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Because vegan diets exclude dairy products, many people might be afraid that this can lead to calcium deficiency and subsequently higher risk of fractures. Does this happen? What does scientific literature say about this subject?

6

Quick answer: yes, vegan diet can expose to higher risk of fractures.

  1. According to this source, "because vegans do not eat dairy products, their calcium intakes tend to be low.":

Americans are regularly being urged to consume more calcium in order to prevent osteoporosis. It is practically impossible to meet the recommendations without large amounts of cows' milk, calcium-fortified foods, or supplements.

Also, vitamin D intake tends to be low:

Vitamin D appears to be more important for bones in conjunction with lower calcium intakes (typical in most vegan diets) than in diets that have large amounts of calcium.

So, being vegan may lead to having Calcium and/or vitamin D deficiency.

  1. According to this source (study on runners), stress fractures tend to occur more often when Ca and vitamin D supplements are not taken.

    The first study we’ll examine is a 2008 paper by Joan Lappe and coworkers at Creighton University. Lappe et al. randomly assigned 5200 volunteers, all of them female recruits in the Navy, into two groups: The first received calcium and vitamin D supplements daily, to the tune of 200% of their recommended daily value for calcium and 150% for vitamin D The other group received a placebo pill. Vitamin D was included in the supplementation program because it boosts calcium absorption. At the end of the eight-week study, stress fractures had occurred in 172 subjects in the control group (on the placebo) but in only 138 subjects in the experimental group, a relative reduction of 20%.

According to this source, low vitamin D levels may contribute to stress fracture risk.

According to this source, "low calcium intake has been associated with increased fracture rates". Its conclusion clearly states that the relationship between having a vegan diet and fracture rates is not clear:

Vegan diets are at risk for deficiencies in protein, calcium, and vitamin D3. Dairy products are a common source of calcium in the American diet. The relationships among vegan diets, bone loss, and fracture rates have been equivocal.

However, regarding osteoporotic fracture, this study concluded that "vegetarianism is not a serious risk factor for osteoporotic fracture".

From 1. and 2., the conclusion is that a vegan diet may increase risk of fractures, especially when correlated with running (or other sports).

My advice for a vegan/vegetarian in this matter is to use the precautionary principle and ensure normal levels of Calcium and vitamin D (blood analysis and supplements, if needed).

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Not getting enough calcium seems to be the relevant risk factor for vegans.

Here's one nice accessible article reviewing research on this subject here:

Dietition Perspectives on Protein, Calcium and Vegan Bone Health

This suggests that protein is beneficial for bone health, but vegans usually do get enough protein. Vegans are not at risk as long as they get sufficient calcium, but, vegans may indeed need to put in some conscious effort to do that.

What evidence we have for this—and admittedly, it’s very little—isn’t especially favorable for vegans. In the EPIC-Oxford study, vegans had a 30% higher risk for fracture after adjusting for numerous variables like age, smoking, alcohol consumption, and physical activity. After adjusting for calcium intake, however, there was no difference in fracture rates. Vegans who got enough calcium were no more likely to break a bone than milk-drinkers

According to the studies looked at by the authors, drinking milk does not seem to have any benefits for bone health, but eating cheese and yogurt may possibly decrease fracture risk, for reasons presently unknown.

It seems that both health is complicated, and that there are a lot of different studies into the issue from different perspectives. There's a very readable compilation of research data here; too long to include them all in this post, but some of the studies found higher fracture rates for vegans, while others found no difference. There are also sections on absorption of calcium from plant foods (influenced, for example, by levels of oxalate in the food) since dietary calcium, rather than any other factor, seems to be the issue.

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