Garlic (Allium sativum L.): A Comprehensive Reference
1. Identity, Botanical Classification, and Natural Source
Garlic (Allium sativum L.) is a member of the Liliaceae family. The genus Allium contains hundreds of species, including garlic, onion, shallot, leek, and chive, all known for their physiologically active secondary metabolites. Garlic is an ancient crop native to Central and South Asia and northeastern Iran. While originally from Asia, it is also cultivated in China, North Africa (Egypt), Europe, and Mexico. Garlic has compact bulbs, and each bulb consists of several bulblets or cloves covered with a white skin. The plant grows to 25â70 cm with hermaphrodite flowers.
A multifunctional crop, garlic belongs to the Amaryllidaceae family (taxonomy varies by authority) and is of high medicinal, nutraceutical, and culinary importance.
Common Forms and Preparations
Garlic products include garlic oil macerates, essential oils (EOs), garlic powder, and aged garlic extract. Bulbs are extracted and made into tablet, powder, and oil forms; the major active ingredient varies by preparation and is either allicin or S-allylcysteine, an amino acid by-product. Because the active ingredients are volatile and can be destroyed by the act of crushing, the amount of active ingredient in the various forms of garlic varies greatly.
Aged garlic extract (AGE), made from garlic allowed to age for at least 20 months, has more stable active compounds than most forms, and consuming garlic supplements in this form appears to confer the greatest health benefits and freedom from adverse effects. The long aging process converts the harsh, unstable allicin into gentler but still bioactive compounds, most notably S-allylcysteine (SAC), which is stable, odorless, and absorbs well into the bloodstream, where it can be measured in plasma.
As studies have not conclusively shown which phytochemical components are responsible for which beneficial effects, many garlic supplements are currently available on the market, including garlic powders, oil macerates, steam-distilled oils, and aged garlic extracts; each dose form varies in phytochemical content because of the different methods of preparation.
2. Traditional and Historical Use
Garlic has been integral to human culture and medicine for more than 5,000 years, serving as both a culinary staple and therapeutic agent; its ethnobotanical and medicinal significance spans civilizations, from its origins in Central Asia to its global dissemination through trade and cultural exchange. Interest in the potential benefits of garlic has origins in antiquity and is one of the earliest documented examples of plants employed for treatment of disease and maintenance of health; garlic was in use at the beginning of recorded history and was found in Egyptian pyramids and ancient Greek temples.
Ancient Egypt
Ancient Egyptian texts document the dual role of garlic as a food and sacred offering, consumed by pyramid builders to enhance strength and endurance, and used in rituals to symbolize protection and vitality. In ancient Egypt, garlic was sometimes termed the "plant of immortality," a testament to its sacred status and its use in rituals, mummification, and as nourishment for laborers building the pyramids.
Ancient Greece and Rome
Hippocrates, widely regarded as the father of Medicine, made garlic a part of his therapeutic armamentarium, advocating its use for pulmonary complaints, as a cleansing or purgative agent, and for abdominal growths, particularly uterine. As in Greece, the Romans perceived garlic as an aid to strength and endurance; it was fed to both soldiers and sailors and was part of a ship's manifest when it set out to sea. The leading medical authority was the Greek, Dioscorides, who served as the chief physician for Nero's army and was the author of a five-volume treatise that recommended garlic because it "cleans the arteries."
China and India
Garlic is commonly consumed and has a long history of being utilized as a traditional medicine in China. Chinese medicine has historically been associated with the use of combinations of herbs to form a healing tonic, rather than the administration of single agents, and Allium was evidently frequently used in combination therapy. Fatigue, headache, and insomnia were often treated with garlic in traditional Chinese medicine. In ancient Indian medicine, garlic was a valuable remedy used as a tonic, as a roborant, and to cure a lack of appetite, common weakness, cough, skin disease, rheumatism, and hemorrhoids; in the Vedas â the Indian holy book â garlic was mentioned among other medicinal plants.
Iran and the Middle East
Medicinally, garlic was an integral part of Iranian traditional medicine, where it was prescribed for respiratory infections, digestive issues, and as a general health tonic due to its antimicrobial and anti-inflammatory properties.
