Food choices play an important role in managing blood glucose levels on a day-to-day basis and can also influence the progression of type 2 diabetes.  Diets rich in polyphenols offer some protection against the development of certain diseases, such as cancer, cardiovascular disease, type 2 diabetes and neurodegenerative diseases. This article examines:

• What are polyphenols?
• Which foods are high in polyphenols?
• The link between diabetes and polyphenols
• Which polyphenols are best for diabetes?
• Which foods to eat to boost your intake of diabetes-protective polyphenols.

What are polyphenols?

Polyphenols are compounds naturally occurring in plant-foods including fruit, vegetables, wholegrains, legumes, herbs, spices, tea and coffee. Polyphenols help protect plants from environmental stress such as ultraviolet radiation, pests, insects and disease. There are thousands of different polyphenols, which are classified into subgroups based on their chemical structure. Some of the better-known subgroups include:

• Phenolic acids
• Flavonoids
• Stilbenes
• Lignans

Flavonoids

Flavonoids are the most abundant polyphenol in our diets, and just to make things more confusing, they are divided into several sub-categories including:

• Flavones
• Flavanones
• Flavanols
• Flavonols
• Flavan-3-ols
• Isoflavones
• Anthocyanins
• Proanthocyanins

Which foods are high in polyphenols?

Polyphenols are found in a wide variety of plant-based foods. There are greater concentrations of polyphenols in the outer layers of the plants and lower concentrations in the centre. It is difficult to quantify the exact amount of polyphenols in plants as it changes according to:

• Ripeness
• Soil type
• Sun exposure
• Rainfall
• Stress experienced during growth.
• Plant variety (e.g. cherry tomato vs Roma tomato)
• Cooking methods
• Storage and preservation techniques

Polyphenols differ in their bioavailability, which refers to how much the human body can digest and use. The amount of polyphenols in a food, does not reflect the amount beneficial to the human body.

The table below lists some foods that are high in polyphenols:

Phenolic acids	Flavonoids	Stilbenes	Lignans
Blackberries	Apples	Grapes	Flaxseed
Black radish	Cabbage	Red wine	Flaxseed oil
Coffee	Celery
Onion	Cherries
Strawberry	Dark chocolate and cocoa products
Wheat bran	Citrus fruits
Wine	Currants
	Grapes
	Kale
	Legumes
	Olive oil
	Onions
	Pears
	Plums
	Red capsicum
	Red wine
	Soy
	Tea, especially green tea
	Tomatoes

How polyphenols are linked to diabetes

Polyphenols are believed to improve diabetes management and reduce the risk of cardiovascular disease due to their antioxidant and anti-inflammatory properties.

Antioxidants help to reduce the level of free radicals in the body. An excess of free radicals causes oxidative stress, which can make diabetes worse. Oxidative stress can worsen insulin resistance and impair the cells in the pancreas that make insulin.

Low-grade inflammation and oxidative stress can also lead to greater plaque build-up in the arteries, increasing the risk of cardiovascular disease.

 Studies have also indicated polyphenols have protective effects against diabetes-related complications, including diabetes-induced changes in the kidney.

Is one polyphenol better than another for people living with type 2 diabetes?

A 2019 systematic review attempted to answer this by comparing nine studies on the flavonoid subgroup to determine whether certain flavonoids were associated with lower incidences of type 2 diabetes.

Overall, a higher dietary intake of flavonoids was associated with reduced risk of type 2 diabetes and certain flavonoids reduced risk more than others. In order from most significant to least significant were the:

1. Flavanols (tea, grapes, red wine, apples and dark chocolate)
2. Flavonols (apples, onions, cauliflower and cabbage)
3. Flavan-3-ols (cocoa, tea and red wine)
4. Isoflavones (soy and soy-products)

Flavones and flavanones were not found to affect diabetes risk.

More studies are needed as polyphenols are challenging to study due to the difficultly in calculating how much polyphenol is in certain foods, their bioavailability and whether the health benefits are due to the polyphenols, or other components in the food such as vitamins, minerals and fibre.

Research considerations

It is important to remember that a limitation of dietary studies is that we do not eat single components or single foods. In the real-world, we eat foods in the context of a whole diet. This means we must always be cautious making assumptions that one component of food is responsible for positive or negative changes in health. When thinking about the diet and the contribution of individual components, the saying ‘the whole is bigger than the sum of its parts’ is particularly relevant.

How to boost your intake of diabetes-protective polyphenols

If you want to eat more polyphenols that have been shown to be beneficial for diabetes consider the following :

• Add a handful of blueberries to your morning oat porridge
• Finely grate green cabbage to add to salads
• Alternate water with green tea
• Drink coffee or tea with soy milk
• Enjoy a blueberry soy-milk smoothie as a snack
• Prioritise apples, grapes and berries as your preferred fruit choice
• Eat two pieces of fruit every day
• Eat a small quantity of dark chocolate every two to three days
• Swap red meat or chicken for soy-based tofu in stir-fries and curries
• Swap scrambled eggs for spiced scrambled tofu on weekends
• And, of course, eat more vegetables!

The bottom line

A common factor of all healthy diets is that they contain plenty of plant-based, minimally-processed foods. People who eat diets higher in fresh, plant-based foods are likely to have higher intakes of many beneficial nutrients such as polyphenols, fibre, vitamins and minerals, and lower intakes of harmful nutrients such as excessive salt, sugar, energy and saturated fats. At this stage, it is difficult to tell whether it is the polyphenols, other nutrients, diet composition or overall quality of the whole diet that helps to reduce disease risk or progression of type 2 diabetes. Unfortunately, we don’t have enough evidence to start writing red wine and chocolate prescriptions. However, we can be pretty safe in our support for eating a diet high in fruit and vegetables once again.

By Charlotte Lentfer, Accredited Practising Dietitian