by Jack Norris, RD
- Calcium and Vegan Diets: Why the Fuss?
- Animal Protein and Osteoporosis
- Epidemiological Studies
- Metabolic Studies
- What Causes Osteoporosis?
- Calcium and Vitamin D
- Vitamin B12 and Bone Mineral Density
- Selenium and Bone Mineral Density
- Research on Vegans
- Calcium Absorption from Plant Foods
- Calcium Tips
- Dangers of Calcium Supplements
Calcium and Vegan Diets: Why the Fuss?
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.
Enter vegan diets. Because vegans do not eat dairy products, their calcium intakes tend to be low. The Dietitian’s Guide to Vegetarian Diets (2004) lists 45 studies that have surveyed vegetarians’ calcium intakes in Appendix G. The daily calcium intakes in these studies are about:
- Vegans: 500 – 600 mg
- Lacto-ovo vegetarians: 800 – 900 mg
- Non-vegetarians: 1,000 mg
Only one study included supplements. In it, daily calcium intakes were: 840 mg (vegan males), 720 mg (non-vegetarian males), 710 mg (vegan females), and 855 mg (non-vegetarian females).
Animal Protein and Osteoporosis
As you can see above, the calcium intake of vegans tends to be quite a bit lower than lacto-ovo vegetarians, non-vegetarians, and the DRI. Traditionally, the vegan community has responded to this by saying osteoporosis is a disease of calcium loss from the bones, not a lack of calcium in the diet. This was based on two bits of evidence. The first bit is that ecological studies have shown that the countries with the highest intake of dairy products (northern Europe and the USA) have higher rates of hip fractures than do Asian and African countries where much less milk is consumed. This can be explained by the second bit of evidence which is that studies show that after ingesting animal protein, people urinate large amounts of calcium. Therefore, the thinking goes, calcium intake isn’t important for preventing osteoporosis and vegans are protected due to the lack of animal protein in their diets.
Let’s examine this evidence.
While it is true that Asian countries have lower rates of hip fractures than do higher dairy-consuming countries, it turns out that hip fracture rates can be misleading. The first prospective cohort study measuring clinically diagnosed vertebral fractures in an Asian population, the Hong Kong Osteoporosis Study, found that while hip fracture rates were lower in Hong Kong than Sweden, vertebral fracture rates were higher (Bow 2012), as shown in the image below.
The authors of the Hong Kong Osteoporosis Study state:
The observed ethnic differences in fracture incidences may be due to the fact that hip fracture risk was affected by fall risk, whereas the risk of vertebral fracture mostly depends on bone strength. Despite the low hip fracture rate in our population, Hong Kong women had a higher prevalence of osteoporosis [technical parenthetical deleted] than US Caucasian women (35.8% vs. 20%, respectively) and a similar prevalence of about 6% in Hong Kong and US Caucasian men.
In other words, at least in this group of people from Hong Kong, the Chinese don’t have lower rates of osteoporosis.
Here are some excerpts from a 2009 review of the epidemiological studies on protein and bone health (Darling 2009):
Overall, there was very little evidence of a deleterious influence of protein intake on [bone mineral density (BMD)], with most cross-sectional surveys and cohort studies reporting either no influence or a positive influence. Thus, 15 cross-sectional surveys found a statistically significant positive relation between protein intake and at least one BMD site. However, 18 studies found no significant correlation between protein intake and at least one BMD site.
The cohort studies also identified little evidence of any deleterious influence of protein intake on bone. …[N]o studies showed a significant increase in BMD loss with increased protein intake, and only one study showed a significant decrease in BMD loss with increased animal and total protein intakes.”
Overall, the [seven] cohort studies indicated either a benefit or no effect of protein intake on hip fracture relative risk, with only one study reporting a significant increase in risk with increasing animal protein intake and increasing animal to vegetable protein ratio. Three studies found a decreased relative risk of hip fracture with increasing animal, total, and vegetable protein intakes. Two studies found no significant association of animal protein with fracture risk, whereas 2 studies found no association of total protein with fracture risk. Last, 2 studies found no relation between fracture risk and vegetable protein.