Cross-Cultural Observations
Ancient medical texts from Egypt, Greece, Rome, China, and India each prescribed medical applications for garlic; in many cultures, garlic was administered to provide strength and increase work capacity for laborers. It is of interest to note that cultures who developed without contact with one another came to similar conclusions about the efficacy of garlic. Specific preparation methods, such as fermentation and aging, contributed to the therapeutic versatility of garlic, as observed in black garlic traditions.
3. Key Constituents and Active Compounds
Garlic is a common spice with many health benefits, mainly due to its diverse bioactive compounds, such as organic sulfides, saponins, phenolic compounds, and polysaccharides. Garlic contains diverse bioactive compounds, such as allicin, alliin, diallyl sulfide, diallyl disulfide, diallyl trisulfide, ajoene, and S-allyl-cysteine. It is also rich in sulfur-containing phytoconstituents such as ajoenes, vinyldithiins, and flavonoids such as quercetin.
Alliin and Allicin
Thiosulfinates (e.g., allicin) result from the conversion of sulfoxides through an enzymatic reaction of sulfur-substituted cysteine sulfoxides when raw garlic is processed; no thiosulfinates are found in intact garlic. When a garlic clove is crushed or cut, the enzyme alliinase comes into contact with alliin and converts it to allicin (diallyl thiosulfinate). Allicin is thought to be a transient compound that decomposes rapidly into other sulfur-containing compounds and is not found to be an active compound in finished garlic preparations.
Allicin, the most biologically active sulfur-containing compound of garlic, possesses various cardioprotective properties, including reducing blood pressure, regulating blood lipids, preventing atherosclerosis, and protecting against myocardial injury.
S-Allylcysteine (SAC) and Related Compounds
Îł-Glutamyl-S-allyl-L-cysteines are converted into SAC through an enzymatic transformation with Îł-glutamyltranspeptidase when garlic is extracted with an aqueous solution. Free radical scavenging activity was discovered in aged garlic extract but not in fresh garlic extract; S-allyl cysteine and S-allyl mercapto-L-cysteine, the two major components in aged garlic, showed the highest free radical scavenging activity.
Ajoene and Diallyl Sulfides
Allicin instantly decomposes to other compounds, such as diallyl sulfide (DAS), diallyl disulfide (DADS), dithiins, and ajoene. The ajoenes have been shown to be powerful inhibitors of platelet aggregation. The thiosulfinate allicin accounts for approximately 70% of the total thiosulfinates produced and is thought to be the principal bioactive compound responsible for the health-promoting benefits of garlic; most of the cardioprotective and antibacterial effects of garlic appear to be derived from allicin, its metabolites, and to a lesser extent the dialkyl sulfides.
Other Phytochemicals
As a rich natural source of bioactive compounds, garlic contains polysaccharides, saponins, tannins, linalool, geraniol, phellandrene, ÎČ-phellandrene, ajoene, alliin, S-allyl-mercapto cysteine, and ÎČ-phellandrene.
4. Established Mechanisms of Action
Cardiovascular Mechanisms
Garlic's ability to lower blood pressure is multifactorial and includes increasing the availability and activities of nitric oxide, inhibiting ACE thereby reducing pathways that decrease plasma volume and cause vasoconstriction, and increasing the production of HâS that results in hyperpolarization of vascular smooth muscle cells. Moreover, allicin was shown to inhibit angiotensin II, and Îł-glutamyl-S-allylcysteine (GSAC) in garlic may also block ACE and induce vasodilation. Allicin prevents vascular calcification, promotes endothelial cell migration, and lowers blood pressure and plasma lipids.
Clinical studies have found that allicin reduces ET-1 and C-reactive protein (CRP) levels and elevates NO levels, improves endothelial dysfunction, and reduces the incidence of restenosis in patients after PCI.
Antioxidant Mechanisms
Allicin (diallyl thiosulfinate) has been proven to have powerful cardioprotective effects and mediates various pathological processes related to CVD, such as inflammatory factor secretion, myocardial cell apoptosis, and oxidative stress. Research has summarized the biological functions of allicin and its potential mechanisms including antioxidation, anti-inflammation, and anti-apoptosis effects.
Immunological Mechanisms
Garlic appears to enhance the functioning of the immune system by stimulating certain cell types, such as macrophages, lymphocytes, natural killer (NK) cells, dendritic cells, and eosinophils, by mechanisms including modulation of cytokine secretion, immunoglobulin production, phagocytosis, and macrophage activation. A. sativum modulates cytokine secretion, and such modulation may provide a mechanism of action for many of its therapeutic effects.
