Clinical trial shows that berries reduced the blood sugar response to dietary sugar, with implications for lower diabetes and heart risk
by Craig Weatherby

Meals high in sugars and rapidly digested starches—such as white bread, pasta, bagels, and pastries—cause diners' blood sugar levels to spike.

And diets dominated by such meals promote diabetes… which is usually signaled by the decline in blood-sugar control called pre-diabetes syndrome.

Key Points
  • Sugar-sweetened berry mix raised blood sugar less than the same weight of pure sugars.
  • Prior tests show that the antioxidants in berries block enzymes that digest carbohydrates (sugars-starches) and cut rodents' absorption of carbs.
  • The same kinds of antioxidants occur in tea, grapes, coffee, and raw cocoa.
Pre-diabetes syndrome is one of several of the diagnostic signs that define metabolic syndrome (MetS)… an unhealthful state that affects more than 50 million Americans and is linked closely to diabetes and cardiovascular disease (CVD).

Earlier this month, we reported on the results of a Finnish clinical trial, which found that berries altered body chemicals in ways that could help deter the damage done by poor diets, including the several defining characteristics of MetS 
(For more on that, see “Berries May Deter Liver and Metabolic Disorders”).

Some of their fellow Finns just released the encouraging results of a small clinical trial that tested the ability of berries to moderate the body's blood sugar response to dietary sugar.

Their interest was sparked by animal and lab studies showing that the polyphenol antioxidants in berries suppress enzymes (maltase and sucrase) needed to digest and absorb carbohydrates (sugars and starches).

And previous human studies have shown that apple juice and coffee
both naturally rich in polyphenolscan exert beneficial effects on blood sugar following a meal.

The Finns set out to test the idea that even sugar-sweetened berries might yield lower spikes in blood sugar levels compared with pure sugar.

If true, this could hold healthful implications for tea, grapes, coffee, and raw cocoa, which provide similar sets of polyphenols.

Finns' clinical findings give berries another boost
Researchers from Finland's University of Kuopio recruited 12 volunteerseleven women and one man, aged 25–69 yearsfor a randomized, controlled, cross-over trial.

A swift primer on sugar

Most of the sugar in most fruits is sucrose… which consists of one molecule of glucose bound to one molecule of fructose.

High-fructose corn syrup has a similar make up, in that it consists of about 45 percent glucose and 55 percent fructose.

But the fructose is much less tightly bound than in sucrose, and the physiological effects HFCS of seem to be different fromand generally worse thanthose of cane sugar, which are bad enough.

The subjects were screened by blood tests and interviewed about their health history, current medications, alcohol and tobacco use, physical activity, and use of dietary supplements.

Each person was studied during two three-hour meal tests on separate days at least five days apart, and was instructed to avoid berries for a day before each test.

The participants were given one of two things to consume along with a test meal:
  • Mix 1 – A sugar-sweetened berry puree (150 g total weight) made of bilberries, blackcurrants, cranberries and strawberries, and sweetened with 35 grams of sucrose (cane sugar).
  • Mix 2 – A “control” sugar mix containing the same amount of water and proportion of sugars as the berry mix (i.e., 250 ml water, 35 g sucrose, 4.5 g glucose, and 5.1 g fructose). 
The berry puree contained added white cane sugar, which consists entirely of sucrose.

Thus, whatever factors in berries that might moderate blood sugar would have to contend not only with the natural sugars in the berries, but also with the added sucrose.

The most important finding was that, the peak increase in blood sugar was 1·0 mmol/l smaller after the berry meal… a significant difference.

What is MetS?
Metabolic syndrome (MetS) is a cluster of characteristics linked to increased risk of developing cardiovascular disease, stroke, and type 2 diabetes. Some 50 million Americans have MetS.

MetS is defined as having three or more of a half-dozen metabolic risk factors:
  • Abdominal obesity (excessive fat tissue in and around the abdomen).
  • High blood triglycerides, low HDL cholesterol and high LDL cholesterol: a state that fosters plaque buildup in artery walls.
  • Elevated blood pressure.
  • Insulin resistance or glucose intolerance, in which the body can't properly use insulin or blood sugar (also called also called pre-diabetes syndrome)
  • Pro-thrombotic state that promotes dangerous clots (e.g., high fibrinogen or plasminogen activator inhibitor–1 in the blood).
  • Pro-inflammatory state (e.g., elevated C-reactive protein in the blood).
To put this number in perspective, the normal range when fasting is 4 to 6 mmol/l (72-108 mg/dL). People with a fasting blood sugar level consistently above 7 mmol/l (126 mg/dL) are generally considered diabetic.

Also of note, people's blood sugar levels were significantly lower 15 and 30 minutes after eating the berry mix, compared with their response to the sugar mix.

Berries seem to delay and reduce sugar absorption
The peak blood sugar level was reached 45 minutes after participants tested the berry mix and 30 minutes after they ate control sugar mix.

This was likely due to “extended release” of sugar from the berries, as people's digestive processes slowly extract them from the fruits' fibrous matrix.

The researchers concluded that the delayed and smaller rise in average blood sugar level indicate reduced digestion and/or absorption of the sugars from the berry mix.

As they wrote, “The shape of the plasma glucose [blood sugar] curve, with reduced concentrations [levels] in the early phase and a slightly elevated concentration in the later phase, indicates a delayed [blood sugar] response due to berry consumption. Berries also significantly decreased the peak glucose [level].”

But as they stressed, we lack a full understanding of the role of berries in sugar metabolism, and we need studies testing their effects on insulin and other relevant physiological factors.

Until then, these findings add another reason to consider berries a wonderfully healthful food.

  • Lehtonen HM, Suomela JP, Tahvonen R, Vaarno J, Venojärvi M, Viikari J, Kallio H. Berry meals and risk factors associated with metabolic syndrome. Eur J Clin Nutr. 2010 Mar 3. [Epub ahead of print]
  • Louie JC, Atkinson F, Petocz P, Brand-Miller JC. Delayed effects of coffee, tea and sucrose on postprandial glycemia in lean, young, healthy adults. Asia Pac J Clin Nutr. 2008;17(4):657-62.
  • Ostman E, Granfeldt Y, Persson L, Björck I. Vinegar supplementation lowers glucose and insulin responses and increases satiety after a bread meal in healthy subjects. Eur J Clin Nutr. 2005 Sep;59(9):983-8.
  • Törrönen R, Sarkkinen E, Tapola N, Hautaniemi E, Kilpi K, Niskanen L. Berries modify the postprandial plasma glucose response to sucrose in healthy subjects. Br J Nutr. 2010 Apr;103(8):1094-7. Epub 2009 Nov 24.