A recent report suggests that eating garlic - either raw or as a supplement - does not lower cholesterol levels. The news also points out that allicin is the active ingredient to prevent the formation of cholesterol in over 90 animal and human studies in 1990s. The reporter says that the studies were poorly designed and did not provide conclusive proof. 
Dr Christopher Gardner from Stanford University divided 170 people aged 30 to 65 who had moderately high LDL (bad) cholesterol levels for six months into four groups. He asked one group to eat a four-gram clove of garlic each day, the second to take a powdered garlic supplement, the third to take aged garlic extract and the fourth group to take placebos. Gardner found no statistically significant effects of the three forms of garlic on LDL cholesterol concentrations. 
After reading the news, I have a few questions. What are the results of other recent clinical studies on garlic' cholesterol lowering effects (in 2006-2007)? If some studies in 2006-2007 showed positive results, what could be the cause for the mixed results among the studies? Has its cholesterol-lowering mechanism been identified?
Before answering the above questions, let us review what the scientists found about garlic' cholesterol activities in clinical studies of 2006-2007:
Mahmoodi M. and his co-workers from Rafsanjan, Iran, found that oral consumption of 5 g of garlic twice a day for 42 days was beneficial in lowering triglyceride and the total cholesterol levels in a study of 30 subjects with blood cholesterol higher than 245 mg/dl. 
Japanese researchers from Wakunaga Pharmaceutical Co., Ltd developed a monascus garlic fermented extract. This extract contains dimerumic acid and monacolin K. In a study of 15 subjects aged 33-59 suffered from either hypercholesterolemia or hyper-low-density lipoprotein cholesterolemia, they found that intake of this garlic extract for 2-4 weeks could reduce serum total cholesterol (TC) and low-density lipoprotein cholesterol levels significantly. 
In a long-term study, Russian researchers demonstrated that allicor (a long-acting garlic product) reduced total cholesterol and LDL cholesterol in 167 patients with hyperlipidemia. 
However, two groups reported negative results in their clinical studies on the effects of garlic on cholesterol. van Doorn MB and his colleagues from Gaubius Laboratory, Netherlands, did not find any benefits of garlic powder on inflammatory biomarkers, endothelial function, or lipid profile in 90 overweight subjects aged 40-75 with risk factors for cardiac vascular disease.  Zhang L and co-workers from Beijing Institute for Cancer Research found no significant benefits of intake of 200 mg aged garlic extract and 1 mg steam-distilled garlic oil on lipid profiles in a study of subjects with low meat intake and moderate cholesterol concentrations. 
What are the health benefits provided by garlic extracts?
Borek C. at Tufts University School of Medicine, Boston, stated that antioxidant-rich aged garlic extract (AGE or Kyolic), may help reduce the risk of cardiovascular and Alzheimer's disease, as these diseases are associated with high cholesterol in his review article. 
He further explains that AGE increases superoxide dismutase, catalase, glutathione peroxidase, and glutathione levels, and inhibits lipid peroxidation and inflammatory prostaglandins. AGE reduces cholesterol synthesis by inhibiting 3-hydroxy-3-methylglutaryl-CoA reductase. Inhibition of cholesterol, LDL oxidation, and platelet aggregation by AGE, inhibits arterial plaque formation; AGE decreases homocysteine, lowers blood pressure, and increases microcirculation. 
How does garlic extract reduce cholesterol levels?
Singh DK and Porter TD from University of Kentucky found that garlic extract could reduce cholesterol synthesis by up to 75% without significant cellular toxicity. Their study suggests that diallyl disulfide, diallyl trisulfide, and allyl mercaptan are the key compounds for the inhibitory activities. 
Why is the result mixed for the effect of garlic preparations on cholesterol lowering?
Rahman K and Lowe GM. at Liverpool John Moores University, UK, point out that epidemiologic studies show an inverse correlation between garlic consumption and progression of cardiovascular disease. Cardiovascular disease is associated with multiple factors such as raised serum total cholesterol, raised LDL and an increase in LDL oxidation, increased platelet aggregation, hypertension, and smoking.
Numerous in vitro and animal studies have confirmed the ability of garlic to reduce these parameters and suggested its mechanism on cholesterol-lowering activities. From 1993 to March of 2006, 44% of clinical trials have indicated a reduction in total cholesterol, and the most profound effect has been observed in garlic's ability to reduce the ability of platelets to aggregate. However, mixed results have been obtained in the area of blood pressure and oxidative-stress reduction.
The mixed results can simply be explained by different contents of the garlic preparations (i.e. the pills, extracts and dosage forms) or experimental design in the studies.  Please, note that the dose in Mahmoodi's study is 5 g of garlic twice a day for 42 days (positive results) while Zhang L used only 200 mg aged garlic extract and 1 mg steam-distilled garlic oil in his study (negative results).
ALL RIGHTS RESERVED ZHION 2007. This article is for your reference only. Consult with your doctor for any question or taking any supplement.  van Doorn MB et al, Effect of garlic powder on C-reactive protein and plasma lipids in overweight and smoking subjects. Am J Clin Nutr. 2006 Dec;84(6):1324-9.  Mahmoodi M, et al, Study of the effects of raw garlic consumption on the level of lipids and other blood biochemical factors in hyperlipidemic individuals. Pak J Pharm Sci. 2006 Oct;19(4):295-8.  Zhang L, et al, A randomized factorial study of the effects of long-term garlic and micronutrient supplementation and of 2-wk antibiotic treatment for Helicobacter pylori infection on serum cholesterol and lipoproteins. Am J Clin Nutr. 2006 Oct;84(4):912-9.  Sumioka I, et al, Lipid-lowering effect of monascus garlic fermented extract (MGFE) in hyperlipidemic subjects. Hiroshima J Med Sci. 2006 Jun;55(2):59-64.  Sobenin IA, et al, Allicor efficacy in lowering the risk of ischemic heart disease in primary prophylaxis, Ter Arkh. 2005;77(12):9-13.  Borek C. Garlic reduces dementia and heart-disease risk. J Nutr. 2006 Mar;136(3 Suppl):810S-812S.  Singh DK, Porter TD. Inhibition of sterol 4alpha-methyl oxidase is the principal mechanism by which garlic decreases cholesterol synthesis. J Nutr. 2006 Mar;136(3 Suppl):759S-764S.  Rahman K and Lowe GM. Garlic and cardiovascular disease: a critical review. J Nutr. 2006 Mar;136(3 Suppl):736S-740S.  Garlic 'does not cut cholesterol' BBC NEWS 2007/02/27 07:52:03.