Anticancer Mechanisms
Bioactive metabolites of garlic alter the peroxidation of lipid, activity of nitric oxide synthetase, nuclear factor-ÎșB, epidermal growth factor receptor, and protein kinase C, cell cycle, and survival signaling. The production of nitrosamines, typical carcinogens generated during heating and storage, could be inhibited by garlic and the organic allyl sulfides it produces; further, the allyl sulfides in garlic inhibit DNA alkylation, an early step in developing nitrosamine carcinogenesis.
5. Scientific Evidence by Health Area
5.1 Cardiovascular Health â Blood Pressure
Clinical studies that investigated the effects of garlic and garlic-based supplements on blood pressure present findings suggesting that garlic consumption may modestly reduce blood pressure, particularly in individuals with mild hypertension. The efficacy of garlic in lowering blood pressure can vary based on diverse factors such as the type of garlic used (fresh, aged extract, supplements), individual responses, and dosage.
A meta-analysis suggests that garlic intervention has a significant effect on reducing blood pressure in certain populations, including those who are obese, aged 50â60, or have higher diastolic blood pressure; however, the overall impact on hypertension may be limited, and potential gastrointestinal and other adverse effects should be considered in clinical practice.
Further large-scale, long-term clinical trials are warranted to establish the efficacy of garlic in managing hypertension, including the optimal dosage and formulation. Evidence for blood pressure effects is rated as moderate, promising in certain subpopulations, but not yet definitive.
5.2 Cardiovascular Health â Lipids and Cholesterol
The evidence on garlic's lipid-lowering effects is mixed across multiple meta-analyses. An early systematic review by Silagy and Neil (1994), including 16 trials with data from 952 subjects, found that many of the trials had methodological shortcomings. The mean difference in reduction of total cholesterol between garlic-treated subjects and those receiving placebo was â0.77 mmol/l, representing a 12% reduction with garlic therapy beyond the final levels achieved with placebo alone; the reduction was evident after one month of therapy and persisted for at least six months. In dried garlic powders in which the allicin content is standardized, there was no significant difference in the size of the reduction across the dose range of 600â900 mg daily.
A 2009 meta-analysis reached different conclusions: Thirteen trials including 1,056 subjects were eligible for the meta-analysis; overall, administration of garlic did not show any significant difference in effects on all outcome measures examined when compared with placebo, and the available evidence from randomized controlled trials does not demonstrate any beneficial effects of garlic on serum cholesterol.
A later, more comprehensive meta-analysis (Ried et al., 2013) including 39 primary trials found garlic to be effective in reducing total serum cholesterol by 17 ± 6 mg/dL and LDL cholesterol by 9 ± 6 mg/dL in individuals with elevated total cholesterol levels (>200 mg/dL), provided garlic is used for longer than 2 months; an 8% reduction in total serum cholesterol is of clinical relevance and is associated with a 38% reduction in risk of coronary events at 50 years of age.
A 2019 meta-analysis of 29 trials concluded that garlic produced modest reductions in total cholesterol, driven mainly by modest reductions in triacylglycerol levels, without any appreciable lowering in LDL levels or increase in HDL levels.
A 2024 meta-analysis of 21 RCTs in dyslipidemia patients found that garlic consumption significantly reduced total cholesterol (TC) (WMD = â0.64 mmol/L, 95% CI = â0.75 to â0.54, P < 0.001), and triglycerides (TG) (WMD = â0.17 mmol/L, 95% CI = â0.26 to â0.09, P < 0.001).
Overall, the evidence for lipid-lowering effects is mixed to moderate. Different meta-analyses arrive at different conclusions depending on inclusion criteria, garlic preparation studied, and population characteristics. Effects appear most consistent for total cholesterol and triglycerides, with more variable effects on LDL and HDL.
5.3 Blood Glucose and Type 2 Diabetes
A meta-analysis of nine RCTs involving 768 type 2 diabetes (T2DM) patients in which the dose of daily garlic (allicin) supplement ranged from 0.05 g to 1.5 g found a significant reduction in the level of fasting blood glucose at 1â2 weeks, 3â4 weeks, 12 weeks, and 24 weeks in favor of the garlic group; significantly decreased fructosamine and glycated hemoglobin (both at 12 and 24 weeks) were also found in the garlic group.
