Introduction
Bones are truly amazing. A unique combination of flexible collagen and brittle calcium phosphate makes them incredibly strong and durable.
Part of the durability has to do with the fact that our bones are under constant maintenance, being partially broken down and rebuilt on a daily basis. Special bone cells called osteoblasts take calcium and phosphorus from the blood and deposit it as calcium phosphate crystals in the bone. In order for this to happen effectively a second set of cells, termed osteoclasts, must make some space for the new bone.
Figure 1. Relationship between total bone mass and age (from Kamen, 1996)
Osteoclasts remove the crystals and return them to the blood as calcium and phosphorus. It is the outer layer of the bones that are susceptible to these ongoing processes. We build up an optimum bone mineral (calcium phosphate) density (BMD) for the first 25-30 years of our life and spend the remainder of our lives gradually losing it, more quickly in women than in men.
(Figure 1). That is one reason why it is important to maximize your bone density when you are young improving your chances of avoiding or at least delaying osteoporosis.
Why all the fuss about calcium?
Calcium is responsible for nerve cell communication, contraction of muscle cells, function of important enzymes, protein synthesis, blood clotting efficiency and bone formation. Calcium levels in the blood are so important that the body will do almost anything to maintain an optimum concentration. If adequate calcium is not available from the diet then calcium will be removed from the bones and delivered to the blood. Unfortunately for your bones, blood calcium levels take priority.
Figure 2. Typical daily flow of calcium in and out of bone tissue (from Kamen, 1996)
You can imagine, perhaps, a blood calcium budget in your body being run by an accountant who insists on a balanced budget at all costs. The only external source of calcium is the diet (income). Since the digestive tract isn’t 100% efficient we must consume more calcium than our bodies really need. In addition some calcium gets excreted in the urine. So if we use 1000 mg as an example (Figure 2). of a typical daily calcium intake then it is estimated that only about 300 mg of that will enter the blood, and of that 300 mg about 150 will be excreted in the urine and another 150 returned to the digestive tract and eliminated in the feces.
In this budget there is a constant daily input and output of calcium to and from the gastrointesinal tract, the blood, the bones and the body’s other cells. With this in mind think of your bones as your nest-egg of calcium savings (sort of a calcium RRSP to be used when you’re older or on a rainy day). If calcium income at a given time is not adequate to maintain the blood calcium balance then the accountant will borrow from the bones to ensure a steady blood calcium level. Under ideal circumstances this calcium debt will be repaid to the bones.
So what’s the problem?
The problem is that the ideal conditions for repaying the calcium debt to the bones, are too infrequent in many people’s lives. Imagine if you made frequent withdraws from your RRSP and never repaid them. When it came time to retire you would be in for an awful shock. A poor lifestyle, inadequate diet, increased stress and insufficient exercise make it almost impossible for the blood calcium accountant to replace the bone calcium savings. Too many rainy days over an extended period of time and a calcium deficit is created in the bones and they become weak increasing the risk of fracture.
“Well”, says the accountant, “we need more income!” Yes, increased dietary calcium and calcium supplements may help slow the loss of BMD but the majority of research studies show that supplements don’t prevent the loss and they certainly don’t increase your BMD (Kamen, 1996).
Why? Because increasing available calcium for bone formation is complex and many other variables come into play. Certain aspects of diet and lifestyle are almost like having embezzlers within the company (your body) who keep sneaking off with potential “profits” (improved BMD). You can boost calcium “income” as much you wish but if a thief is stealing the profits how do you solve the problem?
Let’s get rid of the thieves!
Coffee, alcohol and smoking all lead to an increase in the acidity of the blood and one way the body compensates for this is by using calcium from the bones, as a buffer to reduce the acidity. Reducing or eliminating these items from your diet would be wise, especially smoking since it causes so much other damage to your body besides brittle bones. These are not the only delicacies that increase blood acidity. Dr. Susan Brown provides an excellent review of this subject (Brown and Jaffe, 2000) and a comprehensive list of a variety of foods and their affect on blood pH (alkalinity/acidity).
A diet that is too high in protein, for example, can increase the acidity of the blood and result in increased urinary excretion of calcium. This is one reason why excessive consumption of meat is not good for the bones (Marsh et al., 1988). As a general rule a 70 kg person only requires about 70 grams of protein per day and there are many other options besides meat. Be wary of milk, which is very high in protein, fat and sugar, and therefore may not be the ideal source of dietary calcium. Green leafy vegetables are your best source of calcium; chard, dandelion, alfalfa and parsley to name just a few.
Soft drinks are typically loaded with phosphoric acid which is a nightmare for pH balance and the high phosphorus content can throw off the calcium:phosphorus ratio and result in poor calcium assimilation. Most soft drinks are also high in sugar and caffeine making for a quadruple whammy. This one form of junk food is impairing healthy bone development in a significant number of young men and women as some studies are already showing (Wyshak and Frisch, 1994).
Calcium is only part of the picture
Calcium may be a key building material but other nutrients are critical to enabling your body to make the most efficient use of the available calcium, including adequate amounts of vitamin D3, magnesium, HCl, zinc and also key minerals such as boron and silicon.
Exercise, especially outdoor weight bearing exercise is essential as well. The benefits of exercise start as early as childhood and continue into our 70’s and 80’s with benefits resulting no matter when the exercise program has begun. Exercise, yoga or meditation will all help to reduce stress leading to healthier bones.
Space does not permit a discussion of all the nutrients that play a role in bone development but they are all consistent with an approach to diet that is nutritionally complete and benefits the entire body, not just the bones. It seems appropriate that if we take good care of the body as a whole, healthy bones and a healthy heart will follow.
Eric Norman is a research scientist investigating heart disease in post-menopausal women using ovarian hormone therapy and in type II diabetics.
References
- Brown, S.E. and R. Jaffe. Acid-alkaline balance and its effect on bone health. International Journal of Integrative Medicine Vol. 2: No.6, 2000.
- Chalmers, P. Geographical variationsin Senile Osteoporosis. J of Bone and Joint Surgery. 1970; 52B:667
- Kamen, B. Hormone Replacement Therapy: Yes or No? Fourth Ed.. Pub. Nutrition Encounter, Novato, Cal. 1996
- Marsh, A.G. et al..Vegetarian lifestyle and bone mineral density. Amer J Clin Nutr. 1988:48:837.
- Wyshak, G. and R.E. Frisch. Carbonated beverages, diettary calcium/phosphorus ratio, and bone fractures in boys and girls. J Adolesc Health 15:210-215. 1994)