A recent review that casts a rosy glow on the trend for fruit smoothies because they can help consumers meet five a day fruit-and-veg targets has caused uproar amongst dentists, who say the sugar and acid content can cause tooth erosion.
According to 2008 figures from the UK's Food Standards, 74 per cent of consumers now say they are aware of the five-a-day advice, and 58 per cent claim to be complying with it. These figures are, however, thought to be somewhat inflated.
Dr Carrie Ruxton, author of the new review, which is published in the Nutrition Bulletin (Blackwells for the British Nutrition Foundation), argues that fruit smoothies should be counted as more than one portion of fruit, rather than the current Department of Health view that they may count as just one, no matter how much in drunk.
However the opinion has been slammed by the British Dental Federation, which has responded by saying that too many smoothies could cause an "erosion epidemic".
Dr Nigel Carter, chief executive of foundation, said: "Fruit smoothies are becoming increasingly popular and the fruit content can make them seem like a good idea. However, they contain very high levels of sugar and acid and so can do a lot of damage to the teeth."
He said that every sip of smoothie puts teeth "under acid attach for up to an hour", which can cause the protective enamel to erode, causing pain, sensitivity, and decay.
Dr Ruxton conducted a comparison of the nutritional content of smoothies and orange juice, based on the UK market leading smoothie brand Innocent.
She saw that smoothies - which are combinations of homogenised fruit and juice - contain more energy, carbohydrate, sugars, fibre and vitamin C per 100g than juice.
Innocent smoothies contain no added sugar, but 12.1g per 100g. Orange juice, by contrast, contains 9g of sugar per 100g.
While Dr Carter said his view is borne out by the scientific research that found links between oral health and a range of serious health conditions, Dr Ruxton actually sought to tackle the question of smoothies and dental health in her article.
"The current UK classification of sugars makes a distinuction between the supposedly cariogenic non-milk extrinsic (NME) sugars present in fruit juice/smoothies and the 'less cariogenic' intrinsic sugars present in whole fruit," she wrote.
"This would mean, in theory, that drinking a smoothie would represented a greater risk to dental health than eating two portions of whole fruit. However this is not borne out by the evidence."
Dr Ruxton drew attention to two human clinical studies she sees as supporting her view: Hussein et al 1996, which looked at plaque pH following consumption of fruits left whole versus those that had been processed; and Issa et all 2003, which used in situ enable slabs to look more precisely at caries risk following whole and processed fruits.
A third study, Beighton et al 2004, looked at saliva samples of volunteers after whole and juices fruit consumption.
Taken together, Dr Ruxton said that fruit in any form has the potential to adversely affect dental health - whether in juice form, smoothie form, or whole fruit.
"The nutritional benefits offered by smoothies far outweight any risks, and re-evaluation of the five-a-day criteria is justified," concluded the researcher.
Smoothies are a relatively recent addition to the retail chiller cabinet, and although no precise market data is available, their share is understood to be growing every year. According to a YouGov survey cited in the Nutrition Bulletin, 37 per cent of UK consumers asked said they occasionally or regularly drank smoothies.
The Nutrition Bulletin study has been welcomed by Innocent, which said: "Dr Shilpee [Innocent's company nutritionalist] has been in to see the Department of Health and Food Standards Agency a few times, and hopefully this latest research will help them to review the current guidelines and help more people hit their five a day target."
The company has not passed comment on the issue of dental health.
Source: Nutrition Bulletin, 33, 129-132
"Smoothies: one portion or two?
Author: CHS Ruxton