Bone health

Summary

Osteoporosis is a serious problem in the UK, where one in two women and one in five men over 50 experience fractures, mostly as a result of low bone strength. The highest rates are seen in the countries that consume the most animal foods. As the consumption of meat and dairy foods increases, so do fracture rates.

The Singapore Chinese Health Study found that a diet rich in meat and refined starchy foods was linked to a much higher risk of hip fracture than a traditional diet, characterised by vegetables, fruit and soya foods.

There is little evidence that increasing calcium intake will prevent fractures. In fact, research shows that the more dairy and animal protein is consumed, the higher the incidence of osteoporosis. The acid-alkaline hypothesis offers a plausible explanation as to how this occurs and although it has been challenged, a high dietary acid load (as seen in a typical Western diet), can increase the acidity of the blood.

The pattern of incidence of osteoporosis around the world certainly suggests that some aspect of the typical Western lifestyle is a significant contributing factor to bone loss. Furthermore, the evidence shows that diets rich in animal protein are harmful to bone health It seems likely that meat is as damaging, if not more so, than milk as it doesn’t even contain calcium – and of course plant-based sources of calcium are best..

Research shows that physical activity (especially weight-bearing) is the most critical factor for maintaining healthy bones, followed by improving diet and lifestyle; this means eating plenty of fresh fruit and vegetables, and cutting down on caffeine and avoiding alcohol and smoking. For more information see Viva!Health’s fully-referenced scientific report Break Free – How to Build Healthy Bones and What Really Matters in the Prevention of Osteoporosis at: www.vivahealth.org.uk/bones

Osteoporosis (meaning porous bones) occurs when calcium is lost from the bones and they become more fragile and prone to fracture. It is sometimes called the silent disease as there are often no symptoms until a fracture occurs. Hip fracture constitutes the most serious complication of osteoporosis and accounts for the majority of fracture-related deaths and healthcare costs among individuals over 50. In the UK, one in two women and one in five men over 50 experience fractures, mostly as a result of decreased bone density (National Osteoporosis Society, 2016).

Many risk factors for osteoporosis have been identified including a low BMI, low bone mineral density, reduced sunlight exposure (essential for vitamin D production in the skin), early menopause, smoking, alcohol consumption, low physical activity levels and obesity – so being underweight or overweight can increase the risk.

In 2012, the WHO Collaborating Centre for Metabolic Bone Diseases, at the University of Sheffield Medical School in the UK published a review of hip fracture incidence worldwide (Kanis et al., 2012). They observed a greater than 10-fold variation in hip fracture risk between countries. The highest levels were seen in North Western Europe (Iceland, UK, Ireland, Denmark, Sweden and Norway) through to central Europe (Belgium, Germany, Austria, Switzerland and Italy) to the south east (Greece, Slovenia) and onwards (to the Lebanon, Oman and Iran). Other high-risk countries for women were Hong Kong, Singapore, Malta and Taiwan.

Regions of moderate risk included Oceania (the islands of the tropical Pacific Ocean, Australia and New Zealand), the Russian Federation, the southern countries of Latin America and North America. However, if you separate the ethnic groups in the US, then Hispanic, Asian and Black populations would be classed low-risk and Caucasian women, high risk.  

Low-risk regions included the northern regions of Latin America, Africa, Jordan and Saudi Arabia, India, China, Indonesia and the Philippines.  

In general, fracture rates are highest in Caucasian women living in temperate climates and are lower in women from Mediterranean and Asian countries and lowest still among women in Africa. Countries in economic transition, such as Hong Kong, have seen significant increases in fracture rates in recent decades (WHO, 2003). The incidence of hip fracture is escalating worldwide and 50 per cent of the total hip fracture incidence is projected to occur in Asia by 2050 (Dai et al., 2014). This shows that environmental factors, such as diet, are responsible.

This view is supported by changes in fracture risk in immigrant populations. For example in the US, black Americans have a lower fracture risk than Caucasians, but a much higher risk than black Africans. A similar scenario is seen among the Japanese population of Hawaii compared to those in Japan and Chinese people living in Singapore compared with mainland China (Kanis et al., 2012).