However, a systematic review of clinical trials noted: despite extensive animal research on garlic's anti-diabetic properties, there is limited evidence of its effectiveness in human investigations; garlic has been shown to affect blood glucose levels in the majority of clinical investigations, but not consistently in diabetic patients, proving that further research is still needed.
The evidence for garlic in diabetes management is preliminary to moderate. Meta-analytic results suggest benefit, but heterogeneity among studies, small sample sizes, and methodological limitations mean firm conclusions cannot yet be drawn.
5.4 The Common Cold and Immune Function
A recent Cochrane review concluded that there is insufficient clinical trial evidence regarding the effects of garlic in preventing or treating the common cold; only a few clinical trials have been conducted, and a 2014 Cochrane review of a single randomized controlled trial involving 146 participants who received either a garlic supplement or a placebo for 12 weeks concluded insufficient evidence. The trial reported 24 occurrences of the common cold in the garlic intervention group compared with 65 in the placebo group (P < 0.001), resulting in fewer days of illness in the garlic group, though the number of days to recovery from an occurrence of the common cold was similar in both groups (4.63 versus 5.63).
A single trial suggested that garlic may prevent occurrences of the common cold, but more studies are needed to validate this finding; claims of effectiveness appear to rely largely on poor-quality evidence.
The evidence for garlic in cold prevention or treatment is weak, limited to a single qualifying RCT at the Cochrane level.
5.5 Antimicrobial Activity
Substantial studies have shown that garlic and its bioactive constituents exhibit antioxidant, anti-inflammatory, antibacterial, antifungal, immunomodulatory, and cardiovascular protective properties. The preponderance of this evidence, however, is from in vitro and animal studies. Very little research has been done on clinical applications for infection, and a 2022 review identified only two studies that suggest a possible benefit, both of which included only small numbers of people and had weaknesses in the research.
5.6 Cancer
The anti-cancer action of garlic is likely the best researched of the many advantageous pharmacological effects, and its use offers significant protection against the risk of developing cancer; a few active metabolites of garlic have been reported to be essential in the destruction of malignant cells due to their multi-targeted activities and lack of significant toxicity. Garlic contains several bioactive molecules with anticancer actions, including diallyl trisulfide, allicin, diallyl disulfide, diallyl sulfide, and allyl mercaptan; the effects of various garlic-derived products have been evaluated against skin, prostate, ovarian, breast, gastric, and other cancers.
Several experimental studies have demonstrated that diallyl disulfide (DADS) exhibits anti-tumor activity against many types of tumor cells, including gynecological cancers, hematological cancers, lung cancer, neural cancer, skin cancer, prostate cancer, and gastrointestinal cancers.
The majority of anticancer evidence is from in vitro and animal studies. Human clinical trials demonstrating cancer prevention or treatment through garlic supplementation remain limited and are not of sufficient quality or scale to draw firm clinical conclusions.
5.7 Atherosclerosis
In patients with coronary artery disease combined with diabetes, oral administration of allicin capsules resulted in a significant improvement in flow-mediated dilation (FMD) and NO levels, and a decline in ICAM-1 level and incidence of major adverse cardiovascular events in the allicin group compared to the control group, which may be related to allicin's improvement in endothelial function. Research has covered allicin's cardioprotective effects concerning various CVDs, such as atherosclerosis, hypertension, myocardial infarction, arrhythmia, cardiac hypertrophy, heart failure, and cardiotoxicity. Evidence in this area remains promising but largely mechanistic and early-phase clinical.
6. Body Systems and Health Areas Associated with Garlic
- Cardiovascular system: Blood pressure reduction, lipid modulation, antiatherosclerotic effects, antiplatelet/antithrombotic activity, endothelial function improvement.
- Metabolic/endocrine: Blood glucose regulation, insulin sensitization, potential adjunctive role in T2DM management.
- Immune system: Stimulation of macrophages, NK cells, lymphocytes; modulation of cytokine secretion; possible reduction of cold incidence (preliminary).
- Antimicrobial: Antibacterial, antifungal, antiviral properties documented in vitro; limited clinical confirmation for systemic infection.
- Oncology: Chemopreventive and cytotoxic activity across numerous cancer types in preclinical settings; insufficient human trial evidence to date.
- Hepatic/renal: Garlic and its bioactive constituents exhibit hepatoprotective, digestive system protective, and renal protective properties in experimental models.
