Epidemiological studies find that whole-grain intake is protective against cancer, cardiovascular disease, diabetes and obesity. Potential mechanisms for this protection are diverse since whole grains are rich in nutrients and phytochemicals. First, whole grains are concentrated sources of dietary fibre, resistant starch and oligosaccharides, carbohydrates that escape digestion in the small intestine and are fermented in the gut, producing short-chain fatty acids (SCFA). SCFA lower colonic pH, serve as an energy source for the colonocytes and may alter blood lipids. These improvements in the gut environment may provide immune protection beyond the gut. Second, whole grains are rich in antioxidants, including trace minerals and phenolic compounds, and these compounds have been linked to disease prevention. Additionally, whole grains mediate insulin and glucose responses. Although lower glycaemic load and glycaemic index have been linked to diabetes and obesity, risk of cancers such as colon and breast cancer have also been linked to high intake of readily-available carbohydrate. Finally, whole grains contain many other compounds that may protect against chronic disease. These compunds include phytate, phyto-oestrogens such as lignan, plant stanols and sterols, and vitamins and minerals. As a consequence of the traditional models of conducting nutrition studies on isolated nutrients, few studies exist on the biological effects of increased whole-grain intake. The few whole-grain feeding studies that are available show improvements in biomarkers with whole-grain consumption, such as weight loss, blood lipid improvement and antioxidant protection.

Large-bowel fermentation of resistant starch produces SCFA that are believed to be important in maintaining visceral function. High-amylose maize starch (HAMS) and acylated starches are sources of resistant starch and are an effective means of increasing colonic SCFA. Cooking increases digestibility of starches but its effects on the capacity of these starches to raise large-bowel SCFA are unknown. We have examined the effects of cooking of HAMS and butyrylated HAMS (HAMSB) on amylolysis in vitro and their capacity to raise caeco-colonic SCFA in rats. The starches were boiled in excess water and microwaved, followed by drying at 100°C. Cooking increased in vitro glucose release for both starches but significantly less from HAMSB. Rat growth rates were unaffected when fed cooked resistant starch. Digesta pH was increased in the caecum and proximal colon of rats fed cooked HAMS. Distal colonic pH was highest in rats fed cooked HAMSB. Factorial analyses (2×2) of caecal SCFA pools showed significant differences between HAMS and HAMSB, and that cooking significantly lowered caecal butyrate pools. Portal venous butyrate concentrations were higher in both HAMSB groups than those fed HAMS. The data suggest that HAMSB is less susceptible to in vitro amylolysis than HAMS following cooking and delivers more butyrate to rat caecum than HAMS. This attribute may be useful in food applications for specific delivery of SCFA to the colon. Preparation of carbohydrates to simulate human food in animal experiments may be important to assess nutritional and physiological effects accurately.

The digestion of four sources of resistant starch (RS) has been studied in twelve healthy volunteers who ate controlled diets for 15 d periods. RS from potato, banana, wheat and maize (17−30 g/d) was compared with a starch-free diet, a diet containing wheat starch that was fully digested in the small intestine, and with 18·4 g NSP from bran/d. RS increased stool wet weight by 1·6 g/d per g RS fed for potato, 1·7 for banana, 2·5 for wheat and 2·7 for maize, but this was significantly less than bran NSP at 4·9 g/g. RS was extensively digested in twenty-seven of thirtyfour diet periods but five subjects were unable to break down one or two of the RS sources. Faecal N and energy excretion were increased. RS decreased NSP breakdown and RS2 (resistant starch granules) tended to prolong transit time. All forms of RS increased faecal total short-chain fatty acid excretion. RS2 (from potato and banana) gave greater proportions of acetate in faeces, and RS3 (retrograded starch from wheat and maize) more propionate. We have concluded that RS2 and RS3 are broken down in the human gut, probably in the colon although in 26% of cases this breakdown was impaired. RS exerts mild laxative properties, predominantly through stimulation of biomass excretion but also through some sparing of NSP breakdown.