There is some research investigating garlic and garlic-derived compounds for potential medicinal effects, but I think, w4u, the evidence is a lot less compelling than you suggest. There are plenty of negative findings and only a handful of possible positive results. The claim that “garlic has been used EFFECTIVELY for thousands of years” is also pretty dubious. For one thing, in the absence of scientific research for most of those years, we have only anecdote to suggest this, and the same kinds of anecdotal evidence would support the same claim for bloodletting, prayer, and virtually every other traditional folk remedy. History is not a reliable guide to which plant-based remedies are effective and which aren’t. There is nothing wrong with investigating such things, but they often fail to live up to this kind of claim.
Crit Rev Food Sci Nutr. 2013;53(7):670-81. doi: 10.1080/10408398.2010.537000.
Garlic in clinical practice: an evidence-based overview.
Li L1, Sun T, Tian J, Yang K, Yi K, Zhang P.
BACKGROUND AND OBJECTIVE:
Garlic has been widely used in clinical practice, and there were many systematic reviews (SRs) describing its effects. But none reviewed the clinical utility comprehensively, so we aimed to evaluate its effects from every aspect of its effects.
We comprehensively searched medical electronic databases, asked the experts in this field, along with reference tracking, and manual searching. We included all kinds of SRs, including Cochrane SRs and non-Cochrane SRs. Two authors independently selected articles for relevant SRs, and extracted data of included SRs, resolved differences by consultation with a third reviewer.
We described nine SRs about garlic. Available evidence showed that garlic can reduce blood pressure (BP) in hypertensive patients and patients with elevated systolic BP (SBP), but not in normotensive subjects. Evidence about the effects of garlic on lipid parameters was contentious, so we cannot make a decision whether garlic is effective enough for reducing total cholesterol (TC), triglycerides (TAG), and high-density lipoprotein (HDL). As evidence was very limited and insufficient, relations between garlic intake and reduced risks of all kinds of cancers, antiglycemic and antithrombotic effects of garlic, the effects of garlic on cardiovascular morbidity and mortality were unclear. Garlic as a preventative or treatment option for the common cold or peripheral arterial occlusive disease or pre-eclampsia and its complications could not be recommended, as only one relatively small trial evaluated the effects separately.
Garlic might be effective in some areas of clinical practice, but the evidence levels were low, so further researches should be well designed using rigorous method to avoid potential biases.
Garlic for the prevention of cardiovascular morbidity and mortality in hypertensive patients.
Stabler SN1, Tejani AM, Huynh F, Fowkes C.
Garlic is widely used by patients for its blood pressure lowering effects. A meta-analysis published in 2008 concluded that garlic consumption lowers blood pressure in hypertensive and normotensive patients. Therefore, it is important to review the currently available evidence to determine whether garlic may also have a beneficial role in the reduction of cardiovascular events and mortality rates in patients with hypertension.
To determine whether the use of garlic as monotherapy, in hypertensive patients, lowers the risk of cardiovascular morbidity and mortality compared to placebo.
A systematic search for trials was conducted in the Cochrane Hypertension Group Specialised Register, CENTRAL, MEDLINE, EMBASE, AGRICOLA, AMED, and CINAHL up to November 2011. A hand search of reference lists of identified reviews was conducted. Experts in the area were also contacted to identify trials not found in the electronic search. Clinicaltrials.gov was searched for ongoing trials.
Randomized, placebo-controlled trials of any garlic preparation versus placebo for the treatment of hypertension were included.
DATA COLLECTION AND ANALYSIS:
Two reviewers independently extracted data and assessed trial quality using the risk of bias tool. Data synthesis and analysis was performed using RevMan 5.
The search identified two randomized controlled trials for inclusion. One trial included 47 hypertensive patients and showed that garlic significantly reduces mean supine systolic blood pressure by 12 mmHg (95% CI 0.56 to 23.44 mmHg, p=0.04) and mean supine diastolic blood pressure by 9 mmHg (95% CI 2.49 to 15.51 mmHg, p=0.007) versus placebo. The authors state that garlic was “free from side effects” and that no serious side effects were reported. There were 3 cases “where a slight smell of garlic was noted.“The second trial could not be meta-analysed as they did not report the number of people randomized to each treatment group. They did report that 200 mg of garlic powder given three times daily, in addition to hydrochlorothiazide-triamterene baseline therapy, produced a mean reduction of systolic blood pressure by 10-11 mmHg and of diastolic blood pressure by 6-8 mmHg versus placebo.Neither trial reported clinical outcomes and insufficient data was provided on adverse events.
There is insufficient evidence to determine if garlic provides a therapeutic advantage versus placebo in terms of reducing the risk of mortality and cardiovascular morbidity in patients diagnosed with hypertension. There is also insufficient evidence to determine the difference in withdrawals due to adverse events between patients treated with garlic or placebo.Based on 2 trials in 87 hypertensive patients, it appears that garlic reduces mean supine systolic and diastolic blood pressure by approximately 10-12 mmHg and 6-9 mmHg, respectively, over and above the effect of placebo but the confidence intervals for these effect estimates are not precise and this difference in blood pressure reduction falls within the known variability in blood pressure measurements. This makes it difficult to determine the true impact of garlic on lowering blood pressure.
