It’s the era of food labels, Heart Foundation ‘ticks’ and nutritional guidelines. After years of government nutrition campaigns, most people now know how important a healthy diet is. Eating too much fat and salt increases the chance of heart disease while certain foods (such as fruit and vegetables) protect against cancer and other diseases. The knock-on effect is that many people now rightfully demand healthy food choices.
However, as evidence of the importance of healthy eating continues to mount, the complexity of nutritional science is also increasing. Scientists now understand not only the role of individual micronutrients (such as calcium) but also how different micronutrients interact with each other in the human body. Scientists have demonstrated that there’s more to nutritional value than just vitamins, and many bioactive substances like polyphenols that are found in food have been identified as potentially protective against disease. Similarly, science has provided ample evidence that ‘oils ain’t oils’ when it comes to heart disease.
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, it’s just that now we’re starting to work out why. This knowledge can also be used to guide the development and use of nutritional supplements, giving people more options for getting the goodness they need.
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 eating habits. For example, in the 1950s the cardiovascular-protective effects of a Mediterranean diet were investigated after the eating habits and low rates of cardiovascular disease in Mediterranean populations were noted.
Five of the ten leading causes of death in developed countries, including coronary heart disease, certain cancers and type 2 diabetes, have now been shown through scientific studies to be associated with nutrition. Unhealthy eating habits increase the risk of these diseases. Conversely, good nutrition protects against them. It’s clear that healthy eating in general is important, but surely with all the science available we can be more specific than that?
As it turns out, we now know a lot more. Increased understanding of food and its component parts has led to increased recognition of the complexity of food, and the ways in which the various components of a whole food such as a piece of fruit or a fillet of fish influence health and disease.
Substances in plant foods
It is difficult to separate out the effects of one single component of a food, as whole foods contain numerous vitamins, minerals and other components that interact and function in different ways. However, advances in nutrition science are enabling a better understanding of food’s component parts. For example, bioactive components of plants (components which have an effect on the body) that may protect against certain diseases have been investigated in recent years. Amongst them, polyphenols are receiving increasing attention in relation to disease prevention.
There are many types of polyphenols found in plant foods. The most common types include:
- 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.
Eating to protect against menopausal symptoms
There is considerable evidence that eating habits in various countries and various ethnic groups influence the incidence of particular diseases and the severity with which individuals experience them. For instance, amongst women in Asian countries, 10–20% experience hot flushes (the most common symptom of menopause) compared to > 70% in North America. In general, Asian women experience later onset and less severe menopausal symptoms.
In comparison to North American women, Asian women consume relatively large amounts of soy products which contain polyphenols called phyto-oestrogens. Phyto-oestrogens are a type of polyphenol from the isoflavone group. They are bioactive substances with numerous disease protective mechanisms, including:
- Anti-oxidant properties which help to protect the body’s cells from damage by free radical molecules;
- Anti-inflammatory properties which help to prevent cells becoming inflamed; and
- Anti-carcinogenic properties which help to inhibit the growth of cancer cells.
The lower rate of menopausal symptoms amongst Asian women led scientists to the hypothesis that consuming phyto-oestrogens in the diet reduces the symptoms of menopause. This may be because when a person consumes isoflavones, they are metabolised in the gut to compounds with a similar structure to oestrogen, supplementing the reduced hormone production experienced in menopause. But, of course, it’s not as simple as that. Metabolism of phyto-oestrogens is largely controlled by gastrointestinal flora which differ between individuals and over time. The types of foods consumed and the duration of exposure to soy (e.g. lifetime or less) further influences an individual’s ability to metabolise phyto-oestrogens to biologically active oestrogen-like forms. However, while the oestrogen-like compounds produced from the breakdown of isoflavones are similar to natural human oestrogen, their effects are weaker.
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).
