Whole fruit contains a number of phytonutrients and other bioactive compounds which are believed to decrease the risk of developing diabetes and other diseases.1,2,3 A recent analysis including three large-scale prospective studies demonstrated that a higher intake of whole fruit decreased risk of type 2 diabetes.3 However, whether the benefits of whole fruit can be replicated by fruit juices is the subject of scientific debate.4,5,6
Rates of fruit juice consumption are on the rise globally, likely at least in part due to the public perception that they represent a healthy alternative to both whole fruit and other, less healthy sugar sweetened beverages.7
This review aims to analyse the recent evidence regarding the impact of 100% fruit juice on the development, control and progression of type 2 diabetes mellitus, thus providing up-to-date advice for dietitians, other healthcare professionals and policy makers.
How ‘healthy’ is fruit juice?
High consumption of whole fruit is associated with reduced or neutral risk of diabetes.3 It is thought that the benefits of fruit juice are less than those of whole fruit due to its relative deficiency in fibre.8
It is also likely that many of the beneficial vitamins, minerals and phytochemicals found in whole fruit are at least partially destroyed in the processing.
However, fruit juice does still contain other antioxidants and polyphenols,9 meaning that its ‘healthy’ label is not completely without merit. It has also been shown that fruit fibre itself is not significantly related to the lower risk of diabetes.9,10,11,12,13,14,15,16
Attempts to identify specific components of whole fruit that yield benefits in diabetes has proven difficult; several supplementation trials, for example with beta-carotene and vitamin C, have reported null associations on adverse metabolic traits, including diabetes mellitus.17,18,19 It is therefore plausible that an inverse association with diabetes only occurs in the presence of a highly complex mixture of micronutrients, as is found in whole fruit.20
Does fruit juice consumption increase the risk of developing diabetes?
Evidence for the impact of fruit juice consumption on the risk of developing diabetes is somewhat limited and conflicting, with both positive3, 4, 21, 22 and null associations reported. 6, 23, 24, 25, 26, 27 Several studies have suggested discrepancies in benefit according to the type of fruit juice consumed.3,6,22,24, 25,28
In spite of these conflicting results, it is believed that relative lack of fibre and other phytochemicals in fruit juice may contribute to diabetes risk. Furthermore, the high sugar load delivered by fruit juice in a liquid state may be an important mechanism by which fruit juices could contribute to an increased risk of developing diabetes.29
Liquids do not possess the same satiating effect of solid food. This may lead to the consumption of excess calories, which often are not fully compensated for by dietary restriction.30,31,32
It is likely that frequent consumption of fruit juices contributes to a higher dietary glycaemic load, which has been positively associated with diabetes.10
Moreover, fruit juice may alter nervous system energy signalling, thus leading to dependence and habituation, in turn contributing to calorie overconsumption and ultimately to an increased risk of developing metabolic syndrome.33
The association with diabetes is more clear cut when it comes to sugar-sweetened fruit juices or ‘fruit drinks’, with several prospective studies demonstrating increased risk of type 2 diabetes development with increased consumption.24,28
What effect does fruit juice have on glucose control?
While several randomised controlled trial studies have been conducted to investigate the effect of fruit juice consumption on glycaemic control, the results have again been conflicting.
A recent meta-analysis concluded that fruit juice consumption did not significantly affect fasting glucose and insulin concentrations.2 However, older studies have shown that fruit juices lead to more rapid and larger changes in serum levels of glucose and insulin than whole fruits.34,35
Another more recent study also concluded that liquid calories may result in more rapid and larger changes in serum levels of glucose and insulin than whole fruit.3
A meta-analysis by Post et al determined that increasing consumption of dietary fibres can reduce fasting glucose concentration and HbA1C,36 therefore it is plausible that the reduced fibre content of fruit juice would explain its detrimental effect on serum glucose levels as compared to whole fruit. It is also possible that fruit juice consumption might increase dietary consumption of sugar and calories, influencing the total effects of fruit juice on glucose control.2 Further studies are required to fully investigate the effect of fruit juice on HbA1c levels.2
Does fruit juice have an impact on diabetes complications?
Several studies have indicated that fruit juice could actually reduce cardiovascular risk. The polyphenols contained in fruit juice have been shown to improve the antioxidant status and immune function of trial participants, which may result in a reduced risk of cardiovascular disease.37,38
Certain juice types in particular have been hypothesised to be of benefit in reducing cardiovascular risk. Low calorie cranberry juice may improve levels of circulating triglycerides, CRP and glucose, as well as ameliorating insulin resistance and diastolic blood pressure.39
Pomegranate juice has also been shown to reduce insulin resistance, enhance beta-cell function, and decrease fasting serum glucose in type 2 diabetes patients.40
However, other studies have reported conflicting results, showing that fruit juice consumption had no significant favourable effect on lipid abnormalities, which often occurs together with insulin resistance.41,42
Moreover, recent data would suggest that excessive fructose consumption from 100% fruit juice may be associated with liver injury and metabolic syndrome.43
The hepatic metabolism of fructose leads to the storage of intrahepatic lipids, as well as the inhibition of mitochondrial beta-oxidation of long-chain fatty acids, triglyceride formation, insulin resistance and hyperglycaemia.43
Fruit juice is considered by the public, and by many healthcare professionals, to be a healthy beverage. It is recommended by the HSE, the WHO and other health agencies in many countries as a suitable alternative to whole fruit in making up a person’s ‘five a day’. This is despite fruit juice having a similar energy and sugar content to many soft drinks eg. a 250ml serving of apple juice typically contains 110kcal and 26g of sugar whereas a 250ml serving of cola typically contains 105kcal and 26·5g of sugar.44
Gill et al recently surveyed a group of 2,005 adults in the UK and asked them to estimate the sugar content of a variety of soft drinks, smoothies and fruit juices. They found that the sugar content of fruit juices and smoothies was underestimated by 48% on average, whereas the sugar content of carbonated drinks was overestimated by 12%.48 This indicates that it is likely that many other people also overestimate the health benefits of 100% fruit juice.
While the direct effects of fruit juice consumption on diabetes risk and glycaemic control are conflicting, it is possible that the excess calories consumed by those drinking fruit juices offsets any health benefits obtained from fruit juice.
In the US, recent publications have called for the elimination of fruit juice from childrens’ diets in an attempt to reduce rates of childhood obesity.45 The American Academy of Pediatrics46 and the American Heart Association47 recommend that children aged from one to six years old consume no more than four to 6oz per day of 100% fruit juice.
The effect of fruit juice on glucose control is not new information (Figure 1). In spite of this, many patients and healthcare professionals are unaware of the harms of this purportedly healthy beverage.