The role of diet was revealed in the Singapore Chinese Health Study which investigated dietary patterns and fracture risk among 63,257 Chinese men and women (Dai et al., 2014). Two distinct dietary patterns were identified: the vegetable-fruit-soy pattern, characterised by vegetables, fruit and soya foods, and the meat-dim-sum pattern, rich in meat and refined starchy foods. Results showed that compared to the meaty diet, the Chinese diet rich in vegetables, fruit and soya products was associated with a substantially lower risk of hip fracture.

In The China Study, one of the most comprehensive nutritional studies ever undertaken, Professor T. Colin Campbell said that there is little evidence to show that increasing calcium intake will prevent fractures. In fact, research is moving in the opposite direction, showing that the more dairy and animal protein that is consumed, the higher the incidence of osteoporosis (Campbell and Campbell, 2005).  Unfortunately, most medical advice focuses on calcium intake rather than looking for the reasons for calcium loss which include salt, caffeine, tobacco, lack of exercise and maybe alcohol as well as animal protein.

The acid-alkaline hypothesis

The hypothesis that a high animal protein diet could be a risk factor for osteoporosis dates back to research conducted more than 40 years ago (Barzel and Jowsey, 1969). The hypothesis proposes that as food is digested, acids are released into the blood and the body tries to neutralise the acid by drawing calcium from the bones. This calcium is then excreted in the urine (the calciuric response). Animal proteins from meat, dairy, fish and eggs are thought to have a particularly bad effect because of the greater amount of sulphur-containing amino acids they contain compared to most plant proteins. Sulphur-containing amino acids give rise to sulphuric acid when they are broken down in metabolism.

We are probably better adapted to the type of diet our ancestors were exposed to during millions of years of evolution than to the diet we have been eating since the agricultural revolution 10,000 years ago – or since industrialisation only 200 years ago (Frassetto et al., 2001). Modern diets are more acid-forming than the alkalising foods that would have been consumed. Consider a cheeseburger with fries and a fizzy drink compared to nuts, seeds, fruit, leaves and water with the occasional piece of meat and/or fish… Consequently, humans are not adapted to contemporary acid-forming diets which contribute to modern epidemics of chronic disease (Scialla and Anderson, 2013).

A substantial body of evidence links animal protein to a decrease in bone mineral density. A study, looking at hip fracture incidence in 33 different countries in relation to consumption of plant and animal protein, found that the countries with the lowest fracture rates also had the lowest intakes of animal protein (Frassetto et al., 2000). In 10 of the 11 countries with the highest fracture rates, animal protein intake exceeded plant protein intake. The authors said that hip fracture incidence is directly related to animal protein intake and suggested that bone integrity is compromised by acid that results from the metabolism of animal protein. They suggested that the moderation of animal food intake, coupled to an increased ratio of vegetable to animal food consumption, may confer a protective effect.  

Another study of 1,035 elderly women found that those with a high ratio of animal to vegetable protein intake had a greater risk of hip fracture compared to those with a low ratio (Sellmeyer et al., 2001). A further study of 757 young girls in urban Beijing in China, compared the effects of protein intakes from animal and plant sources on bone mass accrual over five years (Zhang et al., 2010). Results showed that protein from animal foods, particularly meat, had negative effects on bone mineral content

Another study compared the effects of animal and plant protein in the diets of overweight and obese postmenopausal women dieting (Campbell and Tang, 2010). They found that the energy-restricted diet with meat promoted bone loss compared with the energy-restricted diet without meat. They warned that for postmenopausal women trying to lose weight, choosing a diet containing meat may reduce bone mineral density and increase the risk of osteoporosis. This extends the findings of an earlier study which examined the levels of bone loss in 1,600 older women and found that vegetarians had lost only 18 per cent bone mineral compared to omnivores who had lost 35 per cent bone mineral by the age of 80 (Marsh et al., 1988).

