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Cola, But Not Other Beverages, Contribute to Osteo

The report that cola drinks, but not other soft drinks, can raise the risk of osteoporosis opened a new debate about the dangers of sugary, caffeinated drinks. However, this study produced more questions than answers. Why were other soft drinks, besides colas, not linked with osteoporosis? Why were women affected but not men? What makes cola drinks bad for bone health? This article fully discusses the Framingham Study that produced the report and why cola may cause weaker bones.
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The Framingham Study

The Framingham Osteoporosis Study is often used to determine the effects of foods, supplements and lifestyle choices on the risk of osteoporosis in the elderly. It is best described as the benchmark study in osteoporosis.

In a 2006 paper published in The American Journal of Clinical Nutrition, a group of researchers from the US Department of Agriculture Human Nutrition Research Center on Aging at Tufts University in Boston took data from the Framingham Study to determine the effects of regular soft drinks consumption on bone mineral density.

The researchers compared the frequency of soft drinks consumption with X-ray examinations of the spine and hip bones in 1,413 women and 1,125 men involved in the Framingham Study.

The results of the study showed that

  • cola consumption was associated with lower bone mineral density in women
  • cola consumption did not affect bone mineral density among men even though they consumed more soft drinks than women
  • the greatest reduction in bone mineral density occurred at the hip (a 4% reduction in bone mineral density at the hip was observed in women who drank 3 or more colas per day)
  • diet cola also produced similar risks of osteoporotic fractures
  • decaffeinated cola was also linked to lower (but less significant) bone mineral density
  • non-cola carbonated beverages did not affect bone mineral density
  • cola consumers did not have significantly higher phosphorus intake when compared to those who did not take cola drinks
  • calcium-to-phosphorus ratio was, however, lower among cola consumers than non-consumers

The researchers, therefore, concluded that cola, but not other carbonated soft drinks, consumption was strongly linked to low bone mineral density in women.

Three Theories

Just as the Framingham cola study answered the question about which soft drinks can increase the risk of osteoporosis, it raises even more questions. For example, why are women affected and men are not? Why does cola make the hip bones more fragile but not the bones of the spine?

And more importantly, what makes cola bad for our bones?

There is no consensus about the reasons why cola drinks may reduce bone mineral density. Howwever, the three leading theories to account for this effect are discussed below.

These theories involve the differences in the contents of cola and non-cola drinks as well as the dietary choices of cola drinkers.

Cola or Milk?

Some health experts believe that the observed effect of cola drinks on bone mineral density in the study discussed above is due to the choice of cola drinks over healthier drinks such as milk and dairy products.

Because colas are sugary and, generally, taste better than milk, it is quite possible that more people are abandoning milk and dairy drinks for cola drinks. In fact, epidemiological studies show that most young people prefer soft drinks, especially colas, to milk.

Therefore, the drink of choice to go with meals has shifted from milk to colas and other soft drinks.

However, proponents of this theory believe that cola drinks do not directly affect the bone. Rather, these drinks just do not contain calcium.

This means that the people who replaced of milk with cola drinks in their diets have lower dietary calcium intakes. When calcium intake is low, it can lead to lower bone mineral density later in life.

This theory seems to suggest that cola is not really the culprit. Rather any soft drink that replaces milk should produce the same calcium-lowering effect and also raise the risk of osteoporotic bone fractures. However, the Framingham cola study did not find that non-cola carbonated beverages lowered bone mineral density.

The Caffeine in Colas

The second theory to explain the effect of cola drinks on bone mineral density viewed the Framingham results from the differences between cola drinks and non-cola soft drinks.

First, it should be noted that the process of carbonation is not the reason why colas affect the bone.

A 2005 study published in the British Journal of Nutrition compared the effects of non-carbonated mineral water and carbonated mineral water on bone mineral density in two groups of healthy postmenopausal women.

At the end of the 8-week duration of the study, blood and urine tests showed that there was no difference in bone turnover between the two groups.

The two distinguishing contents of cola drinks are caffeine and phosphoric acid.

