Vitamin pills may start to lose effects fast if kept in warm rooms

The experts said that crystalline substances – including vitamin C, some forms of vitamin B and other dietary supplements – are prone to a process called deliquescence, in which humidity causes a water-soluble solid to dissolve.

Keeping vitamins and supplements away from warm, humid environments can help ensure they stay effective, the American researchers said.

Lisa Mauer, an associate professor of food science at Purdue University in Indiana, said: "You might see salt or sugar start to cake in the summer, and that's a sign of deliquescence.

"You can also get chemical instabilities, which are a little more problematic if you're consuming a dietary supplement with vitamin C for that vitamin C content."

Prof Mauer said salt, sugar and powdered drink mixes commonly caked, making their measurement more difficult but not rendering them useless.

But she said chemical changes became more than just a nuisance in vitamins and dietary supplements. "If you get some moisture present or ingredients dissolve, they'll decrease the quality and shelf life of the product and decrease the nutrient delivery.

"You can even get complete loss of the ingredients. It depends on the conditions. It depends on the formulations. Within a very short time – in a week – you can get complete loss of vitamin C in some products that have deliquesced."

Prof Mauer said storing vitamins or supplements in containers with lids did not always help protect them.

"Opening and closing a package (in a bathroom] will change the atmosphere in it … you add a little bit of moisture each time."

The research was published in the Journal of Agricultural and Food Chemistry.

Dee Atkinson, director of the natural health specialist Napiers, which has stores in Glasgow and Edinburgh, said: "I usually ask my patients to keep their supplements in a dark cupboard."