Tinned fruit on your shopping list?

"Eat to Live, Not Live to Eat"
John H Weisburger, 2000

The evidence for beneficial effects of fruit and vegetables on prevention of many chronic diseases is overwhelmingly convincing, and as a consequence public health authorities and governments worldwide recommend we should eat a minimum of 5 servings per day. Regarding fruit intake studies and promotion campaigns, most often no distinction is made between fresh and processed forms (ie frozen, dried, canned or purees/juices) in the interests of obtaining data on what people may include in their regular diet, and also encouraging people to meet the recommendations with this wider range of options. Nevertheless in almost all countries the target goals are far from being met. There are fewer studies where the different fruit forms have been individually assessed for health outcomes, even though it’s known that inevitably the processing may affect nutritional properties to varying degrees. In addition, with the commercial offerings many additional unhealthy ingredients may be incorporated. For example, packaged banana chips seen in supermarket aisles start off with basically no saturated fat in the fresh fruit, but in the interests of increasing market share and profitability they can finish up with more than 15%. The attractions of processed products are many, ranging from greatly increased storability, lower cost (the fresh market demands perfectly coloured, blemish-free fruit with a large proportion being unsuitable and sent to food processors), greater availability outside the harvest season, palatability (eg plantains), different tastes (eg dried fruit where moisture levels are reduced leading to increased sweetness), reduction in bitterness (eg quinces), faster preparation and ease of use with skins and/or seeds removed, and cultural traditions (eg olive oil). However, usually the processing steps have negative effects, and the question must be asked whether grouping all these food forms together in studies and recommendations as ‘fruit’ is a valid assumption from a nutritional perspective?

The following summary of a recent study by a research group from Oxford, Cambridge and University College London (PLOS One (2015) February 25, 1-15) addresses this issue with respect to canned fruit.

Dietary recommendations to promote health include fresh, frozen and tinned fruit, but few studies have examined the health benefits of tinned fruit. We therefore studied the association between tinned fruit consumption and mortality. We followed up participants from three prospective cohorts in the UK: 22,421 participants from the European Prospective Investigation into Cancer and Nutrition (EPIC)-Norfolk cohort (1993–2012), 52,625 participants from the EPIC-Oxford cohort (1993–2012), and 7440 participants from the Whitehall II cohort (1991–2012), all reporting no history of heart attack, stroke, or cancer when entering these studies. We estimated the association between frequency of tinned fruit consumption and all-cause mortality using Cox regression models within each cohort, and pooled hazard ratios across cohorts using random-effects meta-analysis. Tinned fruit consumption was assessed with validated food frequency questionnaires including specific questions about tinned fruit.

During 1,305,330 person years of follow-up, 8857 deaths occurred. After adjustment for lifestyle factors and risk markers the pooled hazard ratios of all-cause mortality compared with the reference group of tinned fruit consumption less often than one serving per month were: 5% greater for one to three servings per month, 10% for one serving per week, and 13% for two or more servings per week. Analysis of cause-specific mortality showed that tinned fruit consumption was associated with mortality from cardiovascular causes and from non-cardiovascular, non-cancer causes. In a pooled analysis of three prospective cohorts from the United Kingdom self-reported tinned fruit consumption in the 1990s was weakly but positively associated with mortality during long-term follow-up.

These findings raise questions about the evidence underlying dietary recommendations to promote tinned fruit consumption as part of a healthy diet.

