Garlic Oil And Gender—Effects On Plasma Cholesterol And Glucose Levels - Brief Article

Nutrition Research Newsletter, July, 2001

Reduction in coronary heart disease (CHD) risk is an objective of affluent societies and has become a major reason for garlic consumption. The pharmacologic activities of garlic, including its claimed lipid-lowering effects, depend upon sulfur-rich compounds, most notably allicin. Allicin and its derivatives give rise to the characteristic odor and taste of garlic. Allicin is formed instantaneously when garlic is crushed because cellular disruption brings the enzyme alliinase into contact with its substrate alliin. Numerous studies have demonstrated biological activity for allicin and a variety of its derivatives. However, in vivo, such garlic derivatives have no detectable metabolites in blood or urine so that direct pharmacokinetic studies are lacking, and the active components of garlic deliverable from allicin remain uncertain. Many animal studies have suggested that garlic favorably influences CHD risk factors. Of the greater than 60 human trials of the lipid-lowering effects of garlic, most provided alliin/allicin standardized GP tablets, and most were positive in outcome. However, the current knowledge of the response to garlic is limited with respect to effective doses, types of garlic preparation and human subject categories. In this recently published study, researchers have sought to monitor any beneficial influence of garlic preparations on a range of CHD risk factors. The aim was to test the hypothesis that garlic supplements would reduce specified risk factors for CHD in male and female subjects at low risk of CHD.

Subjects were 51 volunteers, randomized to compare garlic oil (GO) and placebo, and 27 recruited separately to receive garlic powder (GP). Subjects were asked to maintain their usual diet throughout the study period. Volunteers were allocated randomly, in a double-blind manner. The GO dose was determined to deliver 8.2 mg of allyl sulfide/d. Subjects receiving the GP received a daily dose of 1 g GP, which released 7.8 g of allicin/g in water. Thus the two treatments provided similar allicin-equivalent doses. Reviews of subjects were at 0, 4 and 6 weeks, at the end of the intervention phase at 11 weeks and at 14-15 and 17-18 weeks after the start of the intervention phase during the dose washout phase.

Sixty subjects remained in the study to the end of the 11-week treatment period. Over 95% of questionnaires were returned in each group at the end of treatment. In matters of taste, the placebo capsules were rated as "nice" by three subjects and "OK" by 17 respondents. GO capsules were rated as "OK" by 12 subjects, "unpleasant" by seven and "really bad" by one subject. GP was also "OK" for 12 subjects, and "unpleasant" for seven.

There was a mean reduction between baseline and the end of treatment of -0.24 mmol/L in plasma total cholesterol in subjects taking GO, and an average reduction in plasma total cholesterol of -0.04 mmol/L in subjects taking placebo. At the end of the intervention period, measured effects of garlic over placebo for the cholesterol-related variables and glucose were in opposite directions for men and women. Significant differences in effect of GO between men and women were observed for HDL-C and TC/HDL-C. Women showed favorable effects in terms of CHD risk, whereas men had small adverse effects. There was a significant difference in GO effect for glucose, with a reduction seen for men and an increase for women.

The main hypothesis was not supported by statistical tests. One plausible explanation is that garlic lacks lipid-lowering efficacy, as was concluded for other recent negative garlic studies and that latest meta-analysis. The main findings in the present study were that traditional GO capsules had no significant effects but that gender differences may occur in the influence of garlic on CHD risk factors. Support for the value of garlic supplementation in the reduction of CHD risk in normal subjects is lacking.

X. Zhang, D. Lowe, P. Giles, S. Fell, M. Connock, D Maslin. Gender May Affect the Action of Garlic Oil on Plasma Cholesterol and Glucose Levels of Normal Subjects. J Nutr 131:1471-1478 (May 2001) [Correspondence: David J. Maslin. E-mail: D.Maslin@wlv.ac.uk].