Caffeine has been linked to lower bone mineral density in different studies. Because caffeine can produce acidic metabolites in the body, it can leach calcium from the bones as the mineral is stripped from bones and moved to the blood to neutralize acidifying caffeine metabolites.

In addition, caffeine promotes diuresis. Therefore, it can increase the urinary excretion of calcium.

By promoting calcium loss through the urine, caffeine can reduce the amount of calcium available to maintain the right bone mineral density.

The Phosphoric Acid in Cola Drinks

However, the phosphoric acid in cola drinks has attracted the most blame for being responsible for the ability of cola drinks to lower bone mineral density. And this is rather ironic because phosphorus, along with calcium, is responsible for most of human bone mass.

A number of reasons have been given to account for the effect of phosphoric acid on bone mineral density.

First, phosphoric acid is known to bind to calcium and magnesium in the digestive tract. Salts of calcium and phosphorus (calcium phosphate) are known as apatites and they make up the bulk of the bone. However, such salts are only good for bone health as long as they are formed in the bone.

However, the salts of calcium and phosphoric acid formed in the gut are not readily absorbed in the intestine.

Therefore, the consumption of phosphoric acid may reduce the absorption of calcium (and magnesium).

Because both minerals are important to bone health, phosphoric acid can potentially lower the amount of calcium and magnesium available for bone mineralization.

Secondly, the increased consumption of phosphoric acid from cola drinks may lower the body’s calcium-phosphorus ratio. This means that less calcium is available for bone mineralization than phosphorus.

Since both calcium and phosphorus are needed to form the basic units of the bone, a lower calcium-phosphorus ratio does not improve bone formation.

Lastly, the acidifying effect of phosphoric acid has also been mentioned as a cause of bone loss.

The high acidity of phosphoric acid is employed in dentistry for cleaning and etching the teeth. The industrial uses of this compound, such as rust removal, as exploit its acidic nature. Therefore, some experts have argued that phosphoric acid may acidify the blood enough for the body to neutralize it by stripping calcium from the bones.

Flaws in the Phosphoric Acid Theory

However, there are a number of holes in the argument for phosphoric acid as the culprit in cola drinks.

For one, the amounts of phosphoric acid found in cola drinks are actually low and definitely lower than the level of phosphoric acid in commonly consumed foods such as cheese (a dairy product) and chicken.

Secondly, the Framingham cola study found that there was no difference between the phosphorus intakes of cola and non-cola drinkers. This means that the phosphoric acid contents of cola drinks were not responsible for the lower calcium-phosphorus ratio of cola drinkers.

It is possible that people who drink cola are substituting milk with these drinks. However, non-cola drinkers also sacrifice milk for non-cola soft drinks and, therefore, should experience similar bone loss.

The most likely explanation then would be the caffeine content of cola drinks.

It would seem that the diuretic effect of the caffeine in colas is significant enough to promote urinary calcium excretion and, therefore, lower the calcium-phosphorus ratio.

In fact, a 2001 study published in The American Journal of Clinical Nutrition found that calcium loss from soft drinks was solely due to their caffeine contents and not phosphoric acid.

In this study, the researchers compared the effects of various drinks on the rate of urinary calcium excretion in a group of women aged 20 – 40 years. These women were divided into groups according to the drinks they received.

The drinks tested were water, milk, caffeinated drink, non-caffeinated drink, a drink with phosphoric acid and a drink with citric acid (the phosphoric acid alternative used in soft drinks).

The researchers found that only milk (surprise, surprise) and the caffeinated drinks increased urinary calcium excretion relative to water.

In fact, their results showed that the rate of calcium loss from caffeinated drinks with phosphoric acid was the same as the rate of calcium loss from drinks with caffeine and no phosphoric acid.

Therefore, it would seem that caffeine, and not phosphoric acid, is responsible for the lower calcium levels in cola drinkers. But even that conclusion has been disputed. An earlier study published in the same journal in 1990 showed that the body compensated for caffeine-induced calcium loss by lowering its normal daily calcium losses.

However, it is possible that this compensation is not enough to stop the depletion of calcium from the bones by caffeine.

Without solid evidence against the phosphoric acid content of cola drinks, the best explanation for the lower bone mineral density reported among cola drinkers is the combined effect of caffeine on calcium loss, the displacement of milk from the diets of cola drinkers and, perhaps, their sedentary lifestyle.