Long-term hormone replacement therapy, while effective in reducing menopausal symptoms such as hot flushes, is associated with adverse health effects, including abnormal blood clotting, breast cancer, stroke and coronary heart disease. Health professionals have also raised concerns about the long-term safety of supplemental forms of phyto-oestrogens, however the available evidence does not support these concerns and dietary polyphenols are currently considered safe. Although not recommended as routine therapy, natural therapies like phyto-oestrogens are preferred by some women experiencing menopause. The increasing scientific understanding of phyto-oestrogens has created a wider range of choices for menopausal women, allowing them to make informed decisions about the type of therapy they use and specific changes to their diets that may be beneficial.
Polyphenol consumption has also been associated with reduced cancer risk. For example, the soy-bean isoflavone called genistein has been associated with lower rates of breast cancer. Studies examining the rates of breast cancer in different countries and geographic locations with particular eating habits 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 people are exposed to early in life is thought to be crucial in producing the observed cancer-protective effect.
The mechanisms through which polyphenols exert a cancer-protective effect relate to the way they influence DNA methylation, an event that helps DNA remember where it’s been. Every cell in your body contains the same DNA, even though they may have different functions. DNA methylation helps a liver cell, for example, remember that it’s a liver cell and function accordingly. If DNA methylation occurs abnormally, cells do not reproduce exact copies of the genetic material they contain. Abnormal DNA methylation may prevent the correct copying of cancer-suppressing genes into a new cell, and this has been linked to cancer development. Polyphenols have been shown to regulate gene expression in the cell cycle, including the expression of genes that suppress cancer growth, and in doing this they are believed to protect against cancer.
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 (the polyphenol in green tea) has a protective effect in relation to some cancers. Similarly, drinking coffee, which contains the polyphenol cholorogenic acid, has been associated with a 4% decreased risk of pancreatic cancer in men and women with each cup per day increase in coffee consumption. However, the risk or benefit of increasing daily intake of coffee was not studied beyond 6 cups per day, and it is not recommended that individuals drink more coffee than this.
Consumption of coffee and/or caffeine has also been associated with decreased risk of stroke, type 2 diabetes mellitus, and Parkinson’s disease in post-menopausal women not using hormone therapy. Consumption of chocolate, the cocoa of which contains high concentrations of polyphenols, is also associated with a reduced risk of cancer and other diseases including cardiovascular disease.
Cardiovascular disease and omega fatty acids
Cardiovascular disease is a leading cause of mortality in developed countries and has received much research attention. Many foods (in particular fat and salt) have been associated with increased cardiovascular risk. 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.
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. It is thought that omega-3 fatty acids (and other plant-derived fats such as oleic acid) protect the cardiovascular system by preventing inflammation in blood vessels.
However, it might not be omega-3 fatty acids alone that protect against cardiovascular disease. For example, omega-3 fatty acid-rich foods like fish also contain high concentrations of the micronutrient selenium, which is thought to help protect against disease. This makes it difficult to determine how much omega-3 fatty acid and selenium contribute to the protection provided by the food as a whole. Overall evidence of omega-3 fatty acid’s benefits is sufficient that the American Heart Foundation recommends increased consumption as additional therapy for individuals with cardiovascular risk (e.g. those who have experienced a heart attack).
Omega-6 fatty acids are polyunsaturated fats commonly found in corn, sunflower oil and other plant products. They are essential components of human nutrition and fulfil important functions in the brain and reproductive organs. However, unlike omega-3 fatty acids, some types of omega-6 fats actually promote inflammation which may contribute to cardiovascular disease. Demonstrating the complex nature of nutrition and health, these two fatty acids are chemically related and omega-6 fatty acids can, theoretically, be broken down to omega-3 fatty acids (although this doesn’t occur in humans).
The total intake of omega-3 and omega-6 fatty acids is not the only dietary aspect to consider: the balance between these two fatty acids is important to human nutrition. 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, even if overall fat consumption remains unchanged. 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 acid. The ratio of omega-6 fatty acids consumed in developed countries is currently considered so high that it increases the risk of death and illness.
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 now enables consumers to make more specific nutritional choices, which should be good news for individuals wanting to improve their health by improving their nutritional intake.
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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