Canadian researchers randomly assigned acute hospital patients to receive either vitamin C or vitamin D supplements for seven to 10 days.

They found that those who were administered with vitamin C showed a rapid and clinically significant improvement in their state of mood. However, no such change was reported in the vitamin D patients.

The double-blind clinical trial took place at the Jewish General Hospital in Montreal, Canada and the results were published in the journal Nutrition.

Team member Dr L John Hoffer, of the Lady Davis Institute for Medical Research, said: ‘The lack of any effect of vitamin D on mood is good evidence we are not dealing with a placebo response.

‘This looks like a true biological effect. Our finding definitely requires follow up in larger studies in other centres,’ he said.

‘The treatment is safe, simple and cheap, and could have major clinical practice implications.’

Vitamin C rich foods include citrus fruits, green peppers, strawberries, tomatoes and broccoli.

Earlier studies revealed that the majority of A&E patients have below average levels of vitamins C and D in their blood.

‘About one in five acute-care patients in our hospital have vitamin C levels so low as to be compatible with scurvy,’ said Dr Hoffer.

The same researchers said last year that the red vegetable juice can increase physical endurance.

The study focused on men aged 19 to 38 cycling on exercise bikes.

Drinking half a litre of beetroot juice a day for a week enabled them to cycle 16 per cent longer before getting tired out.

Now the scientists believe they understand how the beetroot boost works.

They found that drinking beetroot juice doubled the amount of nitrate in the blood of volunteers, and reduced the rate at which muscles used their main source of energy.

Beetroot juice helped muscles work more efficiently and lowered their oxygen uptake.

Study leader Professor Andy Jones, from the University of Exeter’s School of Sport and Health Sciences, said: ‘Since our first study came out we have seen growing interest in the benefits of drinking beetroot juice in the world of professional sport.

‘I expect this study to attract even more attention from athletes.’

Drinking beetroot was found to increase blood nitrate levels and reduce muscle usage of adenosine triphosphate, the body’s chief energy source.

Oxygen uptake by muscles was also lowered, said the study published in the Journal of Applied Physiology.