Our analysis suggests that consumption of 2 or more cola beverages per day was associated with an increased risk of chronic kidney disease. We saw no increase in risk associated with consumption of noncola carbonated beverages, or with noncola caffeinated beverages.
Although we know of no previous study that assesses a link between cola beverages and kidney disease, there is evidence supporting the plausibility of our findings. One difference between cola and noncola carbonated beverages is that, while noncola beverages have been predominantly acidified using citric acid, cola beverages are generally acidified using phosphoric acid. Phosphorus may have an effect on the risk of kidney disease. Case reports have linked sodium phosphate bowel cleansing preparations to acute and sometimes irreversible renal failure,20,21
characterized by calcium phosphate deposits in the distal tubules and collection ducts or nephrocalcinosis.22–24
Although the level of phosphorus from colas is far lower than that from bowel preparations, long-term cola intake may lead to kidney damage, especially in the presence of underlying renal dysfunction.
Laboratory studies have shown that high phosphorus diets can cause nephrocalcinosis in rats.25,26
Diets high in phosphate may increase plasma phosphorus levels, with phosphate in colas perhaps being more bioavailable.27,28
In a randomized trial among men with kidney stones, recurrence of kidney stones was higher among those who continued to drink soft drinks containing phosphoric acid, compared with those who drank beverages acidified by citric acid.13
Cola consumption can result in physiologic changes in urine composition that are conducive to oxalate kidney stone formation.15,16,29,30
We saw no association with cola among individuals with history of kidney stones, although the sample size for this subanalysis was small.
The odds ratios for a cola association were higher among those with a history of diabetes, although the test for interaction was not statistically significant. A recent study has shown a positive association between cola beverages and incidence of hypertension.11
In our analysis we saw no evidence of effect modification by hypertension, nor did hypertension confound our results.
We cannot rule out the possibility that the observed effect may have been due to caffeine, but we did not see an increasing risk with increasing consumption of caffeine from noncola sources.
This study had several limitations. We were not able to examine dietary differences between cases and controls or between cola and noncola drinkers. Cola drinkers may also differ from noncola drinkers in other ways that could lead to residual confounding. Among controls, cola drinkers were younger, less likely to have a history of diabetes, and more likely to have a history of kidney stones compared with noncola drinkers. We controlled for age and diabetes, and history of kidney stones was not a confounder.
The high proportion of proxy respondents among cases is a further limitation. However, the estimated effect of cola was stronger after excluding proxy respondents, suggesting that their inclusion attenuated the estimated risk.
Beverage consumption was self-reported and thus subject to potential bias. However, since a link between carbonated beverage intake and risk of kidney disease was unknown at the time of the study, it is unlikely that cases would have differentially reported their beverage consumption. In this analysis, only colas were consistently associated with chronic kidney disease, which makes reporting bias an unlikely explanation for our findings.
Participants reported average beverage intake over their adult life. While these reports may reflect more recent consumption, there is no reason to believe that those who currently drank colas had not done so in the past. Colas have been available since the late 1800s and were first popularized in the southern United States.
Cola consumption is common and chronic kidney disease is a substantial public health burden. Our preliminary result of an association between cola consumption and risk of chronic kidney disease deserves to be explored in more detailed studies.