Mol Nutr Food Res. 2007 Nov;51(11):1382-5.
Clinical effectiveness of garlic (Allium sativum).
Pittler MH1, Ernst E.
The objective of this review is to update and assess the clinical evidence based on rigorous trials of the effectiveness of garlic (A. sativum). Systematic searches were carried out in Medline, Embase, Amed, the Cochrane Database of Systematic Reviews, Natural Standard, and the Natural Medicines Comprehensive Database (search date December 2006). Our own files, the bibliographies of relevant papers and the contents pages of all issues of the review journal FACT were searched for further studies. No language restrictions were imposed. To be included, trials were required to state that they were randomized and double blind. Systematic reviews and meta-analyses of garlic were included if based on the results of randomized, double-blind trials. The literature searches identified six relevant systematic reviews and meta-analysis and double-blind randomized trials (RCT) that were published subsequently. These relate to cancer, common cold, hypercholesterolemia, hypertension, peripheral arterial disease and pre-eclampsia. The evidence based on rigorous clinical trials of garlic is not convincing. For hypercholesterolemia, the reported effects are small and may therefore not be of clinical relevance. For reducing blood pressure, few studies are available and the reported effects are too small to be clinically meaningful. For all other conditions not enough data are available for clinical recommendations.
J Nutr. 2016 Feb;146(2):389S-96S. doi: 10.3945/jn.114.202192. Epub 2016 Jan 13.
Garlic Lowers Blood Pressure in Hypertensive Individuals, Regulates Serum Cholesterol, and Stimulates Immunity: An Updated Meta-analysis and Review.
Garlic has been shown to have cardiovascular protective and immunomodulatory properties.
We updated a previous meta-analysis on the effect of garlic on blood pressure and reviewed the effect of garlic on cholesterol and immunity.
We searched the Medline database for randomized controlled trials (RCTs) published between 1955 and December 2013 on the effect of garlic preparations on blood pressure. In addition, we reviewed the effect of garlic on cholesterol and immunity.
Our updated meta-analysis on the effect of garlic on blood pressure, which included 20 trials with 970 participants, showed a mean ± SE decrease in systolic blood pressure (SBP) of 5.1 ± 2.2 mm Hg (P < 0.001) and a mean ± SE decrease in diastolic blood pressure (DBP) of 2.5 ± 1.6 mm Hg (P < 0.002) compared with placebo. Subgroup analysis of trials in hypertensive subjects (SBP/DBP ≥140/90 mm Hg) at baseline revealed a larger significant reduction in SBP of 8.7 ± 2.2 mm Hg (P < 0.001; n = 10) and in DBP of 6.1 ± 1.3 mm Hg (P < 0.001; n = 6). A previously published meta-analysis on the effect of garlic on blood lipids, which included 39 primary RCTs and 2300 adults treated for a minimum of 2 wk, suggested garlic to be effective in reducing total and LDL cholesterol by 10% if taken for >2 mo by individuals with slightly elevated concentrations [e.g., total cholesterol >200 mg/dL (>5.5 mmol/L)]. Garlic has immunomodulating effects by increasing macrophage activity, natural killer cells, and the production of T and B cells. Clinical trials have shown garlic to significantly reduce the number, duration, and severity of upper respiratory infections.
Our review suggests that garlic supplements have the potential to lower blood pressure in hypertensive individuals, to regulate slightly elevated cholesterol concentrations, and to stimulate the immune system. Garlic supplements are highly tolerated and may be considered as a complementary treatment option for hypertension, slightly elevated cholesterol, and stimulation of immunity. Future long-term trials are needed to elucidate the effect of garlic on cardiovascular morbidity and mortality.
Public Health Nutr. 2016 Feb;19(2):308-17. doi: 10.1017/S1368980015001263. Epub 2015 May 6.
Garlic consumption and colorectal cancer risk in man: a systematic review and meta-analysis.
Chiavarini M1, Minelli L1, Fabiani R2.
Colorectal cancer shows large incidence variations worldwide that have been attributed to different dietary factors. We conducted a meta-analysis on the relationship between garlic consumption and colorectal cancer risk.
We systematically reviewed publications obtained by searching ISI Web of Knowledge, MEDLINE and EMBASE literature databases. We extracted the risk estimate of the highest and the lowest reported categories of intake from each study and conducted meta-analysis using a random-effects model.
The pooled analysis of all fourteen studies, seven cohort and seven case-control, indicated that garlic consumption was not associated with colorectal cancer risk (OR=0·93; 95 % CI 0·82, 1·06, P=0·281; I 2=83·6 %, P≤0·001). Separate analyses on the basis of cancer sites and sex also revealed no statistically significant effects on cancer risk. However, when separately analysed on the basis of study type, we found that garlic was associated with an approximately 37 % reduction in colorectal cancer risk in the case-control studies (combined risk estimate=0·63, 95 % CI 0·48, 0·82, P=0·001; I 2=75·6 %, P≤0·001).
Our results suggest that consumption of garlic is not associated with a reduced colorectal cancer risk.