Friday, December 26, 2008

Osteoporosis: Don't Just Increase Calcium, Decrease Sodium

Osteoporosis is a loss of bone due to a failure to make bone or excessive destruction of bone through natural processes. Bone is constantly remodeled through life to compensate for microscopic trauma. The bones repair themselves after microscopic or visible trauma by a process of bone destruction known as resorption. The process of resorption is always followed by new bone formation. When resorption and formation get out of balance, osteoporosis, or weak bone results.

The bones are not only the body’s largest depot of calcium, they are also the body’s largest depot of sodium. Every cell in the body maintains a negative electrical charge, but the electrical charge across the membrane of a bone is cell is very low, only -2 to -4 mV, compared to -90 mV in a cell in the heart. This very small negative charge keeps it from being overwhelmed with toxic levels of sodium. That is, a bone cell is not overwhelmed with sodium until it is injured or becomes “tired.”

Cells of bone are programmed to destroy themselves with they are injured. A fracture quickly causes affected bone cells to absorb sodium and acquire a positive charge of +20 to +40 mV. The positive charge on an injured bone cell is ten to twenty times the magnitude of the negative charge on a healthy bone cell. The positive charge fractured bone alters electrical balance throughout the body. In fact, broken bones can be detected not just on X-ray but also by EKG. Abnormalities in sodium metabolism in bone ripple through the entire body.

If sodium levels in the bloodstream are high, the rest of the body cannot absorb small amounts of sodium to offset the huge amounts of sodium going to injured bone. Bone cells are stuck with a positive charge. The injured, positively charge bone cell cannot attract amino acids or respond to hormones. It has to rely on inefficient anaerobic (oxygen-free) respiration to make energy. Anaerobic respiration fills the cell with lactic acid. The only way it can lower its charge is by releasing calcium.

Calcium, of course, is key to the strength of bone. As injured cells attempt to heal themselves by lowering their electrical charge, the bone becomes weaker. If the cells are completely unsuccessful at repairing their metabolic machinery by release of calcium, they die.

The “hole” in the bone must be cleaned up by specialized cells called osteoclasts. These are giant cells with multiple nuclei and redundant mitochondrial “energy factories” that make them resistant to metabolic stress. The “hole” is filled by restoration cells called osteoblasts. These are smaller cells with a single nucleus and a single set of mitochondria that are very sensitive to metabolic stress.

The cells that destroy bone are much more efficient and resilient than the cells that make new bone. Under conditions of metabolic stress, such as high sodium and low potassium, bone-destroying osteoclasts predominate over bone-making osteoblasts. But if the bone is partially successful at restoring their metabolic machinery by release of calcium, it remains alive, partially decalcified and vulnerable to fracture, not to be replaced for months or years as it progressively weakens.

Critics of this observation note that the kidneys conserve calcium even if it is released by bone. Over 95 percent of calcium released by bone is recycled back into the bloodstream by the kidneys. Over 99 percent of sodium, however, is also recycled back into the bloodstream by the kidneys, so bone cell disorganization progresses unless dietary intake of sodium is restricted. Most of the research regarding the relationship between sodium and osteoporosis has focused on prevention.

At least one study suggests that an average intake of 16,000 mg or sodium per day increases the risk of osteoporosis by 300 to 400 percent. The Scientific Advisory Committee on Nutrition of the UK estimates that preventing osteoporosis requires restricting sodium to 6,000 mg per day. Exact recommendations of sodium intake for preventing osteoporosis are not possible because salt sensitivity varies from person to person and from day to day in individuals. Correcting osteoporosis, however, requires much lower consumption of sodium, certainly less than 1,500 mg per day and optimally about 360 mg per day.

I believe that sodium restriction is more important than calcium supplementation in treating osteoporosis, but sodium restriction without potassium augmentation will not work. I also believe that moderate sodium restriction is not likely to be helpful.

When researchers put volunteers on a “low” salt diet containing 2000 mg of sodium a day, calcium excretion (measured by the concentration of calcium in 24-hour samples of their urine) was essentially the same as when they consumed a “normal” diet containing 3300 mg of sodium a day. Giving supplemental potassium, however, significantly increased retention of calcium. I believe this is because potassium allows bone cells to “recharge” so that they do not need to expel calcium to reduce their concentrations of sodium.

Opponents of sodium restriction point out that the effects of salt consumption on calcium retention are short term, lasting only about 12 hours. They are right. However, if you consume salty foods every 12 hours, you constantly tire the cells of your bones.

What you can do about osteoporosis? If you consume at least 6,000 mg of potassium per day from food, you will observe improvement in osteoporosis if you restrict consumption of sodium to about 600 mg per day, regardless of your calcium or magnesium consumption. Even less sodium, however, is more effective.

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