through which omega-3 fatty acids (and other plant-derived fats such as oleic acid) are proposed to exert their cardiovascular protective effects is through mediation of endothelial cell responsiveness to inflammatory stimuli. However, the relationship is not well understood and may arise indipendently of this specific fatty acid. For example, because omega-3 fatty acid-rich foods like fish also contain high concentrations of other potentially protective micronutrients such as selenium, it is not clear what contribution each component makes to the protection observed.5 While the exact quantitative effects have not been determined, there is enough evidence of their benefit that increased omega-3 fat consumption is now recommended by the American Heart Foundation as adjunct therapy for individuals with cardiovascular risk (e.g. those who have experienced a myocardial infarct).21

Omega-6 fatty acids are polyunsaturated fats commonly found in corn, sunflower oil and other plant products.21 They are essential components of human nutrition and fulfil important functions in the brain and reproductive organs. However, some types also promote inflammation. Conversely, omega-3 fatty acids reduce inflammation. These two fatty acids are chemically related and omega-6 fatty acids can, theoretically, be broken down to omega-3 fatty acids.21 The human body lacks the required conversion enzyme,21 and so omega-3 and omega-6 fatty acids are obtained exclusively from dietary sources.11 

In relation to human nutrition, the ‘right’ quantity of omega-3 fatty acids is assessed relative to omega-6 fatty acid consumption. A higher ratio of omega-3 to omega-6 fatty acids (as consumed in a Mediterranean-style diet, for example) is considered protective in relation to cardiovascular and other diseases,5 even if overall fat consumption remains unchanged.11 However, Western diets typically contain a higher proportion of omega-6 fatty acids: in the United States, the typical diet contains 14–25 times more omega-6 fatty acids.22 The ratio of omega-6 fatty acids consumed in developed countries is currently considered so high that it increases mortality and morbidity risk.5


Advances in nutrition science have yielded new knowledge regarding the associations between diet and disease. While the age-old advice to consume a diet based mainly on plant foods including whole grains and limited animal-derived foods remains true, more nuanced messages are now possible and necessary. This knowledge enables health practitioners to make more specific nutritional recommendations, which should be good news for patients wanting to improve their health by improving their nutritional intake.

More information

For more information on nutrition, including information on types and composition of food, nutrition and people, conditions related to nutrition, and diets and recipes, as well as some useful videos and tools, see Nutrition.


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  22. Ehrlich SD. Omega-6 fatty acids [online]. Baltimore, MD: University of Maryland Medical Centre; 26 June 2009 [cited 17 August 2011]. Available from: URL link

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Posted On: 15 September, 2011
Modified On: 28 August, 2014

Created by: myVMC