So, for children and adolescents, while a good protein intake is important for bone development, research suggests that large intakes of animal protein may counter this positive effect. In a study looking at long-term protein intake, dietary acid load and bone status in children, it was concluded that the positive effect of protein could be negated, at least partly, by a high renal acid load (Alexy et al., 2005). These findings support the health benefit of a diet rich in alkali-yielding fruit and vegetables and the authors recommend an integrative approach saying that focusing on single nutrients is not sufficient. Other studies showing that animal protein-based diets with the same amount of protein as a vegetarian diet can increase the risk for uric acid tones (Breslau et al., 1988) have led some to suggest that high calcium losses in the urine caused by animal protein may be a risk factor for the development of osteoporosis.

A number of studies have examined the role of the dietary acid load in people with chronic kidney disease. The evidence supports a direct relationship between a high dietary acid load and chronic kidney disease progression, bone loss and sarcopenia (loss of skeletal muscle). However, due to a wide variety of techniques and terminology used to quantify the dietary acid load, this theory is not widely appreciated by nephrologists (Scialla and Anderson, 2013). A number of critical reviews of the acid-alkaline hypothesis have been published (Darling et al., 2009; Fenton et al., 2009; Fenton et al., 2011). These reviews argue that a causal association between dietary acid load and osteoporosis is not supported by the research.

One critic argues that if bone is the main source of calcium from which diet-related acid is buffered, all the bone in the body would be dissolved in just a few years (Bonjour, 2005). It is also argued that homeostatic mechanisms (‘housekeeping’ systems that attempt to keep everything running normally) including renal acid excretion, would not permit a steady-state low-grade metabolic acidosis caused by a typical Western diet. In other words, the body has ways of redressing the balance when, for example, the diet increases acid levels in the blood, and even small increases are countered by these mechanisms – well that’s the theory anyway. However, it has been demonstrated that a high dietary acid load, which lies within the ranges seen in a typical American or European diet, can increase the acidity of the blood (Frassetto and Sebastian, 2013). So, on the one hand we are told that we can compensate for the acidifying effects of a high-protein diet, while on the other hand, the research shows that we may not be able to balance it out completely. It may be that the truth lies somewhere between these two apparently irreconcilable arguments.

Buffers are chemical substances that can minimise changes in a liquid when it becomes more acidic or alkaline. To maintain equilibrium whilst there is an increased amount of acid in the body, at least three compensatory responses are activated: buffering from the bone (and to some degree skeletal muscle), increased ventilation to eliminate carbon dioxide, and in the kidney, bicarbonate is generated and reabsorbed into the blood while excess hydrogen ions are secreted into the urine. In healthy people, these buffering systems all have a tremendous capacity to maintain the blood pH (acid-alkali balance) within a very narrow margin (Kerstetter, 2009). However, the major reservoir of alkalis (in the form of alkaline salts of calcium) is the skeleton, which provides the buffer needed to maintain blood pH and plasma bicarbonate concentrations (Pizzorno et al., 2010). While kidney metabolism represents a major mechanism by which metabolic acid loads are handled by the body, if the kidneys are overwhelmed or compromised (kidney function declines with age), calcium from the bones may be called on to compensate for the increasingly acidic environment and an alkalising diet could help redress the balance (Dargent-Molina et al., 2008; Frassetto and Sebastian, 2013). So, under certain conditions, the acid-alkaline hypothesis may provide a plausible mechanism in which a vegan diet, rich in fruit and vegetables, could help promote and preserve bone health. This may go some way in explaining the apparently contradictory evidence concerning animal protein, meat and bone health.

The acid-alkaline hypothesis has also been challenged on the basis of a series of short-term experimental studies suggesting that high-protein diets are not harmful to bone health and might actually be beneficial by improving calcium absorption (Kerstetter et al., 2003). However, while high-protein diets may increase calcium absorption, they also increase calcium excretion in the urine. Therefore, the positive effects of protein intake on calcium absorption may only be beneficial under conditions of adequate calcium intake (Mangano et al., 2014).

