The key enzyme was contained in melons
Scientists may have uncovered a natural way to combat stress - eat a melon.
The key ingredient is an enzyme called superoxide dismutase, thought to have beneficial antioxidant properties which prevent damage to the body's tissues.
Volunteers given a capsule containing the enzyme reported fewer symptoms of stress and fatigue than those given a dummy capsule.
The French study is published in BioMed Central's open access Nutrition Journal.
The researchers found a strong placebo effect in the 35 volunteers who received the dummy capsules, which were filled with inactive starch.
However, this effect only lasted for the first seven days of the study.
In contrast, the positive effects on perceived stress and fatigue reported in the group of 35 who took the enzyme capsules were much greater - and longer lasting.
Taking the enzyme appeared to boost concentration, cut feelings of weariness and irritability and improve problems with sleeping.
The researchers said the placebo effect might have been relatively high because the people who took part in the study had everyday levels of fatigue and stress which were not out of the ordinary.
They suggest the results might have been more pronounced if people with higher levels of fatigue and stress had taken part.
It is thought that the enzyme may help to minimise the damage caused by a chemical process known as oxidative stress, which releases harmful atoms called free radicals into the body's tissues.
More work needed
Lead researcher Marie-Anne Milesi, from the commercial health products company Seppic, said: "Several studies have shown that there is a link between psychological stress and intracellular oxidative stress.
"We wanted to test whether augmenting the body's ability to deal with oxidative species might help a person's ability to resist burnout.
"It will be interesting to confirm these effects and better understand the action of antioxidants on stress in further studies with a larger number of volunteers and a longer duration."
Dr Laura Wyness, a senior nutrition scientist at the British Nutrition Foundation, said the study emphasised that fruit and vegetables were packed with compounds which, in combination, have a beneficial effect on health.
However, she said a bigger trial would be needed before firm conclusions could be drawn about the benefits of this particular enzyme supplement.