Fruit Antioxidants May Help Deter Diabetes

Diabetes may be the biggest threat to Americans’ collective health over the next few decades, with rates rising at an alarming pace.

According to the National Diabetes Fact Sheet, 23.6 million children and adults in the United States – 7.8% of the population – have diabetes.

Out of this estimated total, about 5.7 million people with diabetes don’t even know they have it, as their cases are undiagnosed.

Worse, some 57 million people have the cluster of metabolic signs called pre-diabetes, and 1.6 million new cases of diabetes are diagnosed in adults annually.

The UN’s World Health Organization estimates that the number of cases worldwide may double by the year 2025, due to aging, unhealthy diets, obesity, and sedentary lifestyles.

The main problem is excessive intake of sugars and starches from sodas, sweets, and white-flour breads and pastries.

Repeated “spikes” in blood sugar levels increase the risk of developing metabolic syndrome (pre-diabetes) and diabetes.

Conversely, blunting of blood sugar spikes in response to meals is believed to reduce the risk of developing diabetes. Legumes and other foods high in so-called “resistant’ starches possess the power to curb spikes in blood sugar, and their effects can last for many hours following a meal.

But by no means are all sweet foods undesirable from a blood-sugar standpoint. In fact, recent studies suggested that apples produce a net positive impact on blood sugar levels.

Sorting foods by blood-sugar impact
The most common measure of a food’s ability to raise blood sugar levels is its Glycemic Index, or GI.

But a more accurate measure is its Glycemic Load, or GL, which ranks a food based both on the GI of its particular sugars and starches, and on the amounts it contains.

Here’s how diabetes-savvy health reporter David Mendosa explains the difference between a food’s GI and GL:
“The carbohydrate in watermelon, for example, has a high GI. But there isn’t a lot of it, so watermelon’s glycemic load [GL] is relatively low.” (Mendosa D 2008)

And as David says, “A GL of 20 or more is high, a GL of 11 to 19 inclusive is medium, and a GL of 10 or less is low.”

Fruits may taste sweet, but most contain relatively little sugar compared with their other major constituents … mostly fiber, with small amounts of protein.

For example, wild blueberries have a low GL of just 5, while strawberries and apples have even lower GLs, ranking at 1 and 2 respectively.

By comparison, oranges’ GL ranking ranges from 3 to 6, grapes range from 7 to 9, bananas range from 11 to 16, and raisins come in at a whopping 28.

And three studies published last year suggest that the polyphenols in apples and berries may actually help blunt blood sugar rises caused by other foods in a meal.

Let’s take a look at these three investigations, which include one of each major kind: one cell (test tube) study, one epidemiological study, and one small clinical trial.

Studies seem to deem berries anti-diabetic
We’ll work our way forward from oldest to newest, starting with a Greek epidemiological study from February 2010, continue with a clinical trial from April 2010, and finish by describing a cell study published last month.

Study #1 – Greeks link higher antioxidant intake to reduced diabetes risk
Diets high in food-borne antioxidants may reduce the rate of diabetes, according to an analysis of data collected from participants in the famed ATTICA study (Psaltopoulou T et al. 2010).

The authors based this study on a random sample of ATTICA study participants … 551 men and 467 women from all parts of the Attica region in Greece.

Participants reported their average intake of various foods, and the antioxidant capacity of their blood was measured using three standard tests (FRAP, TRAP, and TEAC).

The volunteers were categorized as non-diabetic, pre-diabetic (i.e., diagnosed with insulin resistance), or diabetic.

The results linked higher antioxidant intake with lower blood sugar levels (glycemic indices) in healthy, pre-diabetic, and diabetic people.

This positive association did not hold in obese individuals, but it applied to non-obese people of all ages, genders, and physical activity levels.

The researchers speculated that the health association between higher intake of food-borne polyphenols and healthier blood sugar levels might be due to the ability of these compounds to reduce oxidative stress from free radicals (unstable oxygen compounds).

Recent studies suggest that oxidative stress promotes diabetes. In fact, the pancreatic islet cells that produce insulin have low levels of free-radical quenching enzymes, making them particularly susceptible to oxidative stress.

We also know that inflammation and oxidative stress are interrelated, and the Greek scientists also noted that participants with lower antioxidant intakes had higher levels of inflammation-related chemicals in their blood.

And in a prior study, the same Greek team found that ATTICA study participants with high antioxidant capacity in their blood (due to polyphenol intake) had higher levels of a hormone called adiponectin.

Higher adiponectin levels are generally associated with reduced accumulation of pro-inflammatory abdominal fat.

As the Greek researchers wrote, “An adiponectin-mediated route through which antioxidant-rich foods exert beneficial effects against inflammation [diabetes] and cardiovascular diseases can be thus hypothesized.”

Study #2 – Finnish clinical trial finds berries blunt blood sugar rises
The results of a Finnish clinical trial suggest that polyphenol-rich berries can blunt blood sugar rises caused by eating sugar (sucrose).

The researchers noted that in previous human studies, beverages rich in polyphenols (apple juice and coffee) reduced people’s blood sugar rises from meals, possibly due to an ability of polyphenols to slow sucrose digestion and/or absorption.

The Finnish trial was designed to compare the glycemic (blood sugar) effect of pure sucrose (table sugar) to that of a berry puree made of bilberries, blackcurrants, cranberries, and strawberries, sweetened with sucrose (Törrönen R et al. 2010).

The randomized, controlled, cross-over trial involved 12 healthy subjects (eleven women and one man, aged 25–69 years), who ingested either 150 grams (5.3 oz) of the berry purée or pure sucrose.

Each subject was studied in two meal tests on separate days, at least five days apart.

In the evening before the test meals, the subjects were instructed to avoid berries, and to consume a meal of choice and repeat that meal before the second test.

After consumption of the berry meal, the participants’ blood glucose levels were significantly lower at 15 and 30 minutes and significantly higher at 150 minutes, compared with the control meal.

The peak glucose concentration was reached at 45 minutes after the berry-supplemented meal and at 30 minutes after the control (sugar-supplemented) meal.

The peak increase from the baseline was significantly smaller after the berry meal.

The researchers presumed that the participants’ delayed and attenuated glycemic response indicates reduced digestion and/or absorption of sucrose from the berry meal.

They stressed that further studies assessing berries’ effects on insulin and other hormonal responses are needed.

Study #3 – UK team finds that polyphenols attenuate sugar absorption
Researchers from the UK’s University of Leeds conducted a test-tube study indicating that polyphenols from strawberries and apples may delay sugar uptake by blocking transport of glucose through human intestinal cells (Manzano S et al. 2010).

This would explain why polyphenol-rich fruits seemed to blunting post-meal glucose spikes in the Finnish clinical trial described above.

As the Leeds team wrote, “Our results indicate that unsweetened beverages rich in these dietary PPTs [polyphenols, phenolic acids, and tannins] might provide a dietary mechanism to dampen, blunt or regulate intestinal sugar absorption, a potentially important factor in the management of diabetes and the metabolic syndrome”. (Manzano S et al. 2010)

Many polyphenols, phenolic acids and tannins inhibit the actions of key sugar-transport compounds called SGLT1 and GLUT2.

Apples and berries are high in polyphenols, including anthocyanins, flavonols, flavanols, and ellagic acids.

The researchers also investigated the effects of individual polyphenols from apples and strawberries, and were able to pinpoint the ones with the most effect … findings that hold implications for development of calorie-free supplements or drugs that could give people the ability to blunt blood sugar spikes without needing to eat fruit.

Sources

photo credit: justinknol cc