In the health sciences, double-blind randomised controlled trials are the gold standard for evaluating the effectiveness or otherwise of a particular treatment or intervention. Unfortunately in studying the effect of diet on chronic diseases which may take several decades to develop, these are very difficult/impossible to organise because of logistical, ethical or cost limitations. Usually the best we can do is rely on prospective epidemiological studies where a number of non-diseased participants are entered into a trial at baseline, relevant features are recorded (eg diet, gender, age, smoking status etc) and they are then followed for a given time to see who develops a certain outcome. The features of these participants are then compared with those who remain disease free to try and gauge which factors may be related to outcome. This design is what was followed in the present study, and greater confidence can be placed on findings than in case control studies where a given number of cases are selected for a study and then matched with suitable controls to consider relationships. Major problems present in these types of study are reliable collection of data, the relevance of the study duration and possible unaccounted factors which may have influenced the results. In the case control design there can be real problems with the information provided by cases being influenced by the fact that they subsequently developed the disease during the study period (reverse causality). Controls might also not be well-matched in terms of factors that are relevant. In both design types there is also the possibility that some participants may alter their diet during the study period independently of disease outcome, so unless dietary information is updated throughout the study it can lead to erroneous conclusions. More confident interpretations can be made in both designs when findings are repeated in different settings by different research groups, with different study criteria and goals, and different racial participants. The authors state that their study is the first, certainly on this scale, to consider the effect of tinned fruit separately from all fruit combined, so it will now be important to see if their findings can be replicated by other research groups in other communities.

One of their subsidiary findings was that the tinned forms of particular fruits (apples, oranges, bananas, peaches and strawberries) also had significant adverse outcomes on all-cause mortality. This is important because different fruits have variable nutrient concentrations/profiles and these may be differentially affected during the canning process. But the absence of significant effects for other fruits cannot be overly interpreted. It may reflect a relative lack of data as the bulk of fruit consumption in the UK is these common fruits – tinned peaches for example are amongst the lowest cost fruit products available, and participants who ate most tinned fruit reported the lowest incomes. Far fewer mangos, avocados and litchis etc are eaten.

As the first large study to assess the health effects of canning fruit (there have been some studies on the other processed forms) and given it was not designed to investigate mechanisms, the authors nevertheless felt they should suggest several possibilities for exploration in future studies. Cardiovascular disease and cancer may take many decades to develop, and across the 3 cohorts (average age at entry 47-52, range 20-97) those reporting the highest tinned fruit consumption were generally older, reported lower education levels and were more physically inactive. As tinned fruit are the most acidic of canned foods, the authors suggested some of these older participants may have been exposed to lead and other heavy metals used in the manufacture of tin cans in those earlier periods. Nowadays regulatory standards are in place to minimise this exposure to sub-toxic levels. Bisphenol A is also widely used to coat the interior of food cans to prevent corrosion, and in one study a single serving of canned soup for 5 days showed a 1200% increase in urinary levels compared to non-canned soup. Urinary bisphenol A levels have also been positively associated with coronary artery disease in the US National Health and Nutrition Examination Survey and in the EPIC-Norfolk cohort. This substance is known to cause disruption of the endocrine system but most world authorities have now ruled that the regulated public exposure levels are not a general health risk. The authors also commented that a significant factor may be nutritional, as ingredients in canned fruit may have undergone varying degrees of heat labile degradation during processing eg loss of vitamin C. In addition the food industry usually adds a variety of other substances, some of which are known to be unhealthy eg sugars, fats and salt. Furthermore, many E-coded ingredients (eg acidity regulators, colouring and emulsification agents, stabilisers, preservatives etc) are also routinely added which may interact in various possibly undocumented ways with important fruit nutrients.

Although the authors reported a significant positive correlation between tinned fruit consumption and all-cause mortality, it was not a large effect with a maximum 13% increase. This small effect could have been influenced by the EPIC-Oxford cohort, the largest of the three, who were generally health-conscious with a large proportion of vegetarians. However instead of focussing on this unwanted feature, probably the more important finding is the absence of a healthy effect as tinned fruit is widely promoted as an equivalent alternative to other fruit forms (fresh, frozen, dried and juice). If this negative effect is confirmed in future studies it would mean that the well-documented preventive effect of fruit collectively has been diminished by the tinned component, and that the observed benefits (largely due to the fresh forms as these are the major component) would be even greater without them. Overall, this is the first large well-conducted study on this story, and we now need further evidence to help build confidence in the findings. In the meantime, if you’re interested in your health it might be best to adopt the maxim that ‘fresh is best’ when possible.

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