On the ever-growing list of pros and cons of plant-based diets, the higher risk of bone fractures has been reinforced on the list of possible drawbacks.
Vegans and vegetarians may be at greater risk for bone fractures than meat eaters, according to a large, longitudinal study published Sunday in the journal BMC Medicine.
Fractures in adulthood and older ages are common, but previous studies have shown that vegetarians have lower bone mineral density than non-vegetarians.
Bone density is "a measure of the amount of minerals (mostly calcium and phosphorus) contained in a certain volume of bone," according to the US National Cancer Institute.
Substantially lower intakes of dietary calcium and protein have also been reported among non-meat eaters. Despite this prior research, the associations between vegetarian diets and fracture risks have been unclear until now, the study said.
"This is the first comprehensive study and the largest study to date to look at the risks of both total fractures (fractures occurring anywhere in the body) and fractures at different sites in people of different habitual dietary habits," said the study's lead author, Tammy Tong, a nutritional epidemiologist at the Nuffield Department of Population Health at the University of Oxford, via email.
There were 4.1 more cases in vegetarians and 19.4 more cases in vegans for every 1,000 people over a period of 10 years.
Diet and bone strength
Nearly 55,000 relatively healthy adults from the UK answered a questionnaire on diet, socio-demographic characteristics, lifestyle and medical history between 1993 and 2001. Researchers categorized them by diet then and at follow-up in 2010: meat eaters, fish eaters (pescatarians), vegetarians (no meat or fish but dairy and/or eggs) and vegans (nothing from animals).
The authors found 3,941 total fractures by 2016. In comparison to meat eaters, vegans with lower calcium and protein intakes on average had a 43% higher risk of fractures anywhere and in the hips, legs and vertebrae.
Vegetarians and pescatarians had a higher risk of hip fractures than meat eaters, but the risk was partly reduced when the researchers considered body mass index and sufficient consumption of calcium and protein.
However, the risk was still higher for vegans with those factors taken into account.
Limitations of the study included that most of the participants were White European and women.
"The results from this, given the limited participants, cannot be generalized to ... other populations and further study is needed," said Katherine Tucker, a professor of nutritional epidemiology at the University of Massachusetts, Lowell, who wasn't involved in the study.
The authors also had no data on any calcium supplementation or on the causes of fractures, and nutrient intake was self-reported instead of objectively measured.
Additionally, BMI could partially explain the findings, the authors said. Vegans and vegetarians tend to have lower BMI, as was the case in this study. Low BMI has been associated with fractures in some areas, potentially due to factors like less cushioning against force when a person falls.
Nutritional risks of plant-based diets
Because the risk differences in vegans remained after factoring in BMI and sufficient intakes of calcium and protein (by milligram), other factors that weren't investigated may be important — such as the differences between protein and calcium from animals and from plants.
"Most of these things are better absorbed from animal foods," Tucker said. "Some vegetarians say, 'Well, if you look at the food tables, I have enough calcium.' But calcium in whole grains is bound by phytates and in green leafy vegetables, it's bound by oxalates."
Phytates and oxalates are compounds that bind minerals like calcium, zinc and iron, so the minerals aren't released and absorbed as easily in the gut. That means that although spinach and other leafy greens are high in calcium, the oxalates in them prevent the body from absorbing that calcium as much as it could from dairy.