“Fruit juice ‘is diabetes risk’” is the headline in The Sun . “A daily glass of ‘healthy’ orange juice could actually increase the risk of diabetes,” the newspaper says. Women who drink a daily glass of fruit juice are 18% more likely to develop type 2 diabetes, but those who eat three pieces of fruit instead actually reduce their risk by the same amount, the newspaper adds.
This study – a large and, on balance, a well-conducted one – suggests a link between fruit juice and risk of type 2 diabetes. Diabetes is a complex condition, unlikely to be caused by a single factor. In light of this fact and some limitations with the study it is difficult to quantify the contribution that fruit juices make to risk, or the mechanisms by which this might happen, and the findings warrant further study.
Where did the story come from?
Dr Lydia Bazzano and colleagues from Tulane University School of Public Health and Tropical Medicine in Louisiana and other medical and academic centres across the USA carried out this study. The research was supported by the National Institutes of Health. One of the researchers received a grant from the Office of Research on Women’s Health and Office of Dietary Supplements. It was published in the peer-reviewed medical journal: Diabetes Care .
What kind of scientific study was this?
This was a cohort study of over 70,000 female nurses followed for 18 years to determine the links between diet and risk of various outcomes. The study has published many parts of its results over time, and in this particular paper the researchers report on the association between all fruit and vegetables, particular types of fruit or vegetable, and fruit juice with the onset of type 2 diabetes during the 18 years of follow-up.
The study included 121,700 nurses aged between 30 and 55 years old, living across 11 different states in the USA. They were sent an initial questionnaire to collect data about their medical history, lifestyle, diet and other health practices. A follow-up questionnaire was sent every two years after that, and detailed dietary information has been collected since 1980. The questionnaires also asked whether the women had a diagnosis of diabetes. Those who responded yes were sent a further questionnaire to ask more about their symptoms so that an independent diagnosis could be made according to accepted criteria (on the basis of the responses). Women were included in this analysis if they completed the 1984 questionnaire, provided a sufficient amount of information (less than 12 questions blank), ate between 600 and 1500kcal and did not have cardiovascular disease, cancer or diabetes in 1984.
In total, 71,346 women were available for analysis and they were followed up with more questionnaires, including detailed food frequency questionnaires, at various time points until 2002. Since 1984, the food questionnaires included 16 questions on fruit consumption, 28 on vegetable consumption and three on potato consumption. Frequency of intake (ranging from never up to six times a day) and the size of portions were reported. The responses were used to calculate average daily intake and total intakes. Intake of other beverages, including cola or sweetened drinks, was also recorded.
The researchers assessed the risk of type 2 diabetes being reported during follow-up, and the intake of fruit and vegetables (not including fruit juice). They then looked at any associations with specific food groups, e.g. green leafy vegetables, legumes, fruit juices. In the end, they did not include potatoes in any of their analyses, suggesting that they have a different energy and nutrient density and are more likely to be found in fast foods. Women were followed up until death, diagnosis of diabetes or June 1 2002 – whichever came sooner.
What were the results of the study?
Overall, women who consumed more fruit and vegetables were older, less likely to be smokers, did more exercise and were more likely to use hormone replacement treatment than those who did not eat fruit and vegetables as frequently.
During the 18 years of follow-up, there were 4,529 new cases of type 2 diabetes. There was no link between total fruit and vegetable intake and risk of developing the disease, or with total vegetables on their own. Intake of total fruit and green leafy vegetables appeared to reduce the risk of type 2 diabetes.
When exploring the link with fruit juice, having more than three cups per month of apple or grapefruit juice increased the risk of type 2 diabetes compared with having less than one cup a month. Similarly, drinking one or more cups of orange juice per day increased the risk of diabetes by about 24% compared with drinking less than one cup a month. They also found that drinking carbonated beverages, colas (sugar sweetened and low-calorie) and fruit punch increased the risk of diabetes between 4 and 11% per increase in daily single servings.
What interpretations did the researchers draw from these results?
The researchers conclude that the study has shown a positive association between fruit juice consumption and diabetes risk. They say that this may be related to the lack of fibre and high sugar load, among other factors.
What does the NHS Knowledge Service make of this study?
This large and long-term cohort study is well-conducted and provides evidence of a link between intake of fruit juice and incidence of type 2 diabetes. The greatest limitations of the study – which the researchers discuss – are problems with measurement (e.g. misreporting of food intake) and potentially failing to account for other factors that may be responsible for the association. Over time, food consumption has changed, and the use of the same food frequency questionnaire throughout the study may not have captured this. As the study sample was nurses, the researchers suggest that the likelihood of them misreporting their diabetes diagnosis is limited.
These findings warrant further investigation. The link between fruit juice consumption and risk of type 2 diabetes may, in part, be related to the high sugar load that is delivered through the juice (in the absence of other fruit components that would be eaten with solid fruits). Some juices also have added sugar, and it is known that a high sugar intake is linked to an increased risk of diabetes. As the researchers state, the results have implications for recommendations that 100% fruit juice can be considered to be a serving of fruit.