In a large observational study (2014), researchers in Sweden found milk consumption to be positively associated with early death in both men and women, as well as fractures—especially hip fractures—in women. While statistically significant, differences weren’t large enough to indicate dairy plays more than a small role in early death or fractures. In analyses of three sub-cohorts, milk consumption was also found to be associated with oxidative stress in men and women as well as inflammation in men (Michaëlsson 2014).
As mentioned above, there is a theory that protein increases osteoporosis by increasing a loss of calcium through the urine. The idea is that protein, especially through the sulfur-containing amino acids, increases the acid in the blood which, in turn, gets shuttled to the urine, increasing the renal acid load. In order to neutralize that acid, calcium is used as a buffer and then urinated out with the acid.
There has been an enormous amount of research on protein and bone health and this view of protein causing osteoporosis has been refuted. Here are some of the main points of a 2012 literature review (Calvez 2012) of clinical trials on the subject:
- Many clinical trials show that adding purified proteins to the diet increases calcium excretion through the urine. But when whole foods are eaten, this effect is not strong. And phosphorus, in which meat and dairy are rich, counteracts the increase of calcium in the urine between 40 and 65%.
- High protein diets increase acid excretion in the urine, but this can be handled by the body’s acid buffer system without the need for calcium.
- In low-calcium, but not high-calcium diets, higher protein intakes probably increase calcium absorption from the digestive tract causing an increase in calcium excretion in the urine.
- Fruits and vegetables are beneficial to bone health, probably due to their high potassium and magnesium content. This could cause confounding in protein studies because diets high in protein are often low in fruits and vegetables.
- There is some evidence that a beneficial effect of protein on bones is only seen when calcium intake and vitamin D status is adequate.
- Maintenance of adequate bone strength and density with aging is dependent on adequate muscle mass which is dependent on adequate intake of protein.
- An increase in IGF-1 is most likely the mechanism for increased bone health with higher protein intakes.
Although HP [high protein] diets induce an increase in net acid and urinary calcium excretion, they do not seem to be linked to impaired calcium balance and no clinical data support the hypothesis of a detrimental effect of HP diet on bone health, except in the context of inadequate calcium supply.
Another 2009 meta-analysis found that among five well-designed studies measuring calcium balance, net acid excretion was not associated with either decreased calcium balance or a marker of bone deterioration (Fenton 2009).
What Causes Osteoporosis?
If animal protein doesn’t cause osteoporosis, what does?
Genetics likely play a strong role in osteoporosis, and, possibly related, estrogen levels in women. Among factors that can be controlled by lifestyle and diet, there has been evidence for the following:
- Weight-bearing exercise throughout one’s lifetime
- Higher body weight
- Adequate intake of calcium, vitamin D, vitamin K, phosphorus, potassium, magnesium, and boron
- High sodium and caffeine intake
- Too little protein
- Excessive vitamin A (retinol, not beta-carotene)
- Possibly vitamin B12 deficiency
Calcium and Vitamin D
Some background: When calcium levels in the blood drop, parathyroid hormone (PTH) is released. PTH causes calcium to be released from the bones, thus raising the low calcium levels in the blood. Osteoporosis may result from chronically high levels of PTH. The conversion of 25-hydroxyvitamin D to calcitriol is also somewhat regulated by PTH levels (Groff 2000). Calcitriol increases absorption of calcium and phosphorus (another major component of bones) from the intestines and decreases their excretion in the urine. In so doing, calcium levels in the blood rise and PTH levels drop.
Research has shown that, on average, Americans are getting enough calcium. A 2003 report from the Nurses Health Study showed vitamin D to be more important than calcium intake for preventing hip fractures in postmenopausal women (Feskanich 2003). In 2007, a meta-analysis of prospective studies and randomized controlled trials found that calcium intake and calcium supplements were not associated with a lower risk for hip fractures (Bischoff-Ferrari 2007).