More Studies on the Effects of Soft Drinks on the Bones

Mirroring the Framingham Cola Study

A 2003 study published in the Journal of Bone and Mineral Research investigated the effects of carbonated soft drinks on bone mineral density among adolescents. Therefore, where the Framingham study pooled adults, this study recruited young people.

The researchers recruited 1,335 boys and girls aged 12 – 15 years from 36 schools in Northern Ireland.

The researchers gathered data on the adolescents’ beverage consumption and compared these with the results of their bone mineral density tests.

The results of the study showed that

  • the consumption of carbonated soft drinks was associated with lower bone mineral density among girls but not boys
  • both non-cola and diet drinks were associated with lower bone mineral density among girls especially at their dominant heels
  • cola and non-diet drinks were not associated with lower bone mineral density in either sex

While this study continued to highlight the trend that soft drinks were only associated with lower bone mineral density among females, it failed to find such association with cola drinks.

Therefore, it is very likely that other factors besides the combination of caffeine, high sugar and phosphoric acid are responsible for the low bone mineral density caused by soft drinks.

Phosphoric Acid and Hypocalcemia

Even though the phosphoric acid in soft drinks has not been totally linked to reduced bone mineral density, there are indications that it can cause low calcium levels or hypocalcemia.

A 1996 study published in The Journal of Pediatrics found that soft drink phosphoric acid can cause hypocalcemia in children. The researchers compared the soft drinks intakes of 57 children with hypocalcemia (less than 2.2 mmol/L) to those of 171 children with normal calcium levels (greater than 2.2 mmol/L).

The results of the study showed that there was a significant association between hypocalcemia and the consumption of at least 1.5 L per week of soft drinks containing phosphoric acid.

A similar study was published in 1999 in the Journal of Clinical Epidemiology. In this case, the researchers compared the effect of soft drinks phosphoric acid on serum calcium levels in postmenopausal women.

The results of this study showed that women who regularly consumed (one or more bottles per day) soft drinks containing phosphoric acid had hypocalcemia, hyperphosphaturia (high blood phosphate level) and high serum levels of parathyroid hormone.

The researchers, therefore, concluded that the consumption of soft drinks with phosphoric acid was a risk factor for hypocalcemia among postmenopausal women.

The two studies described above are relevant because chronic hypocalcemia will definitely increase the risk of osteoporotic bone fractures later in life.

Bringing Protein into the Debate

A 2008 study published in The American Journal of Clinical Nutrition investigated the association between long-term consumption of soft drinks and changes in bone structures in children and adolescents.

By comparing data from a 4-year study involving 228 healthy children and adolescents, the researchers determined the rate of soft drinks consumption from dietary records and its effect on bone modelling and remodeling from the results of bone scans.

The results of their studies showed that

  • long-term consumption of all soft drinks (caffeinated or not) was associated with lower bone mineral content as well as lower bone strength
  • long-term consumption of caffeinated soft drinks was associated with lower bone tensile strength (the ability to take on strains without fracturing) and smaller bone circumference
  • milk intake was associated with higher bone strength
  • soft drink consumption lowered the consumption of milk and dietary proteins

The researchers, therefore, concluded that long-term soft drinks consumption can reduce bone mineral density and bone strength in children.

The researchers agreed that the replacement of milk with soft drinks in the children’s diets was partly responsible for the poor bone health observed.

However, they believed that the lower total protein intake, associated with increased soft drinks consumption, was the major factor responsible for the reduced bone mineral density and bone strength caused by these drinks.

Lower protein intake translates to lower level of insulin-like growth factor (IGF-1).

Because IGF-1 is involved in bone and tissue development, the effect of soft drinks on protein intake may be the most important factor responsible for weaker bones.

Sources


http://www.ncbi.nlm.nih.gov/pubmed/17023723

http://www.dukehealth.org/health_library/health_articles/diet_soda

http://clivesimpkins.blogs.com/clive_simpkins/2010/03/harvard-med-school-on-carbonated-drinks-and-osteoporosis.html

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