A Chinese mushroom improves the fitness of middle-aged and elderly people, research suggests.
Some mushrooms are thought to be good for health
Tests showed people aged between 40 and 70 who lived a sedentary lifestyle became fitter after taking an extract of the Cordyceps mushroom.
The research presented to the American Physiological Society indicated an improvement in the ability to exercise and a reduction in tiredness.
However, UK experts said scientific proof for the remedy was lacking.
The mushroom became a Chinese remedy around 1,500 years ago after herdsmen in the Himalayas noticed a significant increase in their herds' strength and agility after eating it.
It is now marketed in the West.
The performance of Chinese women athletes in setting several new records at a tournament in 1993 was due to high-altitude training and using a tonic derived from the mushroom, their coach said.
Researchers at Pharmanex in California, which produces a remedy called CordyMax, tried it out on 131 volunteers.
Some were given the remedy and some a placebo over a 12-week period.
Researchers measured exercise capacity, endurance performance and metabolic alterations before, during and after receiving the remedy or placebo.
Volume of oxygen consumption went up 5.5% in the group given the remedy, but only 2.2% in the others, suggesting an increase in aerobic capacity.
The time taken to complete a one-mile walk was reduced by 29 seconds in the CordyMax group but increased slightly in the others.
And diastolic blood pressure fell by 3.2% among people taking the remedy.
The researchers said: "This study provides scientific evidence that CordyMax is effective in enhancing aerobic exercise capability, endurance exercise performance, and exercise metabolism and alleviating fatigue in healthy humans."
Dr Jidong Wu, a lecturer at Middlesex University and president of the Association of Traditional Chinese Medicine, said there had not been scientific trials to establish the effectiveness of the mushroom.
He said: "There are not many clinical trials which are accredited by Westerners, but according to the ancient Chinese literature it is a tonic herb.
"In clinical practice, people taking it feel better, but scientifically how much, we don't know."
The mushroom was thought to improve the performance of the lungs and kidneys, he said.