Data conflict: Polyphenol supplementation improves fat oxidation but not insulin sensitivity

By Lynda Searby

- Last updated on GMT

'Our hypothesis was that these improvements in lipid metabolism would translate into improved insulin sensitivity but they didn’t.' ©iStock
'Our hypothesis was that these improvements in lipid metabolism would translate into improved insulin sensitivity but they didn’t.' ©iStock

Related tags: Insulin sensitivity, Obesity, Adipose tissue

Researchers have found the metabolic effects of supplementation with the polyphenols epigallocatechin-3-gallate (EGCG) and resveratrol don’t lead to improved insulin sensitivity in overweight men and women, adding to conflicting data around the effects of polyphenols on the metabolic system.

Insulin resistance is a major risk factor for the progression of obesity towards chronic metabolic illness, including cardiovascular disease and type 2 diabetes. Reduced lipid storage and mitochondrial capacity are closely associated with insulin resistance.

Animal studies have indicated that EGCG - an extract of green tea, and resveratrol - present in grape skin, red wine and peanuts, could assist in the prevention of weight gain and insulin resistance.

These findings have gained some credence in human studies, which have shown that supplementation with both EGCG and resveratrol improved markers of insulin sensitivity. In addition, the same team of Dutch researchers had previously found that three day supplementation with this combination increased energy expenditure and improved capacity to switch from fat to carbohydrate oxidation in men. However, conflicting data in humans have also been reported.

Longer term trial

This randomised, double-blind study investigated the longer term effect of EGCG and resveratrol supplementation on metabolic profile, mitochondrial capacity, fat oxidation, lipolysis and insulin sensitivity. The researchers found that 12 weeks of EGCG and resveratrol supplementation increased mitochondrial capacity and stimulated fat oxidation, but this did not translate into increased insulin sensitivity in a group of overweight men and women.

“Our hypothesis was that these improvements in lipid metabolism would translate into improved insulin sensitivity but they didn’t,” ​Dr Ellen Braak, lead researcher and professor in physiology of fat metabolism at Maastricht University, told NutraIngredients.

Braak said the findings didn’t surprise her, and that there may be several explanations for the apparent absence of any link between improved fat oxidation and insulin sensitivity.

“EGCG and resveratrol supplementation may have a beneficial effect in a more compromised group of pre-diabetics,” ​she noted.

She said that the key learning from this study is that “the body’s metabolic systems may be quite different depending on the group of subjects studied” ​and that “when you improve fat oxidation it doesn’t automatically improve insulin sensitivity”.

Method

grape wine vineyard
©iStock

The 38 overweight men and women were given either a placebo or 282 mg of EGCG and 80mg of resveratrol per day for 12 weeks. Before and after the intervention, oxidative capacity and gene expression were assessed in skeletal muscle. Fasting and postprandial (HFMM) lipid metabolism and tissue-specific insulin sensitivity were also assessed.

The results showed that EGCG-resveratrol supplementation did not alter food intake, body weight, body fat percentage or body fat distribution. It did, however, increase skeletal muscle oxidative capacity (mitochondrial capacity) and fat oxidation, although this did not result in improved insulin sensitivity.

Whilst this wasn’t the outcome the researchers were expecting, they concluded that “the improved mitochondrial capacity and fat oxidation may improve physical condition and play a role in the prevention of weight gain and the worsening of insulin resistance in the long term”​.

Dr Braak said that a logical next step could be a study in pre-diabetic subjects.

Source: The American Journal of Clinical Nutrition

May 18, 2016. (doi: 10.3945/ ajcn.115.122937)

“Combined epigallocatechin-3-gallate and resveratrol supplementation for 12 wk increases mitochondrial capacity and fat oxidation, but not insulin sensitivity, in obese humans: a randomized controlled trial”

Authors:Jasper Most​, Silvie Timmers​, Ines Warnke​, Johan WE Jocken​, Mark van Boekschoten​, Philip de Groot​, Igor Bendik​, Patrick Schrauwen​, Gijs H Goossens​, and Ellen E Blaak

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