Vitamin B12 and Bone Mineral Density
Taking vitamin B12 might also be important for bone mineral density.
Low vitamin B12 status can result in elevated homocysteine which might harm bones. A cross-sectional study from Slovakia compared lacto-ovo-vegetarian women to omnivores (Krivosikova 2009). The vegetarians had lower B12 levels (246 vs. 302 pmol/l) and higher homocysteine levels (16.5 vs. 12.5 µmol/l). In the four bone locations examined, vegetarians had lower bone mineral density than omnivores in two locations, and their higher homocysteine levels were negatively associated with bone mineral density in three locations. The bone mineral density associations appeared not to be adjusted for body mass index which, while not statistically different between the vegetarians and omnivores (23.9 vs. 25.0, respectively), is an important factor impacting bone mineral density.
A cross-sectional study from Germany found that male vegans and lacto-ovo-vegetarians had similar markers of bone turnover to each other, but higher markers than male omnivores (Herrmann 2009). The vegans had a rather high average methylmalonic acid level of 727 nmol/l, indicative of significant vitamin B12 deficiency. None of the participants were taking B12, calcium, or vitamin D supplements. Upon stratifying the subjects into two groups based on their B12 status, the group with poor B12 status had higher markers of bone turnover. However, since almost all the people in the poor B12 status group were vegan or vegetarian (62 out of 63), and few vegans or vegetarians were in the replete B12 group (6 out of 66), it’s not clear that the increased bone turnover markers were due to poor B12 status or some other factor common to vegans and vegetarians.
Selenium and Bone Mineral Density
In a prospective study from the U.K. of 1,144 post-menopausal women with normal thyroid function, higher selenium levels were associated with greater bone mineral density (Hoag, 2012). Although selenium metabolism is involved in thyroid function, the study found that the associations between selenium and bone mineral density were independent of thyroid function. After 6 years of follow-up, selenium levels were not associated with bone fractures. Because selenium intakes can be low in vegan diets in some countries, it’s important that vegans in those countries ensure an adequate source (see Selenium).
Research on Vegans
Bone Mineral Density
In 2009, researchers from Vietnam and Australia did a meta-analysis looking at the bone mineral density of vegetarians (Ho-Pham 2009b). They concluded that “[T]here is a modest effect of vegetarian diets, particularly a vegan diet, on [bone mineral density], but the effect size is unlikely to result in a clinically important increase in fracture risk.”
A Denmark study found that vegans had higher levels of some, but not all, markers of bone turnover, even though they had relatively similar calcium and vitamin D intakes to omnivores (Hansen 2018). It’s not clear if these markers were of clinical significance.
In 2007, the first study looking at vegan bone fracture rates was released (Appleby 2007). The EPIC-Oxford study recruited 57,000 participants, including over 1,000 vegans and almost 10,000 lacto-ovo vegetarians, from 1993 to 2000. They were asked to fill out a questionnaire to measure what they ate. About 5 years after entering the study, they were sent a follow-up questionnaire asking if they had suffered any bone fractures.
After adjusting for age alone, the vegans had a 37% higher fracture rate than meat-eaters (RR 1.37, CI 1.07–1.74). After adjusting for age, smoking, alcohol consumption, body mass, physical activity, marital status, births, and hormone replacement, vegans had a 30% higher fracture rate (RR 1.30, CI 1.02-1.66). Meat-eaters, fish-eaters, and lacto-ovo fracture rates did not differ in any of the analyses performed.
