Over recent years there have allegations that Cola beverage consumption has been contributing to the osteoporosis epidemic. The large beverage companies vigorously deny the allegation and have produced studies which they claim disproves the allegation while on the other side the proponents of the allegation claim that research proves the allegation. One way to clarify the situation is to examine the chemistry of the process to see whether it supports either side of the argument or as commonly occurs with arguments of this type is it somewhere in between the two extremes.

The cola beverages contain phosphoric acid as the acidulant. This acid therefore has the potential to contribute to bone mineral. The bone mineral is hydroxyapatite ( or dicalcium hydroxy phosphate ). Three components are required to form this mineral, these are calcium, phosphate and base. If any one of these components are missing the mineral hydroxyapatite can not be formed. The calcium and phosphate are obvious food components but base requires some explanation. Foods contain a variety of acids for the body these acids fall into two groups;

The metabolisable acids. The acids which can be broken down and completely destroyed For example citric acid, acetic acid, lactic acid etc.

And

The non metabolisable or fixed acids which are not destroyed by the body. For example hydrochloric acid, sulphuric acid, phosphoric acid etc.

The acids may be present as either the free acid or salts of the acid.

The body must keep the composition and properties of body fluid within certain parameters or damage to the body and organs will occur. The process of maintaining the system within these parameters is known as homeostasis.

The parameter of interest with respect to the colas is pH. The bodies pH is controlled by its acid/base balance. Not all acids are created equal. Many of the acids in food can be completely metabolised to carbon dioxide and water. The acids in the diet which cannot be completely metabolised are the problem ones.

The non metabolisable acids are sulphuric, hydrochloric, phosphoric etc. There is no problem with non metabolisable acids if there is sufficient dietary base available to neutralise them.

Any excess of base is not a problem as the body is able to produce acids such as citric, lactic, acetic etc to neutralise it.

The dietary base as present in food is primarily the sodium, potassium, and calcium salts of the metabolisable acids. Most fruit juices are strongly acid with acids and salts of acids present. As most of the acids present in fruit juices are citric acid or other metabolisable acids most fruit juices behave as dietary bases.

When an excess of non-metabolisable dietary acid is present the body uses bone mineral as the base to neutralise this excess acid. The acid is excreted as a mixture of the free acid and salts of that acid. As a result bone mineral is lost from the body when an excess of non-metabolisable dietary acid is consumed.

For the chemistry connoisseur the following equations are the overall ones relating to bone deposition and excess acid neutralisation.

2Ca2+ + PO43- + OH- → Ca2 (PO4)(OH)

Ca2(PO4)(OH) + H3PO4 → 2CaHPO4 + H2O

Ca2(PO4)(OH) + 3H3PO4 → 2CaH4(PO4)2 + H2O

With respect to the Colas there are two main consequences arising from the above equations.

With an excess of dietary base the extra phosphate would probably contribute to bone mineralisation.

With insufficient dietary base bone mineralisation would be retarded and with sufficient Cola bone mineral loss would irrevocable.

How much phosphoric acid is too much? Looking at the literature the lowest urinary pH is about 5.5. A 50 % neutralisation of free phosphoric would give approximately this pH. This means that not all the dietary phosphoric acid must be neutralised prevent the adverse effects on bones. It also means that there must be sufficient base in the diet to completely neutralise not less than 50% (probably significantly more) of the total unneutralised dietary phosphoric acid.

There was a considerable scientific literature published on the subject in the 1950s. and it is certainly discussed in a number of standard texts. In the older references the term osteoporosis is not used and term “bone mass loss” is used instead. Many food composition tables dating from the 1960s also include residual dietary acid/base figures. I had originally thought that the discovery of the dietary phosphoric acid problem dated from the mid 1950s from work by et al at Boston however I recently came across a reference to it in a book published in 1927. The original published research would have been prior to this.

The effects of dietary phosphoric and other non-metabolisable acids were certainly well known by beverage technologists and chemists from the late 1960s onwards. I first came across references to the effect in the early 1970s when experimentally working on the ion exchange treatment and base modification of fruit based beverages. At the time the effect seemed to be regarded as something of theoretical interest but of little practical significance. The reason for this attitude was that it was generally believed that there was far more than sufficient dietary base to neutralise any non-metabolisable acids in the average diet. This was possibly a justifiable attitude at the time.

This situation was to change later. There has been an ongoing change in dietary patterns with lower quantities of unprocessed fruit and vegetables being consumed. Compounding the effects of dietary change extensive marketing campaigns by the multination beverage companies have resulted in water and other beverages in the diet being replaced by colas. These factors have conspired to change an effect that had been of theoretical interest only to an effect with significant public health implications.

There have been a number of recent surveys that supposedly show no association of cola consumption with osteoporosis. The first stage in doing a statistical survey is the sample selection. During sample selection it is normal to exclude individuals who are considered to be abnormal. Simply considering persons whose diet was low in fruit and vegetables as abnormal and excluding them would skew the survey results in the favour of a no effect result. There is a second consideration. For the consumption of colas above an appropriate equivalent level of fruit and vegetables not to cause osteoporosis would require a contravention of some fairly basic laws of chemistry and physics.

The large American beverage companies certainly knew about the potential bone problems in the late 1950s. However at that time the effect was of more theoretical that actual importance. This has been changed dramatically over recent times due changing dietary patterns and the aggressive Cola marketing campaigns. As a consequence in the US osteoporosis is now an increasing problem in young people in the late 20s to early 30s age group.