Bone benefits of tea uncertain

Cups of tea may prevent hip fractures in older women, The Times reported on October 27 2007. “In the study of 1,500 women, tea fans lost less bone density in their hips,” it said. It also reports that this improvement in bone density wasn’t caused by the milk in the cups of tea.

This story is based on two analyses of tea drinking and bone density in the hips of women over 70, who were enrolled in a trial investigating the use of calcium for preventing fractures. Firm conclusions cannot be drawn from this study about whether drinking tea can improve your bone density and women should not rely on tea drinking alone to improve their bone health.

Where did the story come from?

Dr Amanda Devine and colleagues from the University of Western Australia, the Sir Charles Gairdner Hospital, and the Edith Cowan University in Australia carried out this research. The study was funded by the Healthway Health Promotion Foundation of Western Australia and the National Health and Medical Research Council of Australia. It was published in the peer-reviewed journal The American Journal of Clinical Nutrition.

What kind of scientific study was this?

This research had two parts, one part was a prospective cohort analysis of women aged 70–85 who had taken part in a randomised controlled trial investigating the effects of taking calcium supplements on the risk of fractures over a five year period. The other part was a cross sectional analysis.

When they enrolled in the study, the women answered questionnaires about themselves including their levels of physical activity, socio-economic status, smoking, and medical history, and their height and weight was measured.

In the first part of the study, 275 women were randomly selected to complete a questionnaire about beverage consumption, which asked how many cups of tea (green or black) they drank in the preceding 24 hours; herbal teas were not included as they do not come from the same plant as green and black tea. Researchers measured the bone density of the hips of these women at one and five years after the study began, and calculated how much it changed over this four year period. Only 164 women provided all the information necessary to be included in the cohort analyses. The cross sectional part of the study was carried out in year five of the trial, when researchers asked 1,027 women about their average tea intake over the last 12 months, and measured these women’s bone density.

In both parts of the study, the researchers used statistical analyses to look at the relationship between tea intake and bone density. They adjusted these analyses to take into account which treatment the women were receiving in the randomised controlled trial (calcium or placebo), and other factors that might affect bone density, such as age, body mass index, years since menopause, smoking, physical activity, socioeconomic status, and consumption of coffee, alcohol, and calcium in the diet.

What were the results of the study?

In the cross sectional analyses, researchers found that women who drank tea had 3% higher hipbone density than non-tea drinkers. In the cohort analyses, they found that women who drank tea lost less bone density (1.6%) over four years that women who did not drink tea (who lost 4% bone density). The difference between tea drinkers and non-tea drinkers remained significant after researchers took into account other factors that might affect bone mineral density.

What interpretations did the researchers draw from these results?

Researchers concluded that drinking tea helped maintain bone density in the hip in elderly women.

What does the NHS Knowledge Service make of this study?

This study has a number of limitations, which limit the conclusions that can be drawn from the results:

• The main limitation of this study is that the cross sectional analyses cannot prove that tea drinking causes women to have higher bone density, because as it looked at tea drinking and bone density at the same time point, we do not know whether the women who drank tea had higher bone densities than the non-tea drinkers before they started drinking tea. The cohort analysis does provide a bit more evidence that the tea drinking may be related to preventing bone density loss, as it looked at women’s bone density over a period of time after ascertaining their tea drinking. However, only a small number of women were included in this analysis, which reduces its reliability.
• In the cohort part of the study, the levels of tea drinking were only assessed for the previous 24 hours, which may not have been representative of the women’s normal tea consumption over their lifetime. In the cross sectional analyses, women were asked to remember their tea consumption over the past 12 months, which may have been difficult to recall accurately.
• As women were not randomised to drinking tea or not drinking tea, there are likely to be differences in the characteristics of these two groups of women (other than tea drinking) that may account for the results. The researchers report that in the cross sectional analysis, tea drinkers consumed more calories, calcium, and alcohol, but drank less coffee and smoked for fewer years than non-tea drinkers. In the cohort analysis, tea drinkers smoked less and drank less coffee than non-tea drinkers. The authors did try to take these factors into account in their analyses, but there are many other factors that could be playing a part, for example, a woman’s genetic make up.
• Overall, the differences in bone mineral density between tea drinkers and non-tea drinkers was relatively small, and we do not know whether these differences would translate into a reduced risk of fractures among the tea drinkers. The researchers themselves acknowledge this fact, and report another prospective study that found no relationship between tea drinking and fracture risk.

Women should not rely on tea drinking to improve their bone health, they should instead ensure that they have adequate calcium intake and take regular weight bearing exercise, both of which are known to help maintain bone density.

Sir Muir Gray adds...

As a confirmed tea drinker, I am reassured by this study, but even if it had shown an adverse effect, I would have carried on tea drinking and increased my exercise levels, as exercise can prevent bone loss.