We focus on supplementing our diets with calcium, but we don't stop to question why so much of the calcium we consume is wasted. Normally, we only absorb about 10% of the calcium we ingest (1). A focus on the dynamics that impact on calcium absorption may therefore be more profitable for those who are concerned about maintaining and building healthy bones.
Among the factors that influence calcium absorption are three major regulators of calcium metabolism in the body. The parathyroid glands produce a hormone (PTH) which moves calcium from the bones into the bloodstream. It also signals the kidneys conserve calcium and other minerals from the urine. Additionally, PTH signals the kidneys to produce calcitriol, which is formed from vitamin D, and which, among other functions, signals the small intestine to absorb more calcium (2). The thyroid gland secretes calcitonin, which increases bone mineralization, and decreases the rate at which the bone is broken down.
Any factors which interfere with, or alter the delicate balance maintained by these body systems are likely to have a negative impact on bone mineralization. The following twelve items, many of which stem from our modern diet, are likely, sometimes surprising culprits interfering with calcium metabolism:
1. A diet high in phytic acid, which is found in the bran of whole grains, is likely to interfere with calcium absorption. This acid binds to a variety of minerals including calcium, to form insoluble salts, called phytates, which are wasted from the body. Probably because grains are a relatively new food, from an evolutionary perspective, it appears that we have not yet developed digestive tracts which can break down these phytates(3).
2. A diet high in sodium may also interfere with calcium absorption. Some researchers believe that dietary sodium levels were extremely low in the past, compared to modern diets (4) and increased sodium intake can result in increased calcium excretion (5).
3. Vitamin D is formed by an interaction between the sun's rays, and skin oils. Without supplementation, we may be at risk of inadequate vitamin D, if we spend most daylight hours inside. As mentioned earlier, this vitamin is a regulating factor in calcium absorption. Circulating levels of vitamin D throughout the year were probably higher in stone age men and women because they were more likely to be exposed to the sun's rays. There is evidence suggesting that many elderly in Western countries are deficient in vitamin D (6). Treating every elderly lady in northern latitudes with vitamin D tablets is probably not a bad idea.
4. Our sedentary life ways also interfere with mineralizing our bones. Our ancestors were probably much more active than we are. Impact stress on bone, as in walking and jogging, tends to increase production of calcitonin, which leads to increased deposition of calcium in the bones (7). Exercise induced stress increases the cross sectional area and perhaps (equivocal data) bone mineral density (8). It is important to note that cycling and swimming are not as beneficial because these activities are not as likely to cause calcitonin production.
5. While the modern diet, in the western world, usually contains ample calcium, it may offer inadequate magnesium. Studies of our ancestors' pre-agricultural diets indicate that magnesium was probably consumed at about a 1:1 ratio with calcium (source note?). Thus, that would be the approximate ratio our bodies evolved with. As the Ca:Mg ratio is 12:1 in dairy, those consuming this much vaunted source of calcium might only experience the equivocal benefit that is reported in the medical literature(9). The a:Mg ratio in post-agricultural diets is about 4:1 (10). Because both calcium and magnesium compete for the same absorption mechanisms, the imbalanced intake associated with our modern diet may well lead to magnesium deficiency. One feature of magnesium deficiency is the inhibition of osteoblasts which are cells that build and maintain bones. One of the authors (Ron) increased his bone density significantly by taking magnesium supplements alone.
6. While there is no evidence that high levels of meat consumption are detrimental, as long as there are adequate minerals in the diet, there are reports of isolated, fractionated animo acids from milk or eggs, which have been found to correlate with calcium loss. Some suggest that we should be especially leery of whey protein.
7. Sugar has been implicated in upsets to the calcium/phosphorus balance. Melvin Page reported that sugar increases blood calcium through inciting reabsorbtion from bone tissue (11). A recent rat study is of some relevance. Although sugar was not implicated in osteoporotic changes, Li et al. found clear indications of depleted bone density in their animal study of dietary sugar (12).
8. Phosphoric acid, as found in soft drinks, has also been argued to interfere with calcium absorption (13). In order to utilize calcium well, in addition to vitamin D, we need 1.5 grams phosphorus and 0.5 grams magnesium for each gram of calcium. We normally get plenty of phosphorus; frequently more than enough. Although we need phosphorous, it is abundant in our diets, and increased intake drives up our requirements for calcium and magnesium (14) so it is important to limit soft drinks as they contain considerable quantities of phosphorous and sugar.
9. Coffee reduces inositol levels in the blood. Inositol is a regulating factor in calcium metabolism. It inhibits formation of the cells (osteoclasts) that draw calcium from the bones into the bloodstream (15). It also exerts a direct influence on transport of calcium into cells (16,17).
10. Although the dynamic is not well understood, tobacco smoking also appears to interfere with bone mineralization. Some researchers report that osteoblast formation is inhibited by nicotine (18). Study of animals given chronic doses of nicotine support this perspective by demonstrating reductions in bone mass (19). Studies of large groups show reductions in bone mass of smokers, but there is some debate as to the cause (20).
11. Undiagnosed celiac disease is another cause of calcium depletion of the bones. Recent blood screening data suggest that 95% of the celiacs in the USA are undiagnosed. For those who have the condition and are undiagnosed there is malabsorption of the fat soluble minerals and vitamins, especially calcium and vitamin D (21). Adult diagnosed celiac disease is usually associated with at least some degree of reduced bone density (22). Poorly managed celiac disease is also a risk factor for osteoporosis. It appears that magnesium supplementation may be more helpful to celiac patients than calcium supplementation (9, 23). Loren Cordain has recently published a discussion of human consumption of cereal grains which delineates the problems associated with human consumption of cereal grains, whether one has celiac disease or not (24)
12. A diet dominated by acidic foods will tend to raise blood ph which must be buffered. Calcium is leached from the bones to buffer the blood and keep it within the ph range required for homeostasis (25).
13. Oxalic acid (found in beans and a variety of vegetables) tends to inhibit calcium absorption through binding to minerals in the intestinal lumen (26).
If you are concerned about bone mineralization, the above factors which interfere with calcium absorption suggest the following strategies: