Tea contains many chemical compounds, some of which have a positive effect on the human body by decreasing the risk of cardio vascular disease, atherosclerosis, and cancer. Several components of tea, specifically caffeine, fluoride, flavonoids, and phytoestrogen, have varying effects on bone mineral density (BMD) and fracture risk. Studies of the effect of tea on BMD have demonstrated conflicting results, perhaps related to variations in study design. Variations among the three major types of tea (i.e., oolong, black, and green) may also affect study results. Wu and associates studied the effect of drinking different types of tea on BMD in Chinese men and women while considering a number of potential confounding factors.
Starting in 1996, persons older than 20 years who had not taken any medication that would affect bone remodeling completed a detailed demographic and personal life-habits questionnaire that also asked about tea ingestion. Of these subjects, 1,037 who reached 30 years of age (the age at which the physiologic peak bone mass is usually achieved) underwent determination of BMD by dual-energy x-ray absorptiometry and calculation of body mass index. Subjects were grouped according to duration and characteristics of tea consumption.
A positive linear effect was found between the patients' BMD and the duration of habitual tea consumption, with no increase in BMD among persons who reported regular tea drinking for less than five years. This was true even after consideration of all confounding covariates. No significant difference was found among subjects who drank green, oolong, or black tea (the latter is the most common type of tea consumed in the West).
The authors conclude that the duration of tea consumption, not the actual amount consumed, determines its impact on BMD. A positive relationship between tea drinking and BMD may require more than six years of tea consumption, with more than 10 years of consumption showing a maximally significant increase in BMD of the total body, lumbar spine, and hip regions in men and women.