Fruit and looking after your gut

Most of us are aware from extensive public media coverage that dietary fibre is an important component of a healthy diet, and numerous published research studies have shown high intake is related to lower risk of developing coronary heart disease, stroke, hypertension, diabetes, obesity, and certain gastrointestinal diseases, while enhancing immune function. The 2019 Global Burden of Disease Study on health effects of dietary risks in 195 countries estimated the level of daily fibre intake from all sources (fruits, vegetables, cereals and legumes) which minimises the risk from all causes of mortality as 24 g/d, comparable to an Australian NHMRC recommendation of 30 and 25g/d for men and women respectively. A 2017 CSIRO study of fruit and vegetable intake in 145,000 adults found only 24% of women and 15% of men met the Australian Guidelines for both – 300g/d fruit and 375450g/d vegetables. These intake estimates are inevitably over-stated as it was a voluntary on-line survey, and more rigorously controlled studies have shown such respondents bias their answers towards what is publicly acceptable or recommended, and many who recognise they’re way below don’t participate. In Australia, the main sources of fibre are fruit, vegetables, bread and cereals, but the widespread consumption of refined and processed bread and cereal products (eg white bread, white rice and non-wholemeal spaghetti) that have much less fibre than whole grain sources compromises attainment of these goals, and what often counts as fruit consumption includes fruit juices where almost all the fibre has been removed.

Past research indicates that fruit may be more effective than vegetables in minimising some chronic disease risks, and the following is a summary of a Harvard study of middle-aged US nurses (Gastroenterology (2013) 145, 970–977) that investigated whether particular sources of fibre were better than others in preventing inflammatory bowel disease.

Increased intake of dietary fibre has been proposed to reduce the risk of inflammatory bowel disease (Crohn’s disease [CD] and ulcerative colitis [UC]). However, few prospective studies have examined associations between long-term intake of dietary fibre and risk of incident CD or UC. We collected and analysed data from 170,776 women, followed up over 26 years, who participated in the Nurses’ Health Study, followed up for 3,317,425 person-years. Dietary information was prospectively ascertained via administration of a validated semi-quantitative food frequency questionnaire every 4 years. Self-reported CD and UC were confirmed through review of medical records by gastroenterologists. Cox proportional hazards models, adjusting for potential confounders, were used to calculate hazard ratios. We confirmed 269 incident cases of CD (incidence 8/100,000 person-years) and 338 cases of UC (incidence, 10/100,000 person-years). Compared with the lowest 20% of energy-adjusted cumulative average intake of dietary fibre (median 12.7g/d), intake of the highest 20% (24.3 g/d) was associated with a 40% reduction in risk of CD. This apparent reduction appeared to be greatest for fibre derived from fruits; fibre from cereals, whole grains, or legumes did not modify risk. In contrast, neither total intake of dietary fibre nor intake of fibre from specific sources appeared to be significantly associated with risk of UC. Based on data from the Nurses’ Health Study, long-term intake of dietary fibre, particularly from fruit, is associated with lower risk of CD but not UC. Further studies are needed to determine the mechanisms that mediate this association.

Further information on the study:

The average age of participants at entry to the study was 42-44, and in addition to the 4-yearly diet reviews, study participants had biennial reviews of environmental exposures and health outcomes.
The food frequency questionnaire used to assess dietary fibre intake consisted of 121 food items, later expanded to 136, and individual nutrients derived from these foods were based on USDA tables; there were 15 questions on 20 fruits and 28 on vegetables.
The median fibre intake from fruits in the highest 20% of fruit intake was 6.4 g/d, which is the equivalent of slightly more than 2 medium-sized apples or bananas.
Prospective epidemiological studies are those where consenting subjects are enrolled, and relevant data collected. Then after a certain time the incidence of the disease, diseases or biological markers under study are recorded, and the data are analysed to see if there are any associations with particular lifestyle or biological characteristics eg age, gender, blood pressure, smoking, physical activity, diet etc. The gold standard procedure for establishing cause and effect between risk factors and diseases is the randomised controlled trial, but as so many chronic diseases can take several decades to develop, these are essentially not feasible. The next best approach is to use a prospective study design; case control are less reliable because of problems such as suitable matching of cases, memory recall, reverse causation (people modifying their behaviour with the onset of disease symptoms or diagnosis) and case reports suffer from small sample size.
Unlike many dietary prospective studies where diet is only recorded on entry, the present report updated this every 4 yrs and so was able to take account of any changes throughout the 26 yr period.
Confounders are various factors that could or have been shown to have an effect on the studied variable, eg if disease incidence markedly increases with age, then studying the effect of some other variable or risk factor on the disease will be incorrectly assessed if age is not taken into account.
Fibre consists of those food forms that are not digested in the small intestine and pass through to the colon where they may be fermented or add to stool bulk. Some plant foods have a relatively low percentage fibre content eg 2-6% for fruits and vegetables as water content is 80-95%, and in other drier foods it can be much higher, eg whole grain cereals 5-15%, wheat bran 25-35%, and psyllium husks 70-85%. There are many types of fibre and they are nowadays best understood with more than the familiar classifications of soluble and insoluble.

