In this prospective study of African American women, consumption of sugar-sweetened soft drinks was positively associated with incidence of type 2 diabetes. Women who consumed 2 or more soft drinks per day had a 24% increase in incidence relative to women who drank less than 1 soft drink per month. A similar association was observed for sweetened fruit drinks, with a 31% increase observed for 2 or more drinks per day relative to less than 1 drink per month. Consumption of orange and grapefruit juice and of diet soft drinks was not associated with diabetes risk.
Several possible mechanisms may explain the observed associations between diabetes risk and consumption of sugar-sweetened drinks. The first, which probably accounts for most of the association, is through weight gain. A systematic review of the literature indicates a positive association between greater intakes of sugar-sweetened beverages and weight gain and obesity in both children and adults.12
These beverages are dense in calories and are typically consumed as an addition to usual food intake. Several studies have shown that liquid foods have a low satiety and that when individuals increase consumption of liquid carbohydrates, they may not reduce consumption of solid food in response.13–15
High-fructose corn syrup, which is now the sweetener used in all sugar-sweetened soft drinks consumed in the United States, appears to be particularly effective at promoting weight gain because of its adverse effects on insulin secretion and leptin release, leading to a reduction in the normal inhibitory effect on food intake.16,17
In addition, fructose facilitates the biochemical formation of triacylglycerols more efficiently than does glucose.17,18
Interestingly, orange and grapefruit juice consumption was not associated with an increased risk of diabetes in our study, perhaps because these beverages are typically consumed as part of a meal rather than between meals. The naturally occurring sugars in orange and grapefruit juices (glucose and fructose) may also have different metabolic effects than the high-fructose corn syrup that is added to soft drinks.
A second possible mechanism is through the glycemic effects of the beverages. Both sugar-sweetened soft drinks and fruit drinks contain large amounts of rapidly resorbed carbohydrates, the consumption of which leads to rapid increases in glucose and insulin concentrations.19,20
Although the literature is not consistent, several large studies have found a positive association between glycemic load of the diet and risk of type 2 diabetes.21,22
Sugar-sweetened beverages have a moderate glycemic index per se, but they contribute to a high glycemic load of the overall diet because of the large quantity consumed.23
When BMI was added to the multivariate model, the IRR for high soft drink consumption was reduced to 1.04, suggesting that the association with diabetes risk is mediated by the effects of soft drink consumption on BMI. Our data indicated that women who increased their intake of sugar-sweetened soft drinks had a considerably higher weight gain over a 6-year period than did women who reduced consumption. In contrast, adjustment for BMI had little effect on the association of fruit drinks with diabetes risk, and the relation between intake of these beverages and weight gain was not as strong as for soft drinks.
The consistency of the results observed within categories of age, family history of diabetes, and BMI do not support the hypothesis that adverse effects would be most severe among persons who are already insulin resistant.
The relation of sugar-sweetened beverage consumption to risk of type 2 diabetes was examined in data from the Nurses’ Health Study II, a prospective follow-up of US nurses, almost all of whom are white.5
Positive associations, somewhat stronger than in the present study, were found for both soft drinks and fruit drinks. The weaker associations observed in our study may be due to the higher baseline risk of diabetes experienced by African American women: relative risks associated with a given exposure will be smaller when the baseline risk is greater. In addition, the Nurses’ Health Study II analysis did not adjust for meat intake, a variable that proved to be the strongest dietary confounder in our analysis. A small study of Finnish men and women also found an association of sweetened soft drinks and juice drinks with risk of type 2 diabetes.7
On the other hand, in the Atherosclerosis Risk in Communities (ARIC) study, no association was observed for soft drink consumption and diabetes risk.6
The authors speculated that the reason for the discrepancy may be the older age and higher baseline BMI in the ARIC population. Another study examined soft drink consumption in relation to metabolic syndrome and components of the metabolic syndrome in the Framingham Offspring cohort.24
Of relevance to the present study, persons who consumed 2 or more soft drinks per day had a 32% increased risk of impaired fasting glucose relative to those who consumed less than 1 drink per day.24
Limitations of our study must be considered. The main analyses were based on beverage consumption data collected at baseline. Individuals who changed their drinking patterns may have been misclassified as to exposure, which would have tended to weaken the observed associations. Identification of incident cases of type 2 diabetes relied on the participant’s self-report. A validation study of several hundred BWHS participants who reported incident type 2 diabetes demonstrated that the condition is reported with very few false-positive results. However, some participants may have been unaware of their diabetes and were erroneously misclassified as non-cases. Such misclassification of disease status would have biased results toward the null but would not have caused the association that was observed for soft drink consumption and diabetes risk.
Strengths of the study include the large size of the cohort, the large number of incident diabetes cases, the prospective data collection, and the focus on African American women, a population with particularly high rates of type 2 diabetes. Previous studies have not included enough African American participants for separate analyses. Because our study was based on 10 years of follow-up, it was possible to look at changes in soft drink consumption over time and assess those changes in relation to changes in weight. Physical activity and other potential confounders were taken into account.
Type 2 diabetes is an important public health problem in the United States and particularly so among African American women. Overweight and obesity have been identified as the most powerful risk factors for type 2 diabetes, but losing weight and maintaining a healthy weight have proven to be difficult for many people. Our study suggests that the mechanism for the increase in diabetes risk associated with soft drink consumption is primarily through increased weight. Reducing consumption of soft drinks or switching from sugar-sweetened soft drinks to diet soft drinks is a concrete step that women may find easier to achieve than other approaches to weight loss.
Finally, it should be noted that consumption of fruit drinks conveyed as high an increase in risk as did consumption of soft drinks. Fruit drinks typically contain as many or more calories compared with soft drinks and, like soft drinks, may not decrease satiety to the same extent as solid food. Fruit drinks were consumed more frequently than soft drinks in our study, and in the US population, the proportion of total energy intake from fruit drinks doubled from 1977 to 2001.25
The public should be made aware that these drinks are not a healthy alternative to soft drinks with regard to risk of type 2 diabetes.