Superfoods such as broccoli, lentils and blueberries could be harmful if consumed in large quantities, according research into the impact of the antioxidants they contain.
Scientists have found that too much of the superfoods could mean an imbalance between the number of antioxidants – which slow damage to muscles and cut the risk of heart disease and cancer – and pro-oxidants.
Researchers at Kansas State University’s Cardiorespiratory Exercise Laboratory studied how to improve oxygen delivery to the skeletal muscle during physical activity by using antioxidants.
Their findings show that sometimes antioxidants can impair muscle function and increase tiredness during exercise, rendering them counterproductive.
The researches tested animals with different doses of antioxidants. Those that were given too much showed impaired muscle function, reported the study published in the Journal of Applied Physiology.
Professor David C. Poole, from the laboratory, said: "If you have a person trying to recover from a heart attack and you put them in cardiac rehab, when they walk on a treadmill they might say it's difficult.
"Their muscles get sore and stiff. We try to understand why the blood cells aren't flowing properly and why they can't get oxygen to the muscles, as happens in healthy individuals."
Steven Copp, a doctoral student in anatomy and physiology and a researcher in the lab, added: "Antioxidant is one of those buzz words right now. I think what a lot of people don't realise is that the antioxidant and pro-oxidant balance is really delicate.
“One of the things we've seen in our research is that you can't just give a larger dose of antioxidants and presume that there will be some sort of beneficial effect. In fact, you can actually make a problem worse.”
Professor Poole said antioxidants are largely thought to produce better health, but their studies have shown that antioxidants can actually suppress key signalling mechanisms that are necessary for muscle to function effectively.
“It's really a cautionary note that before we start recommending people get more antioxidants, we need to understand more about how they function in physiological systems and circumstances like exercise,” he said.