General comments

The main goals in the above study were to see if there were any significant associations between different forms of dietary fibre intake and subsequent disease. Once established, subsequent research questions are to ask how these effects are produced. While prospective epidemiological studies have very limited ability in this regard and laboratory and animal studies are usually required, the authors did suggest some possible mechanisms. Patients with inflammatory bowel disease have an abnormal gut microbiome and dietary patterns have been proposed by others to explain more than half the variation in the adult intestinal microbiome – fibre could modify microbial composition and possibly the risk of CD. Also fruit fibre is more rapidly and completely fermented by colonic bacteria than cereal fibre, with the production of short chain fatty acids that can inhibit pro-inflammatory mediators. The larger component of soluble fibre types in fruit could also help maintain a more effective intestinal barrier to limit invasion by bacteria associated with CD. Another mechanism they suggested is that fruit fibre could activate a known cyclic hydrocarbon receptor that mediates protection against environmental antigens.

Interestingly in the same period as this study, an Oxford group also found differential effects of fibre sources on the incidence of diverticular disease (Gut (2014) 63,1450–1456). Diverticulosis is an asymptomatic condition occurring in about half of 60-80yr olds where small pouches form at weak spots in the wall of the colon; the symptomatic condition is called diverticular disease. For this study, 690,075 UK women, average age 60, were enrolled and followed for over 6 yrs. They found 17325 were admitted to hospital or died with diverticular disease. The main sources of dietary fibre (mean intake for all was 13.8g/d) were cereals (42%), fruit (22%), vegetables (19%) and potatoes (15%). After adjustment for confounders, the relative risks for diverticular disease were 16% less for ea 5g/d cereal fibre consumed and 19% less per 5g/d fruit fibre; vegetables and potato had no effect. Fruit fibre was more effective for women with normal BMI (30% less risk for ea 5g/d) compared to those who were overweight or obese (18% less risk).

In all similar nutrition studies it should be remembered that effects found for different consumption levels can only reflect what exists in study populations. Most Westernised people don’t consume anywhere near enough fibre (a 2016 US study found <5% of adults met the recommended level), and when significant effects are reported for the highest level it doesn’t mean that this is the maximum possible. Indeed, in most of these studies a linear effect is seen over the studied ingredient range without any evidence of a plateau. Consequently, any conclusions regarding possible effects at higher levels typically have to remain open-ended, lacking data, or are not mentioned at all. The Homo sapiens genome evolved from primate ancestors over millions of years, and during this period fibre intake was much higher than nowadays; archaeological evidence suggests it was at least 70g/d. Uncultivated fruit and vegetables eaten in hunter-gather times were a major dietary component and had 3-4 times the fibre content of our modern highly-bred lines; cereals were a very small component till the dawn of the agricultural era, about 10,000yrs ago. Importantly for us when contemplating intake beyond recommended minimal levels, the US Institute of Medicine (2019) declared there is insufficient evidence of adverse effects serious enough to warrant setting an upper limit, unlike many other nutrients. However if you decide to increase your fibre intake, it should be done gradually to allow time for the body to adjust.

Consistent with beneficial health effects in the 2 gut studies above, obtaining an increased proportion of daily fibre from fruit and vegetables instead of cereals results in lower exposures to anti-nutrients such as lectins, protease inhibitors and phytate. And on a broader perspective beyond fibre as the sole nutrient under study, there is now a steadily growing number of dietary studies on other chronic diseases in cohorts with higher intakes than national averages. These suggest we should move towards 8-10 servings/d, beyond present government recommendations that are usually not ideal and represent a compromise between where the public is at (too big a change runs increasing risk of the public not engaging), what politicians support, cost and profits, traditional practices, scientific evidence and different health authority perspectives, food industry lobbying, fringe group beliefs etc. A wise approach is not to be too nervous going beyond 2 servings/d fruit consumption (300g/d), plus you get to enjoy more of those you like.