Scientists claim those in age group who drink average of more than four cups of coffee a day are at 56% greater risk of death from all causes

Drinking more than four cups of coffee a day can raise the risk of dying from a host of diseases, scientists claim.

Researchers in the US found that death rates from all causes rose by more than half in people aged under 55 who drank more than 28 cups a week.

The apparently harmful effects of heavy coffee consumption were not seen in older people, according to a study published in the journal Mayo Clinic Proceedings.

The team led by Steven Blair at the University of South Carolina warned that younger people in particular might want to avoid drinking large amounts of coffee.

Previous studies have found conflicting evidence for the impact of coffee on health, with some suggesting it can raise blood pressure, and alter adrenaline and insulin activity in the body, while others point to beneficial effects on the immune system and cognitive performance.

For the latest study, researchers analysed medical and personal history questionnaires filled out by more than 40,000 people aged 20 to 87 between 1979 and 1998. After a typical follow-up period of 17 years, more than 2,500 participants had died.

The study found that those who drank larger amounts of coffee were more likely to smoke and had less healthy lungs and hearts.

Younger men had a greater risk of death even at lower levels of coffee consumption, but the effect only became significant at around 28 cups per week, where the under 55s had a 56% increase in death from all causes.

The impact of coffee appeared to be more serious on women. Those aged under 55 years old had double the risk of dying from all causes than those who did not drink coffee.

Which constituents of coffee may be to blame for the effects are not fully understood. "The exact mechanism between coffee and mortality still needs clarification. Coffee is high in caffeine, which has the potential to stimulate the release of epinephrine, inhibit insulin activity, and increase blood pressure," said Xuemei Sui, a co-author on the study.

The reason the under 55s are most at risk is also uncertain, but related vices, such as heavy drinking, may play a part. "Heavy coffee consumption behaviour might cluster with other unhealthy behaviours such as sleeping late, and eating a poor diet", adds Sui.

Hajra Siraj, a GP in Chelsea and Westminster, said: "Drinking coffee forms a large part of modern living in our society, so it's easy to lose track of how much we're actually consuming, and this study suggests this could have a greater impact on our health than we may realise."