Sue Baic: Strain the facts from all the marketing fiction

Amid all the hype, it can be difficult to separate the fact from the fiction about the health benefits of herbal teas. Certainly, all types of tea are a good source of fluid. To keep adequately hydrated, healthy adults need at least 1.5 to 2 litres of liquid every day - more if one is very active or in a hot climate.

Amid all the hype, it can be difficult to separate the fact from the fiction about the health benefits of herbal teas. Certainly, all types of tea are a good source of fluid. To keep adequately hydrated, healthy adults need at least 1.5 to 2 litres of liquid every day - more if one is very active or in a hot climate.

Water, tea, coffee, juices, milk and soft drinks all contribute to this, but teas are naturally low in sugar and calories, so they're a good alternative for dieters.

Herbal and fruit teas, as opposed to coffee, tea and colas, are also low in caffeine, which can cause side-effects in some individuals. Regular consumers of normal tea seem to develop a degree of tolerance to the diuretic effects of the caffeine and the accompanying fluid has been shown to offset any dehydrating effect. However if you are particularly sensitive to caffeine, herbal alternatives are a good choice.

Teas of all types also contain small amounts of essential vitamins and minerals such as vitamin C and potassium. "True teas" - the black, green or red varieties from the leaves of the tea plant Camellia sinensis - also contain good amounts of phytochemicals known as polyphenols or flavonoids. Research has shown they can lower blood pressure, clotting and protect cells in the body against damage from free radicals. Free radicals are formed as a natural by-product of metabolism and from environmental factors including pollution, smoking and UV light. They are strongly linked with cancer-causing genes and heart disease via damage to and narrowing of the coronary arteries.

The flavonoids in tea also seem to confer some protection against arthritis, osteoporosis, and even cataracts. Most herbal teas contain flavonoids but the quantities are often less and the benefits for heart disease and cancer less fully known at present.

Claims for other health- promoting qualities of herbal teas - such as ginger for nausea, mint for bowel problems and camomile for boosting immunity and relaxation - are largely based on informal observation and alternative medicine rather than on more traditional and tightly controlled scientific study. This doesn't mean they don't work but rather that we don't know for sure yet. For example, cranberry juice was cited as a cure for urinary infections long before the scientists discovered it contained a chemical that prevented harmful bacteria attaching to the bladder wall.

In a recent British study, volunteers drank five cups of camomile tea a day for two weeks. Results suggested there was a rise in antibacterial activity of the body cells and in the levels of production of glycine, an amino acid shown to reduce muscle spasms and act as a nerve relaxant.

But more studies are needed before definitive links with these teas and health can be made and we know what quantities are beneficial for long-term use. Most importantly, we need to know if there are side-effects - especially for sensitive or vulnerable groups such as children, pregnant women and people with existing medical conditions on other medication. And beware of any startling claims to cure conditions such as diabetes, cancer or obesity. If a tea sounds too good to be true, it probably is.

Sue Baic is a registered dietitian and a lecturer in nutrition at Bristol University