Vegetarianism and bone health in women

Abstract

Vegetarian diets typically exclude meat, fish, and poultry (lacto-ovo vegetarian) and may also exclude dairy products and eggs (vegan). This chapter briefly examines potential mechanisms by which vegetarian diets could affect bone health (bone mineral density (BMD) and fracture risk). These include nutrients that may be present in lower amounts in vegetarian versus omnivorous diets and/or that vary across the range of vegetarian diets (e.g., calcium, vitamin D, vitamin B12, protein), as well as substances that may be present in higher amounts in vegetarian diets (e.g., oxalates, phytates, flavonoids). Furthermore, other lifestyle factors with the potential to affect bone (e.g., physical activity, weight status) may also vary between vegetarians and omnivores, and need to be considered when comparing these two groups. Early studies assessing BMD at peripheral sites reported that vegetarians (predominantly Caucasian postmenopausal lacto-ovo vegetarians) had either similar or higher BMD than omnivores, along with similar calcium intakes. A meta-analysis of subsequent studies assessing BMD at the spine and/or hip reported slightly lower BMD in vegetarians than omnivores, but concluded that the difference was not clinically relevant. Many studies in the meta-analysis included Asian vegans, who in most cases had similar calcium intakes to omnivorous controls. To date, only two studies have prospectively examined fracture risk between vegetarians and omnivores. The EPIC-Oxford study followed almost 35,000 subjects (77 % female) for 5 years, and reported that vegans (but not lacto-ovo vegetarians or semi-vegetarians who ate fish) had ˜30 % higher fracture risk than omnivores. Conversely, a 2-year study in a small group of vegan and omnivorous Vietnamese Buddhist nuns found virtually identical rates of incident vertebral fractures. While additional research is required to elucidate mechanisms, long-term adherence to vegan diets appears to be associated with increased fracture risk in Western settings, where calcium intakes of vegans are much lower than those of omnivores.