The human body is a fine-tuned organism that works best only under certain conditions and is very sensitive to any changes in the inner environment. One of the most important characteristics of the body is a stable acid-alkali balance in the blood. The body neutralises any excesses of either acid or alkali to protect this vital balance but if there is too much acid, other systems in the body, such as the skeleton, can suffer. If there's too much acid and calcium from the diet isn't enough to neutralise it, the body needs to draw on its calcium reserves in the muscles and bones. Some of the calcium is then deposited back to the bones but most of it is excreted in the urine together with the acids.

Diet influences the acid–alkali balance in the body and dietary data can be used as an estimate for endogenous acid production – ie the amount of acid produced in the body as a result of the food eaten. Some dietary factors contribute to dietary acid load more than others. Sulphur from sulphur amino acids (protein building blocks) is the main contributor because it is metabolised into sulphuric acid. Sulphur amino acids are highest in animal protein and therefore diets high in animal protein are likely to produce considerable amounts of acid in the body. Another contributor is phosphorus, which is mainly supplied by meat and dairy products. Potassium and magnesium, abundant in plant foods, and calcium, found in plant foods and dairy products, are determinants of alkaline load (Alexy et al., 2007).

A large, seven-year study of 1,035 women found that women with diets high in animal and low in plant protein had an almost four times higher rate of bone loss and their risk of hip fracture was 3.7 times that of women who consumed the least animal protein (Sellmeyer et al., 2001). Findings from another large study of women also showed a clear division – high intake of animal protein caused bone loss while high intake of vegetable protein did not and even contributed to increased bone density (Weikert et al., 2005).

Adequate calcium intake is important but it’s the overall diet that matters. It is a known fact that countries with the highest calcium and animal protein intakes also have the highest fracture rates. The World Health Organization calls this calcium paradox and states: “The paradox (that hip fracture rates are higher in developed countries where calcium intake is higher than in developing countries where calcium intake is lower) clearly calls for an explanation. To date, the accumulated data indicate that the adverse effect of protein, in particular animal (but not vegetable) protein, might outweigh the positive effect of calcium intake on calcium balance.” (WHO/FAO, 2003)

A large-scale study on calcium intake and bone health found that calcium intake above 750 mg a day didn’t offer any protection from fractures and high calcium intakes increased the risk of hip fractures (Warensjö et al., 2011). This study involved over 60,000 women whose diets and health were followed for up to 19 years.

The UK recommended daily intake of calcium is 700 mg for adults and the latest studies revealed that vegans get enough calcium from their diet to meet this recommendation. See chapter Nutrition Status of Vegetarians and Vegans for more information.

An extensive review by Lanou (2009) resulted in a conclusion that: “bones are better served by attending to calcium balance and focusing efforts on increasing fruit and vegetable intakes, limiting animal protein, exercising regularly, getting adequate sunshine or supplemental vitamin D, and getting  500 mg calcium a day from plant sources.”

A number of studies investigating the impact of diet on bone health have demonstrated the beneficial effect of fruit and vegetables on bone mass and bone metabolism in men and women across all age ranges. For example:

  • Adolescent girls consuming more than three servings of fruit and vegetables had healthier and better bones and were losing less calcium in urine than girls consuming less (Tylavsky et al., 2004).

  • A population-based study examined the association between fruit and vegetables intake and bone mineral density in 670 postmenopausal Chinese women (Chen et al., 2006). Analyses showed that high intake of fruit and vegetables was significantly associated with greater bone density at all locations measured.

  • A project based at Human Nutrition Research, Cambridge, UK analysed a series of studies examining the association between fruit and vegetable consumption and bone mineral density (Ashwell et al., 2008). Significant associations were observed between bone health markers (measurements of bone mineral density, bone resorption, loss of calcium in urine etc) and carotenoids (plant pigments) and vitamin E which suggested a positive effect of fruit and vegetable intake on bone health.

  • The Singapore Chinese Health Study enrolled over 63,000 men and women aged 45-74 years between 1993 and 1998 in Singapore (Dai et al., 2014). Their diet has been repeatedly assessed over the years and two dietary patterns have been identified - the vegetable-fruit-soya pattern (mostly cruciferous vegetables, fruit and tofu items) and the meat-dim-sum (meat, processed food). The study also divided each of these patterns into different levels according to what they ate and the results showed that people who ate the most fruit, vegetables and soya had a 34 per cent lower risk of hip fracture compared to people who ate the least. The observed relationship appeared to be direct - the more of the healthy, plant-based foods people ate, the lower their risk of fracture and vice versa.

  • The journal Osteoporosis International published a review of studies on bone health and acid-alkali balance in the body (Lambert H et al., 2015). This review looked at studies where people were given alkaline salts (that naturally occur in fruit and vegetables) to supplement their diet and analysed the results. The researchers found that increasing the intake of alkalis meant reduced losses of calcium through urine and lower rate of bone degradation. They concluded that potassium salts (alkalis) have the potential to prevent osteoporosis and recommend an increased consumption of fruit and vegetables as a means to improve bone health (fruit and vegetables contain potassium and create alkalis during digestion).

A very interesting study of over 150 vegans looked at their diets in relation to bone health (Ströhle et al., 2011). The researchers evaluated participants’ diets individually in terms of their acidifying or alkalising effect on the body. They used two models to work this out to be more precise. The results, regardless of the model used, revealed that vegan diets are characterised by a virtually neutral acid-alkali balance which is very desirable. Calcium intake in vegans was more than 800 mg which is above UK recommendation but lower than in average omnivores. The study authors suggested that given the fact that the low acidifying effect of vegan diets lowers urinary calcium loss, vegans may require less calcium than omnivore adults. In addition, higher phosphate intake is associated with increased calcium retention and the phosphate content of vegan diets could result in lower calcium requirements in vegans.

Ho-Pham et al. (2012) tested bone mineral density of vegan Buddhist nuns and omnivores, their fracture incidence and closely examined their diet. The results were very reassuring – over the two-year study period, vegans maintained bone mineral density much better than omnivores. With age, bone mineral density naturally decreases but omnivores in this study had twice the bone loss of vegans. The study also showed that low body weight, higher intakes of animal protein and fats and corticosteroid use were associated with greater rates of bone loss. What was of specific interest was that calcium intake of the study participants was relatively low, but it did not have adverse effect on bone loss. Indeed, the average dietary calcium intake among the vegan nuns was only 375 mg a day whilst the intake in non-vegetarians was 683 mg, and yet, based on the results, the vegans had better bone health.

There’s plenty of evidence now that a vegan diet not only provides sufficient calcium but that it also ensures the intake of many bone-beneficial nutrients and minimises the potential damage to bones caused by dietary acid load.

For references see The Incredible Vegan Health report

For practical guidance on how to improve your bone health through diet or to see Viva!Health's in-depth report on diet and bone health, see our Break Free resources.