When the results were adjusted for calcium intake, the vegans no longer had a significantly higher fracture rate (RR 1.15, CI .89-1.49). And among the group of subjects who got at least 525 mg of calcium a day (only 55% of the vegans compared to about 95% of the other diet groups), vegans had the same fracture rates as the other diet groups (RR 1.00, CI .69-1.44). The study didn’t give the average calcium intake of those vegans, but it was possible to calculate that at a minimum, their average calcium intake was 640 mg. The authors noted that fracture rates did not correlate with protein or vitamin D intake.
A separate analysis from EPIC-Oxford showed that among all participants (regardless of diet group), calcium intake was related to an increased fracture risk in women (relative risk 1.75 (1.33-2.29) for < 525 mg/day compared to > 1200 mg/day), but not in men (Key 2007).
Adventist Health Study-2 (2013)
Adventist Health Study-2 (AHS-2) published a report of caucasian Seventh-day Adventists living in the USA, comparing the intakes of many different foods and hip fracture risk after 5 years of follow-up (Lousuebsakul-Matthews 2013). The point of the study was not to determine the difference in fracture rates between vegetarians and non-vegetarians and they did not test for statistical significance. However, there was a strong trend towards higher fracture rates in vegans (3.0 per 1,000 person-years) compared to 2.0 for non-vegan vegetarians (including semi-vegetarians) and 1.6 for non-vegetarians.
The study found that eating meat alternatives once a day or more (compared to less than once per week) was associated with a 66% reduced risk of hip fracture in the vegetarians (.34, .12-.95). Even more strongly, eating legumes once a day or more (compared to less than once per week) was associated with a 55% reduced risk in vegetarians (.45, .22-.94).
The authors state, “Protein is recognized for its ability to improve [calcium] balance, suppress parathyroid hormone, increase lean body mass and increase production of the bone growth regulator insulin-like growth factor-1.”
Buddhist Nuns (Vietnam, 2011)
In 2011, a follow-up (Ho-Pham 2011) of an earlier study on vegan Buddhist nuns (Ho-Pham 2009a) was released. After two years, the vertebrae of 88 vegans and 93 omnivores were examined using x-rays. Ten women (five vegans and five omnivores) had sustained a new vertebral fracture after two years; there was no significant difference between the two groups.
Rates of bone mineral density (BMD) change were examined at the lumbar spine and femoral neck with a variety of associations found. Lumbar BMD increased with age, lean body mass, and vegetable fat; and decreased with vegetable protein and steroid use. The authors suggested that the increase in BMD of the lumbar spine was possibly due to osteoarthritis and, therefore, not a healthy phenomenon.
Femoral neck BMD increased with both lean and fat body mass and decreased with age, animal fat, and the ratio of animal protein to vegetable protein. This would indicate that animal protein had a negative impact on bone.
Food questionnaires indicated that the vegans were only eating an average of 1,093 calories, 36 g of protein, and 360 mg of calcium per day. The estimated energy requirement for women their age and size is about 1,600 calories indicating possible underestimation of the questionnaire results. The non-vegan nuns had intakes of 1,429 calories, 62 g of protein, and 590 mg of calcium per day which seems more likely.
Ten fractures in 181 women in two years is high. The rate of vertebral fracture in women over 65 in Hong Kong and Japan is 594/100,000 person-years (Ho-Pham 2009a). The fracture rate in this Buddhist nun study works out to be 2,762/100,000 person-years. The fractures in the Buddhist nun study were determined by x-raying each subject, rather than reporting a bone break, and would likely find more fractures than the study determining the 594/100,000 rate.
In summary, compared to non-vegetarian Buddhist nuns, vegan nuns had a similar rate of vertebral fractures, but it appears that the rates for both groups were relatively high.
Calcium Absorption from Plant Foods
For calcium amounts and absorption rates of various plant foods, please see the table Calcium and Oxalate Content of Foods. Most of the calcium amounts in that table come from the USDA nutrient database, which lists an average of the samples they’ve analyzed. The amount of calcium in plant foods is somewhat dependent on the amount of calcium in the soil (Weaver 2013).