- Nervous system: Neuroprotective properties have been reported, though primarily in preclinical research.
- Oral health: A clinical trial reported that 18 months' use of aged garlic extract reduced periodontitis levels compared to the placebo group.
7. Dosage Forms and Reported Dosages
A typical dosage of garlic is 900 milligrams daily of a garlic powder extract standardized to contain 1.3 percent alliin, providing about 12,000 micrograms of alliin daily, or 4 to 5 milligrams of "allicin potential"; alliin-free aged garlic is taken at a dose of 1 to 7.2 grams (1,000 to 7,200 milligrams) daily.
Most cardiovascular research has used the equivalent of 600 to 900 mg of dried garlic powder per day, which translates roughly to one to two fresh cloves. In one double-blind, randomized, placebo-controlled pilot study of aged garlic extract with warfarin, the study medication was administered at a dose of 5 mL twice a day for 12 weeks.
In a meta-analysis of nine RCTs of garlic in T2DM management, the dose of daily garlic (allicin) supplement ranged from 0.05 g to 1.5 g.
Garlic, taken orally, has been used safely in research studies that lasted as long as 7 years.
The following principal dose forms are recognized in the literature:
- Fresh (raw) garlic cloves: Consumed whole, crushed, or chopped; one to two cloves per day is commonly cited in dietary contexts.
- Dried garlic powder tablets/capsules: In dried garlic powders in which the allicin content is standardized, the dose range studied is 600â900 mg daily.
- Aged garlic extract (AGE): Most commonly studied at 1â7.2 g/day. Aged garlic extract is the form most commonly used in clinical research.
- Garlic oil/oil macerates: Available in soft-gel capsule form; phytochemical content differs from powder forms.
- Garlic essential oil (steam-distilled): Rich in diallyl sulfides but typically lacking allicin precursors.
8. Safety Considerations and Drug Interactions
As a commonly used food, garlic is on the GRAS (Generally Recognized as Safe) list. Generally regarded as safe, garlic can cause mild side effects such as unpleasant breath odor or gastrointestinal discomfort, and it may interact with certain medications, particularly blood thinners.
Bleeding Risk
Taking garlic supplements may increase the risk of bleeding; this is especially important if a person is going to have surgery or if they take medicines, such as anticoagulants or aspirin, that may also affect bleeding. Due to high-level evidence, garlic and hawthorn should be discontinued prior to surgery. High-dose garlic supplements should be discontinued 7â10 days before surgery.
Interaction with Warfarin
Anecdotal cases allege a potential for garlic to interact with warfarin, inhibit platelet aggregation, and subsequently result in bleeding. Documented bleeding events have been linked to the use of products such as cranberry, garlic, ginkgo, and saw palmetto. However, the evidence is complex: one study found no evidence to suggest that garlic consumption either as a supplement or in cooking is associated with more frequent hemorrhagic complications or less control of INR, and data render clinically significant interactions between warfarin and garlic intake unlikely.
A double-blind, randomized, placebo-controlled pilot study of aged garlic extract with warfarin tested aged garlic extract (AGE) with warfarin (Coumadin) in 66 screened patients; 52 patients were randomized, and 48 patients completed the study. The authors found no significant elevation of bleeding risk in the AGE group, though this was a pilot study and the evidence remains limited.
INR should be monitored when starting or discontinuing garlic supplements in patients on warfarin.
Interaction with Other Medications
There is no strong evidence of significant interactions between garlic supplements and statins (rosuvastatin, atorvastatin) in humans; some animal studies have suggested potential pharmacokinetic alterations at high doses, but human data indicate limited or no clinically relevant interactions.
Pregnancy and Breastfeeding
Garlic may not be safe for use during pregnancy or while breastfeeding when taken orally in amounts greater than those found in foods; little is known about the safety of using garlic topically during pregnancy or while breastfeeding.
Gastrointestinal and Other Adverse Effects
Adverse effects observed in clinical trials have included rash and odor. Potential gastrointestinal and other adverse effects should be considered in clinical practice. Garlic preparations were highly tolerable in all trials reviewed by one meta-analysis and were associated with minimal side effects. The most commonly reported adverse effects across the published literature are halitosis, body odor, and mild gastrointestinal upset (nausea, heartburn, flatulence). Garlic supplements are contraindicated in individuals with bleeding disorders, as they may exacerbate clotting problems and increase hemorrhage risk.
References
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