Men and women aged between 40-70 years old found that taking the supplement daily over a period of 12 weeks increased their use of oxygen and endurance during exercise and also relieved tiredness.
It also appeared to reduce body weight by more than 20 per cent, researchers from US firm Pharmanex, the only company licensed to market the product outside China, reported this week.
The Cordyceps mushroom, found primarily in isolated areas of southwestern China, is fermented to produce a patent-protected supplement for human consumption, called CordyMax.
The product has previously been reported to be responsible for the performance of Chinese women athletes, who set several new records at the 1993 Chinese National Games.
Pharmanex is marketing CordyMax for people with busy and hectic lifestyles and the elderly, looking to boost stamina levels without resorting to products that stimulate the nervous system. It could hold appeal for increasingly sedentary, aging populations.
The company carried out a double-blind, randomized trial using 131 sedentary volunteers. Researchers measured exercise capacity, endurance performance, and exercise-related metabolic alterations before, in the middle of, and after the 12-week study in a placebo group and a CordyMax group, taking 3g of the supplement daily.
Peak volume of oxygen (VO2peak) consumption, a way of measuring fitness, was increased by 5.5 per cent in the CordyMax group but not significantly in the placebo group.
Also, the time to VO2peak was increased by 4.1 per cent in the CordyMax group but no change was recorded in the placebo group. This suggests the supplement may increase physical strength for aerobic exercise.
The researchers also reported increased exercise endurance, with the supplement group completing a one-mile walk 29 seconds more quickly, while the placebo group took 19 seconds longer, on average, after the three months.
Those allergic to mushrooms experienced a mild rash but otherwise side effects were minimal.
The scientists did not explain what the active ingredient in the fungus is or how it boosts performance.
The results were reported in the Experimental Biology meeting running since Friday in Washington D.C.