Experiments were carried out in human volunteers to investigate the mechanism by which guar gum improves glucose tolerance. Guar reduced both plasma glucose and insulin responses to an oral glucose load, and delayed gastric emptying. However, there was no correlation between changes in individual blood glucose responses and changes in gastric emptying rates induced by guar. With a steady-state perfusion technique, glucose absorption was found to be significantly reduced during perfusion of the jejunum with solutions containing guar, but returned to control values during subsequent guar-free perfusions. Preperfusing the intestine with guar did not affect electrical measurements of unstirred layer thickness in the human jejunum in vivo. Experiments in vitro established that glucose diffusion out of a guar/glucose mixture was delayed under conditions of constant stirring. We conclude that guar improves glucose tolerance predominantly by reducing glucose absorption in the small intestine. It probably does this by inhibiting the effects of intestinal motility on fluid convection.

To test the hypothesis that fish oil supplementation can inhibit the expression of functionally associated molecules on the surface of human blood monocytes, we randomly assigned 12 healthy adults to receive either an n-3 polyunsaturated fatty acid-rich fish oil supplement for 21 d or to receive no supplement. The percentage of monocytes expressing major histocompatibility complex (MHC) class II molecules (HLA-DR, -DP, and -DQ), intercellular adhesion molecule-1, and leukocyte-function-associated antigen-1, and the intensity of expression of each molecule were quantified before and after the study period. Monocytes were examined immediately after blood sampling and again after incubation in serum-free culture medium for 24 h in the presence of interferon-gamma to up-regulate expression of MHC class II molecules by the monocytes. The intensity of expression of all the monocyte surface molecules examined was significantly reduced after fish oil supplementation (P < 0.025), although there was no change in the percentage of monocytes expressing each molecule. After incubation with interferon-gamma, there was a similar inhibition of surface molecule expression (with the exception of HLA-DQ) by monocytes from the fish oil-supplemented group, and there was a reduction in the percentage of monocytes expressing both HLA-DR and -DP molecules (P < 0.025). No significant changes were observed in the reference group. Dietary supplementation with fish oil can inhibit the expression of surface molecules involved in the function of human antigen-presenting cells, a potential mechanism by which n-3 fatty acids may suppress cell-mediated immune responses.

Polyphenolic compounds are abundant throughout the plant kingdom and are found in a wide variety of human foods. The flavonoids, which are the best defined group of polyphenols in the human diet, themselves comprise a large and complex group, all of which contain a three-ring structure with two aromatic centres and a central oxygenated heterocycle. Recent evidence suggests that significant quantities of quercetin and possibly myricetin and kaempferol are absorbed in the gut. A larger fraction probably remains in the lumen, and thus a substantial proportion of the gastrointestinal mucosa is exposed to biologically significant concentrations of these compounds. A substantial body of experimental work has established that flavonoids can suppress carcinogenesis in animal models and there is considerable interest in the biological effects of these compounds at the cellular level. Flavonoids interact with cellular signal pathways controlling the cell cycle, differentiation and apoptosis. Their potentially antineoplastic effects include antioxidant activity, induction of Phase II enzyme activity, inhibition of protein kinases and interactions with Type II estrogen binding sites. Naturally occurring polyphenolic compounds may play a role in the protective effects of fruits and vegetables against cancers in general, and they appear to have considerable potential for pharmaceutical uses as chemopreventive agents against neoplastic changes in the alimentary tract. Future research should therefore focus on the biological effects of flavonoids in the human body, using biomarkers to define their effects at each stage in the onset of neoplasia.