Growing evidence suggests that calcium and protein may interact in terms of bone health and that the potential harmful effect of a high-protein diet may only be compensated for if there is an adequate calcium intake (Weikert et al., 2005). If there is insufficient calcium in the diet to counter the calciuric effect, calcium may be lost from the bone. At the same time, high calcium intake may have adverse effects (tissue calcification, kidney stones) so is not desirable either.

One study (of just 13 participants) compared a moderate animal protein intake to a high animal protein intake diet (1g per kg body weight compared to 2.1g per kg – the generally accepted daily protein dietary allowance is 0.8g per kg of body weight) and found with 800mg of calcium per day, all participants ended up in negative calcium balance (Kerstetter et al., 2005). This was not anticipated and the authors suggested that that much calcium was not enough to maintain calcium balance. However, because the extra urinary calcium lost in the high-protein diet was found to come from the diet (as opposed to the bone), the authors concluded that, at least in the short term, high-protein diets are not detrimental to bone health.

While, the increased amount of urinary calcium produced by the high-protein diet may reflect enhanced calcium absorption and not bone resorption, under both the moderate and high protein diets tested, the vast majority (over 90 per cent) of the calcium found in the urine still came from the bones. Adult bones constantly undergo remodelling through bone resorption by osteoclasts and bone formation by osteoblasts. In adults, almost the entire human skeleton may be remodelled over a 10-year cycle. While this study suggests that higher protein intakes may not be harmful in the short term, it should be remembered that they can lead to kidney problems and increased levels of IGF-1 which are linked to certain cancers. The long-term impact of high-protein diets on bone health is still unclear and the relative contribution of calcium from the bone and/or diet to protein-induced calciuria remains controversial.

Other studies suggest that high-protein diets may increase calcium absorption and help preserve bone mass by stimulating IGF-1, a potent bone growth stimulator (Mangano et al., 2014). However, increased IGF-1 levels are linked to an increased risk of certain cancers, so high animal protein diets are not desirable and should not be recommended.

This is a complex issue with a wide range of factors involved, not least the role of the kidney. It should be noted that people who consume a high-animal protein diet have an increased risk of kidney disease and continuing to consume high levels of meat, eggs and dairy foods may present a burden on an already overworked kidney. It seems logical that the harder you make the kidneys work, the more likely they are to struggle to meet the challenge. There is a consensus that in people with kidney disease or poor kidney function (resulting from aging), a high dietary acid load may result in acidosis which may then lead to bone and muscle loss.

One study found that a diet incorporating increased fruit and vegetable intake produced substantially less acid than the typical American diet rich in meat and dairy (Scialla and Anderson, 2013). They suggested that in chronic kidney disease and aging, a high dietary acid load (caused by a higher intake of meat, eggs, cheese and cereal grains and a lower intake of fruits and vegetables) may result in low-grade, subclinical acidosis. In these circumstances, efforts to maintain stable blood pH and boost acid excretion from the kidney may lead to bone and muscle loss and further decline in kidney function, but this may be mitigated by alkali. In other words, this unnecessary strain on the kidneys could be avoided simply by eating less acid-forming foods. They concluded more studies are needed to determine the degree of benefit of a foods-based approach to reducing the dietary acid load in patients with early to moderate chronic kidney disease.

The acid-alkaline hypothesis remains controversial. Currently, research that supports both the proponents and opponents of the hypothesis exist (Frassetto and Sebastian, 2013). The pattern of incidence of osteoporosis around the world certainly suggests that some aspect of the typical Western lifestyle is a significant contributing factor to bone loss. Furthermore, the evidence shows that diets rich in animal protein are harmful to bone health but more research is needed. In the meantime, it seems sensible to observe how the Western diet is accompanied by the so-called Western diseases including osteoporosis and limit, if not eliminate, all meat from the diet.

Research suggests that physical activity or exercise (especially weight-bearing) is the most critical factor for maintaining healthy bones, followed by improving diet and lifestyle; this means eating plenty of fresh fruit and vegetables, and cutting down on caffeine and avoiding alcohol and smoking.