Australasian Science Magazine

Anthocyanins in blue and purple fruits can alter carbohydrate digestion, leading to a lower overall glycaemic response. This is important for people trying to control their blood glucose levels. Current research is looking at the ability of anthocyanins in berries to protect against cognitive decline and the onset of Alzheimer’s disease.

By Lisa Ryan

Polyphenols found in plant-based foods may be all we need to live longer.

Like many western countries, Australia has an ageing population. While we all want to age gracefully, it is more important to ensure we live a healthier life. This would allow us to remain independent and improve our overall quality of life.

Evidence is growing that a diet rich in plant-based foods reduces the risk of developing a number of chronic age-related diseases such as cardiovascular disease, cancer, Type 2 diabetes, Alzheimer’s disease, cataracts and age-related functional decline.

Diets that include a variety of fruits and vegetables provide vitamins, minerals, fibre and an array of plant-derived compounds known as phytochemicals. Increasingly phytochemicals are being considered as key components responsible for much of the disease protection conferred by fruits and vegetables.

Phytochemicals are bioactive non-nutrient, non-essential plant chemicals that may provide health benefits beyond basic nutrition. More than 8000 individual phytochemicals have been identified to date, with many others yet to be discovered, making research in this area extremely complex.

Polyphenols are the most diverse group of the phytochemicals, and have been under the most intense investigation over the past decade. Plants produce polyphenols as secondary metabolites involved in a range of different processes, such as resistance against pathogens and parasites, pigmentation, pollination, defence against disease and protection from UV irradiation. It is thought that this extensive protective capability is what enables them to afford protective effects in the human body.

Polyphenols are widely distributed throughout food. Green and black tea is the biggest contributor of polyphenols for many of us. However, some polyphenols in tea, particularly tannins, can adversely affect the absorption of minerals such as iron and other trace elements. Other important sources of polyphenols are coffee, red wine, chocolate and extra virgin olive oil, fruits and vegetables.

By eating a wide range of brightly coloured fruits and vegetables, you can maximise your intake as each colour contains a unique phytochemical profile. Herbs, spices, legumes, cereals, nuts and algae are also important sources of polyphenols.

A number of different polyphenol classes exist but they can be broadly grouped into flavonoid and non-flavonoid compounds. Research suggests that flavonoids are particularly important, contributing to reduced disease risk and mortality. The major flavonoid categories are flavonols, flavones, catechins, flavanones and anthocyanins.

Each individual polyphenol works differently in the body through unique modes of action. Specific features shared by most polyphenols include:

• an ability to act as antioxidants, although the relatively low bioavailability of polyphenols means they may not be major contributors to the overall anti­oxidant status in healthy individuals; and
• interaction with cell membranes, conferring an additional level of protection by providing a physical barrier at the lipid bilayer.

Modes of actions that are unique to individual and groups of polyphenols include:

• interaction with enzymes. Some polyphenols may stimulate or inhibit different enzymes in the body. For example, the ability of anthocyanins in blue and purple fruits and vegetables, such as blueberries, to inhibit the enzymes α-amylase and α-glucosidase in the body can alter carbohydrate digestion, leading to a lower overall glycaemic response. This is important for people trying to control their blood glucose levels.
• interaction with receptors. Isoflavones found in soya products can mimic the action of oestrogen in the human body. This has led to extensive investigation of the use of isoflavones to help reduce menopausal symptoms and the risk of osteoporosis. To date the research has yielded conflicting results.
• interference with DNA replication. Catechins are flavonoids found in high quantities in tea. They can modulate the expression of genes involved in carcinogenesis by down-regulating genes that promote cancer. Other polyphenols can protect DNA from carcinogens or induce programmed cell death in cancerous cells, thereby preventing tumour progression.
• inhibition of lipid absorption and glucose absorption. Epigallocatechin gallate, a catechin found in green tea, reduces lipid digestion and absorption and therefore reduces the amount of dietary fat absorbed into the body. Isoflavones lower LDL cholesterol and total cholesterol levels. A number of different polyphenols found in tea and red wine inhibit glucose absorption and increase insulin sensitivity.
• increase in energy expenditure. Green tea extract may increase metabolic rate after food consumption. This leads to a moderate increase in energy expenditure and aids weight loss. However, there have been inconsistencies between studies, with the form of tea used (loose or bagged), the length of brewing time and the overall polyphenol content contributing to variable findings.
• the production of nitric oxide. Red wine polyphenols induce vasorelaxation by activating or increasing the generation of nitric oxide. This mechanism, as well as the ability to protect against LDL oxidation and platelet aggregation, is one of the proposed reasons behind the cardioprotective effects of polyphenols.

Current research is looking at the ability of anthocyanins in berries to protect against cognitive decline and the onset of Alzheimer’s disease. Studies in rats have consistently shown that memory loss may be prevented by consuming a diet rich in blueberries. Recent human studies in older adults found that memory score was improved in people given blueberry juice. While the exact mechanisms need further research, it does highlight the exciting potential in this field.

There is currently no recommended dietary intake for phytochemicals, but in the future, particularly for the elderly, recommendations may be made for specific phytochemicals.

At present the advice to support the use of supplements, nutraceuticals and functional foods rich in polyphenols is premature. In plants, a variety of polyphenols are present together so it may be the additive, synergistic and complementary effects of co-existing polyphenols that induces any positive health effects.

The potential detrimental effects of large doses of individual compounds taken for long periods of time has rarely been evaluated. Currently much of our knowledge on polyphenols comes from cell culture and animal studies, but this does not accurately reflect the behaviour of these compounds in the human body, particularly as polyphenols are often tested at unrealistic pharmacological doses above and beyond expected human exposure. Furthermore, polyphenols are extensively metabolised in the human body, with the majority of metabolites not accurately characterised to date.

Current evidence indicates that regular consumption of plant-based foods is associated with a reduced risk of developing a number of chronic diseases related to ageing. It is too early to say conclusively that polyphenols are responsible for this protection. However, intervention trials show some promising results for polyphenols in the prevention of cardiovascular disease, Type 2 diabetes, cancer and cognitive decline.

From a public health point of view, if drinking tea and consuming berries daily provides even a modest protective effect on overall disease risk reduction, then consuming them regularly could help reduce morbidity and mortality and also help you age more gracefully.

Dr Lisa Ryan is a registered nutritionist and a senior lecturer in nutrition and dietetics at Monash University.