Ongoing research to develop digestion-resistant starch for human health promotion integrates the disciplines of starch chemistry, agronomy, analytical chemistry, food science, nutrition, pathology, and microbiology. The objectives of this research include identifying components of starch structure that confer digestion resistance, developing novel plants and starches, and modifying foods to incorporate these starches. Furthermore, recent and ongoing studies address the impact of digestion-resistant starches on the prevention and control of chronic human diseases, including diabetes, colon cancer, and obesity. This review provides a transdisciplinary overview of this field, including a description of types of resistant starches; factors in plants that affect digestion resistance; methods for starch analysis; challenges in developing food products with resistant starches; mammalian intestinal and gut bacterial metabolism; potential effects on gut microbiota; and impacts and mechanisms for the prevention and control of colon cancer, diabetes, and obesity. Although this has been an active area of research and considerable progress has been made, many questions regarding how to best use digestion-resistant starches in human diets for disease prevention must be answered before the full potential of resistant starches can be realized.

The major circulating form of vitamin D is 25-hydroxycholecalciferol [25(OH)D3], which is delivered to target tissues in complex with the serum vitamin D binding protein (DBP). We recently observed that mammary cells can metabolize 25(OH)D3 to 1,25-dihydroxycholecalciferol [1,25(OH)(2)D3], the vitamin D receptor (VDR) ligand, and the objective of our study was to elucidate the mechanisms by which the 25(OH)D3-DBP complex is internalized by mammary cells prior to metabolism. Using fluorescent microscopy and temperature-shift techniques, we found that T-47D breast cancer cells rapidly internalize DBP via endocytosis, which is blunted by receptor-associated protein, a specific inhibitor of megalin-mediated endocytosis. Endocytosis of DBP was associated with activation of VDR by 25(OH)D3 but not 1,25(OH)(2)D3 (as measured by induction of the VDR target gene, CYP24). We also found that megalin and its endocytic partner, cubilin, are coexpressed in normal murine mammary tissue, in nontransformed human mammary epithelial cell lines, and in some established human breast cancer cell lines. To our knowledge, our studies are the first to demonstrate that mammary-derived cells express megalin and cubilin, which contribute to the endocytic uptake of 25(OH)D3-DBP and activation of the VDR pathway.

The hepatic enzyme glycine N-methyltransferase (GNMT) plays a major role in the control of methyl group and homocysteine metabolism. Because disruption of these vital pathways is associated with numerous pathologies, understanding GNMT control is important for evaluating methyl group regulation. Recently, gluconeogenic conditions have been shown to modulate homocysteine metabolism and treatment with glucocorticoids and/or all-trans-retinoic acid (RA)-induced active GNMT protein, thereby leading to methyl group loss. This study was conducted to determine the effect of diabetes, alone and in combination with RA, on GNMT regulation. Diabetes and RA increased GNMT activity 87 and 148%, respectively. Moreover, the induction of GNMT activity by diabetes and RA was reflected in its abundance. Cell culture studies demonstrated that pretreatment with insulin prevented GNMT induction by both RA and dexamethasone. There was a significant decline in homocysteine concentrations in diabetic rats, owing in part to a 38% increase in the abundance of the transsulfuration enzyme cystathionine ␤-synthase; treatment of diabetic rats with RA prevented cystathionine ␤-synthase induction. A diabetic state also increased the activity of the folate-independent homocysteine remethylation enzyme betaine-homocysteine S-methyltransferase, whereas the activity of the folatedependent enzyme methionine synthase was diminished 52%. In contrast, RA treatment attenuated the streptozotocin-mediated increase in betaine-homocysteine Smethyltransferase, whereas methionine synthase activity remained diminished. These results indicate that both a diabetic condition and RA treatment have marked effects on the metabolism of methyl groups and homocysteine, a finding that may have significant implications for diabetics and their potential sensitivity to retinoids.

Mice lacking 25-hydroxycholecalciferol [25(OH)D]-1alpha-hydroxylase (CYP27B1) are growth retarded, hypocalcemic, and have poor bone mineralization. We tested whether high dietary cholecalciferol (VD3) could exert effects in the absence of CYP27B1 in vivo. Weanling male wild-type (WT) and CYP27B1 knockout (KO) mice were fed either a 2% calcium (Ca), 20% lactose rescue diet or an AIN93G diet (0.5% Ca, 0.4% phosphorus) containing 1000 (1K, the rodent requirement, 25 microg), 10,000 (10K, 250 microg), or 20,000 (20K, 500 microg) IU VD3/kg diet until 12 wk when blood and tissues were taken. Serum 25(OH)D was >90 nmol/L in the 1K diet group and increased >4-fold in mice fed 10K and 20K diets. The 1K diet impaired growth and caused hypocalcemia in KO mice; the 10K and 20K diets were as effective as the high Ca rescue diet in preventing these outcomes. High VD3 restored expression of vitamin D-regulated genes in intestine (calbindin D(9K)) and kidney (CYP27B1, 24-hydroxylase, calbindin D(9K)) of KO mice. Micro-computed tomography of femora revealed complete recovery of cortical bone in KO mice fed either the rescue or 10K diets but only partial recovery of trabecular bone measures (e.g. 40% lower bone volume, 20% lower trabecular thickness, and 23% increase in trabecular separation). These data show that very high serum 25(OH)D can influence Ca and bone metabolism independent of its conversion to 1,25 dihydroxycholecalciferol. However, neither high dietary Ca nor high dietary VD3 is sufficient to fully recover the phenotype of CYP27B1 KO mice.

Perturbation of folate and methyl group metabolism is associated with a number of pathological conditions, including cardiovascular disease and neoplastic development. Glycine N-methyltransferase (GNMT) is a key protein that functions to regulate the supply and utilization of methyl groups for S-adenosylmethionine (SAM)-dependent transmethylation reactions. Factors or conditions that have the ability to regulate GNMT and the generation of homocysteine, a product of transmethylation, have important implications in the potential perturbation of methyl group metabolism. We showed that retinoid compounds induce active hepatic GNMT, resulting in compromised transmethylation processes. Because retinoids can stimulate gluconeogenesis, a condition known to alter methyl group and homocysteine metabolism, the current study was undertaken to determine the relationship between all-trans-retinoic acid (RA) and gluconeogenic hormones on these metabolic pathways. Intact adrenal function was not required for RA to induce and activate hepatic GNMT; however, treatment of rats with dexamethasone (DEX) was as effective as RA in inducing GNMT in rat liver. The marked increase in plasma total homocysteine levels observed in adrenalectomized rats was reduced to normal levels by treatment with either RA or DEX, indicating that the transsulfuration and/or remethylation pathways may be enhanced. Moreover, coadministration of RA and DEX had an additive effect on GNMT induction. Similar findings were also observed in a rat hepatoma cell culture model using H4IIE cells. Taken together, these results demonstrate that both RA and DEX independently induce GNMT, thereby having substantial implications for the potential interaction of retinoid administration with diabetes.