Do BGs Stick To Cholesterol?
Carbohydrates sometimes come in big packages. They are polymers linked by certain connections that afford specific physical and physiological properties. Beta-glucans (BG), as their name implies, connect individual sugar molecules, via beta linkages, into polymer chains. Oats and barley represent the major grain sources of BG, which is the primary soluble, gummy fibre found in these plants. However, yeast (the brewing and baking type) and various mushrooms also harbor a notable amount of BG, with a different linkage pattern, and which are not soluble. About one-third of yeast BG-derived fibre is soluble fibre.
The buzz over BG centers around evidence that ingestion of BG-rich foods and supplements can favorably alter blood lipid/cholesterol profiles. Based upon a number of studies from the 1960s supporting oat product-induced cholesterol lowering,1 and a petition by Quaker Oats in 1997, the US Food & Drug Administration allowed a heart disease risk-reduction health claim for whole-oat products (oatmeal, oat bran and oat flour). This was associated with a minimum intake of 3g/day of BG. Not all of the more recent studies of BG products have been positive.
In one study with 156 subjects, researchers found three different doses of oatmeal or oat bran products (over 6 weeks) to produce variable reductions in LDL cholesterol,2 with the highest BG dose (6g/day) being no better than the lowest dose of 3.6g/day. A later study put the FDA's recommendation of 3g/day to the test using 62 British men and women subjects.3 Eight weeks of supplementing with oat bran (providing 3g/day of BG) showed no positive effect on any blood lipid marker, even resulting in a decrease in HDL cholesterol and an increase in fasting blood sugar (the latter seen in the wheat bran-supplemented control group also). Barley-derived BG, although much less studied than oat BG, also may have modest lipid-lowering effects.4 Only one clinical study to date has been done with yeast BG (15g/day), which showed a drop in LDL cholesterol (typically not observed with oats) over eight weeks in an open-label (no placebo group) trial.5
The Greening Of Diabetes
Tea spells health. Sales of tea in the US in 2001 reached nearly $5 billion,6 whereas in other countries (particularly in the UK, India, Pan-Asia) volumes per capita are notably higher.
One promising added health benefit brewing for green tea is favorable alterations of carbohydrate (glucose) metabolism. Japanese studies suggest that green tea and its active 'catechins' influence both carbohydrate absorption7 and carbohydrate metabolism.8
A recent report from the US Department of Agriculture supports green tea and its most popular catechin, EGCG (epigallocatechin gallate), for its insulin-enhancing action.9 Using a test tube system that can assess the ability of various agents to 'potentiate' insulin's effects on promoting the entry of glucose into cells, they found EGCG to be the prime mover of glucose into cells.
Another in vitro study showed EGCG may also benefit the diabetic by mimicking the actions of insulin and 'muting' the liver's own production of glucose (gluconeogenesis).10 Shutting down this internal 'glucose faucet' could result in lowering blood sugar. Feeding green tea-enriched diets to animals with type 1-like diabetes (chemically-induced) can improve kidney function.11 In another study, animals with a similar type of diabetes that consumed green tea had lowered blood glucose and an increase in blood antioxidant markers.12 Rather than getting 'steeped' in the promises of these in vitro and animal studies, consumers should await the results of human trials of green tea /catechin effects on carbohydrate metabolism and insulin dysfunction.
Anthony Almada, BSc, MSc, is co-founder of EAS and founder and chief scientific officer of IMAGINutrition in Laguna Niguel, California.
1. Ripsin CM, et al. Oat products and lipid lowering. A meta-analysis. JAMA 1992;267:3317-25.
2. Davidson MH, et al. The hypocholesterolemic effects of beta-glucan in oatmeal and oat bran. A dose-controlled study. JAMA 1991;265:1833-9.
3. Lovegrove JA, et al. Modest doses of beta-glucan do not reduce concentrations of potentially atherogenic lipoproteins. Am J Clin Nutr 2000;72:49-55.
4. McIntosh GH, et al. Barley and wheat foods: influence on plasma cholesterol concentrations in hypercholesterolemic men. Am J Clin Nutr 1991;53:1205-9.
5. Nicolosi R, et al. Plasma lipid changes after supplementation with beta-glucan fibre from yeast. Am J Clin Nutr 1999;70:208-12.
6. Plotkin R. Menu trends: it's tea time. Restaur Hospitality 2002;86:66-8.
7. Kobayashi Y, et al. Green tea polyphenols inhibit the sodium-dependent glucose transporter of intestinal epithelial cells by a competitive mechanism. J Agric Food Chem 2000;48:5618-23.
8. Matsumoto N, et al. Reduction of blood glucose levels by tea catechin. Biosci Biotech Biochem 1993;57:525-7.
9. Anderson RA, Polansky MM. Tea enhances insulin activity. J Agric Food Chem 2002;50:7182-6.
10. Waltner-Law ME, et al. Epigallocatechin gallate, a constituent of green tea, represses hepatic glucose production. J Biol Chem 2002;277:34933-40.
11. Rhee SJ, et al. Effects of green tea catechin on prostaglandin synthesis of renal glomerular and renal dysfunction in streptozotocin-induced diabetic rats. Asia Pacific J Clin Nutr 2002;11:232-6.
12. Sabu MC, et al. Anti-diabetic activity of green tea polyphenols and their role in reducing oxidative stress in experimental diabetes. J Ethnopharmacol 2002;83:109-16.