Gut health is of major public importance and low stool weight, delayed gut transit time and alterations in the gut microbiome along with their associated metabolites, for example, short-chain fatty acids (SCFAs), are key risk factors for gastrointestinal disorders, all of which can be manipulated via the diet. Increased stool weight is one of the major mechanisms underlying the causal relationship between high intakes of dietary fibre and reduced risk of colorectal cancer (Cummings et al. 1992). The relationship is not linear for all fibres, depending on their fermentability, but data from cohort studies indicate that 7 g additional fibre intake/person/day is associated with an expected 8% reduction in colorectal cancer risk (RR 0.92, p = 0.002) (SACN 2015). tavazo

Dried fruits are high in a range of dietary fibres and other bioactive compounds with prebiotic effects (e.g. polyphenols), while some dried fruits (e.g. prunes and apricots) also contain high levels of sorbitol, which has laxative properties and also increases stool weight. Inevitably studies have investigated the impact of dried fruit on faecal weight and transit time. A small, non-randomised, cross-over trial in 16 subjects failed to find a significant effect of three doses of raisins (85, 126 and 168 g/day) on faecal weight or reduced transit time, although this study had considerable limitations including non-randomised design and no wash-out periods between the doses (Spiller et al. 2003).

A more recent, well-designed randomised, cross-over trial in 21 healthy human volunteers found that 50 g dates (3.9 g fibre, 1 g sorbitol)/day for 3 weeks compared with a maltodextrin and dextrose control had a statistically significant benefit on stool frequency with no evidence of gastrointestinal side-effects (Eid et al. 2015). However, in contrast to stool weight, stool frequency is not associated with any known beneficial health effects. There were no statistically significant changes in the growth of selected bacterial groups or SCFA production in this study, but a post-hoc analysis found that volunteers with lower fibre intake (mean 6g/day) showed statistically significant increases in faecal bacterial numbers for six bacterial types, including BifidobacteriumClostridium and Roseburia subspecies, in contrast to volunteers with habitual higher fibre intakes (mean 18.5 g/day) who did not experience any changes in gut microbiome.

A meta-analysis of randomised controlled trials investigating the impact of prunes (dried plums) on stool output found a statistically significant increase in stool frequency of 1 stool/week, with no impact on stool consistency (Lever et al. 2014). Most of the effect was driven by volunteers with constipation with fewer effects in healthy people. A more recent, robust, randomised dose–response trial compared 80 g and 120 g prunes plus 300 ml water/day with a control of 300 ml water/day in 120 subjects habitually passing 3–6 stools/week and with a low fibre intake. The study showed a positive impact of prunes on the primary outcome of stool weight, and on change in stool weight. There was also a significant impact of 80 g prunes/day on increased stool frequency, despite no effect on whole gut transit time. Prunes also resulted in a greater increase in bifidobacteria compared with baseline (Lever et al. 2018).

With a limited number of human studies showing some benefit of traditional dried fruits in some areas of gut health, more studies are warranted to extend our knowledge of the potential beneficial impact for public health, particularly investigating other dried fruits and investigating the relative contribution of fibre and sorbitol to these effects. This highlights a challenge for public health advice that responses to dietary advice in individuals may differ depending upon genetics, lifestyle and typical dietary intake.

Polyphenols and dried fruit

A global analysis of deaths from non-communicable diseases attributable to behavioural and dietary risk factors has suggested that diets low in fruits are the third most important risk factor, behind high blood pressure (first), and active and passive smoking (second), with lifestyle risk factors acting either directly or through conditions such as elevated blood pressure, blood glucose and blood cholesterol. Of the individual dietary risk factors, low fruit consumption was suggested to be more important than high dietary salt, alcohol use, and diets low in nuts and seeds, vegetables, whole grains and fish/seafood (Ezzati and Riboli 2013). One possible explanation for the important role of fruits is their content of phytonutrients, including the major group termed polyphenols. These are naturally-occurring components of foods and beverages that can influence blood pressure and blood glucose, and have been proposed to reduce the risk of metabolic chronic conditions such as cardiovascular disease and type-2 diabetes (T2DM). Comparison of raisins and grapes shows that drying concentrates the content of polyphenols and thus antioxidant activity, and this is supported by comparison with other traditional dried fruits.  Comparison of raisins and prunes with grapes and fresh plums suggests that drying influences the content of individual polyphenols and highlights varietal differences.

The bioavailability of phytonutrients is now generally understood, but their benefits on health and mechanisms of action remain contentious. These were initially investigated in human cells in vitro, complemented by studies in experimental animal models and in healthy and at-risk human volunteers. Though many phytonutrients are antioxidants, this concept is based on in vitro studies assessing antioxidant content rather than functionality. Measuring antioxidant activity essentially indicates the polyphenol content, rather than providing information about its impact on health.

The mechanisms of action of polyphenols are much more complex than direct antioxidant activity and involve effects on blood vessel health through vasodilation, on various oxidative processes in cells such as superoxide production, and on nutrient absorption and energy metabolism, including glucose. Research has shown that polyphenols can blunt postprandial spikes in blood glucose, which is beneficial to health. A study comparing sedentary adults, pre-diabetics and endurance athletes found that the glucose response to raisins was not very different across these groups (Kim et al. 2008). However, there was much greater variation in the insulin response, demonstrating that healthy adults produce less insulin to obtain the same glycaemic response. This needs to be investigated in other population groups such as obese individuals as well as comparison of responses to fresh and dried fruits.

Polyphenols are fermented by the colonic microbiota and microbiome metabolites from polyphenols and other phytonutrients could potentially contribute to health benefits, in addition to the parent compounds present in dried fruit (Cardona et al. 2013). Little is currently known about this for dried fruits and this is another area for future research.

Dried fruit and appetite – the psychology of snacking in relation to obesity

Current trends in eating patterns show that snacking is becoming more ubiquitous. A study reported that the median number of eating occasions was 6 per day (decile range 3–10), reflecting erratic daily eating patterns spread over the day (Gill and Panda 2015). Snacking is associated with concepts such as “junk” food or empty calories, and snack foods are often referred to in the context of loss of appetite control, obesity and overconsumption. Food choices that constitute “healthy snacking” are therefore being recommended, and the identification of foods that help to improve appetite control is a key area of research. An avenue of current research is to investigate whether traditional dried fruits could be a useful strategy to improve appetite control on account of their high palatability and a low Glycaemic Index (GI).

Not all individuals are at risk of weight gain from snacking, but homeostatic and hedonic phenotypes exist within obese populations. Identification of susceptible phenotypes for appetite control and determining the role of snacking behaviour in these phenotypes is the subject of current research. Susceptible phenotypes might express appetite through weak satiation, fragile satiety, obesogenic food choices, or excessive wanting and liking. A low satiety phenotype has been identified, characterised by poorer weight loss outcomes and a greater tendency to snack when given access to high energy density foods (Barkeling et al. 2007; Drapeau et al. 2013). Research has also identified a “binge-eating type” characterised by consumption of a large amount of food in a short period of time (Bruce and Wilfley 1996), and this is present in 25–50% of obese individuals.