The DRI for calcium is roughly based on an average absorption of calcium from foods of 25% (p. 38 in reference DRI Dietary Reference Intakes 2011). This means adults should aim to absorb about 250 mg of calcium per day.
As a calcium dose increases, the percentage absorbed decreases. Beyond the size of the dose, the oxalate level in a plant food is the main determinant of how much calcium can be absorbed.
See Calcium and Oxalate Content of Foods for how much calcium is absorbed from various plant foods, along with references and details for this summary of calcium in plant foods:
- Studies have shown that calcium in fortified soymilk, bok choy, kale, and mustard greens is absorbed well.
- Based on oxalate levels, the calcium in turnip greens, watercress, and broccoli should also be absorbed well.
- Based on oxalate levels, the calcium in collards should be absorbed moderately well.
- Studies have shown that the calcium in spinach and rhubarb is not absorbed well.
- Based on oxalate levels, the calcium in beet greens and swiss chard should not be absorbed well.
- Many non-dairy milks are now fortified with calcium, vitamin D, and/or vitamin B12.
Many orange juices are fortified with calcium.
- Fortified beverages should be well-shaken to make sure the calcium has not settled to the bottom of the carton.
- Calcium supplements can inhibit iron absorption if eaten at the same time.
- In addition to the calcium in the leafy greens listed on the right, leafy greens also contain
vitamin K, which is good for bones.
- The Daily Value for calcium on food labels is 1,000 mg. Therefore, if a
food label says it has 25% of the daily value, it means it has 250 mg of calcium per
Dangers of Calcium Supplements
Some research has linked calcium supplements with an increased risk of cardiovascular disease. These studies have mostly found a link in people getting well over the DRI for calcium (more info), and with the effect stronger in smokers.
A 2013 study from Sweden found that there was no increased risk of up to 1,300 mg of calcium per day, but a significantly increased risk for the group getting ≥ 1,400 mg per day (Michaëlsson 2013).
Another study looking at calcification of the arteries found a cross-sectional correlation at baseline, but no prospective association (Wang 2010).
The Framingham Study measured calcium intakes and followed participants for four years at which time they measured the amount of calcification of their arteries (Samelson 2012). They found no correlation or trends with calcium intake and calcification of the arteries in amounts up to about 3,000 mg per day in either men or women or for calcium supplements of 500 mg per day or more compared to no supplements or up to 500 mg per day. See also No Need to Worry About Calcium and Your Heart from the Tufts Health & Nutrition Letter (November, 2019).
To be safe, it might be best not to increase calcium intake above 1,400 mg per day. People with chronic kidney disease should talk to their physicians about the pros and cons of calcium supplements.
Colorectal cancer is another concern with regard to calcium supplements. As described in Calcium Supplements and Colorectal Cancer, one study found that long-term use of a 1,200 mg per day calcium supplement was associated with an increased risk of one type of polyp that can lead to colorectal cancer. This increased risk with higher calcium intakes was not seen in other studies although the other studies didn’t looked at total calcium intake rather than supplements specifically. The authors of this study suggest that those with serrated polyps of the colon or rectum, especially women and smokers, avoid calcium supplements (Crockett 2018).
There’s been concern that taking calcium supplements can increase the risk of kidney stones, but for the most part, research has shown that taking calcium supplements with meals can actually reduce the risk of calcium-oxalate kidney stones, the most common form of stone. For more information, see the article Oxalate.
Conclusion on Calcium and Vegan Diets
There is no reason to think that vegans are protected from osteoporosis more than other diet groups, and they should strive to meet calcium recommendations. Although it is possible to meet the calcium recommendations by eating greens alone (see chart), the average vegan probably will not meet recommendations without drinking a glass of fortified drink each day, eating calcium-set tofu, or taking a 250 – 300 mg supplement (in addition to eating an otherwise balanced diet). Although it is important to get enough calcium, do not get more than 1,400 mg of calcium per day.
Last updated: December 2020
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