Garlic (Allium sativum)

Overview

• Garlic (Allium sativum) is a bulbous plant in the Amaryllidaceae family, cultivated globally for culinary and medicinal uses.
• The edible clove contains sulfur-containing compounds, notably allicin, responsible for its aroma and biological actions.
• As a supplement, garlic primarily supports cardiovascular, metabolic, and immune health.

Benefits

• Cardiovascular health: Meta-analyses show standardized garlic extracts lower systolic and diastolic blood pressure by 5–10 mm Hg and reduce total cholesterol by 0.2–0.3 mmol L⁻¹ (≈8–10 mg dL⁻¹) in adults with mild hyperlipidemia.
• Metabolic support: Clinical trials demonstrate modest fasting glucose reductions (≈0.3 mmol L⁻¹) and improved insulin sensitivity in pre-diabetic patients taking 600–1200 mg/day of aged garlic extract for 12 weeks.
• Immune modulation: Randomized trials report a 30–40% reduction in common cold incidence and duration with 300 mg daily garlic powder for 12 weeks.
• Antioxidant & anti-inflammatory effects: In vitro and animal studies show garlic's organosulfur compounds up-regulate Nrf2-mediated antioxidant pathways and inhibit NF-κB signaling, reducing oxidative stress and inflammatory markers (e.g., CRP).
• Potential anticancer activity: Observational data link high garlic intake with lower gastric and colorectal cancer incidence, possibly due to diallyl sulfide derivatives' DNA-protective and anti-angiogenic actions.

How It Works

• Allicin (diallyl thiosulfinate) forms when alliinase hydrolyzes alliin after tissue disruption.
• Allicin decomposes into sulfides (diallyl disulfide, diallyl trisulfide, S-allyl-cysteine [SAC]) that penetrate cell membranes.
• These compounds modulate pathways:
  • Nitric-oxide (NO) synthesis: Sulfides activate endothelial nitric-oxide synthase (eNOS), promoting vasodilation and blood pressure reduction.
  • Lipid metabolism: Sulfides inhibit HMG-CoA reductase, the rate-limiting enzyme in cholesterol synthesis, similar to statins.
  • Antioxidant response: Nrf2-Keap1 pathway activation up-regulates glutathione-S-transferase and heme-oxygenase-1, enhancing cellular antioxidant capacity.
  • Immune modulation: Sulfides suppress NF-κB translocation, decreasing pro-inflammatory cytokine production (IL-6, TNF-α).
• Garlic's sulfur metabolites can induce apoptosis in malignant cells via mitochondrial pathway activation and ROS-mediated DNA damage repair inhibition.

Dosage

• Standardized aged-garlic extract (AGE) or garlic powder: 600–1200 mg per day, divided into two doses (morning & evening) with meals to reduce gastrointestinal irritation.
• Allicin-standardized capsules (≥1.3% allicin): 300–600 mg once daily, preferably with food.
• S-allyl-cysteine (SAC) extracts: 200–400 mg per day, split doses.
• Short-term immune support: 300 mg of raw garlic powder or 1–2 g of fresh garlic (≈5–10 g fresh weight) daily for 2–3 weeks.
• Special populations: For hypertension, 800 mg of AGE for 8–12 weeks; for lipid-lowering, 1200 mg/day of aged extract for 12 weeks. Reduce dosage if gastrointestinal discomfort occurs.

Safety & Side Effects

• Garlic is generally well-tolerated, but common side effects include mild gastrointestinal upset (heartburn, flatulence) and "garlic breath."
• Large doses (>5 g/day raw) may cause hemolysis in individuals with G6PD deficiency.
• Contraindications & cautions:
  • Anticoagulant/antiplatelet therapy (warfarin, aspirin, clopidogrel): garlic can potentiate bleeding risk; a 2-fold increase in INR has been reported with >2 g/day.
  • Pre-operative patients: discontinue 1–2 weeks before surgery.
  • Pregnant or lactating women: limited safety data; recommend ≤1 g/day.
  • Thyroid disease: high-dose garlic may interfere with thyroid hormone synthesis.
• Drug interactions: may reduce plasma levels of certain HIV protease inhibitors (e.g., saquinavir) and increase the metabolism of certain statins via CYP3A4 induction. Monitor coagulation parameters when combined with anticoagulants.

Chemistry

• The key active component, allicin, has the molecular formula C₆H₁₀OS₂ and IUPAC name (E)-prop-2-en-1-yl 2-prop-2-enyl disulfide.
• Allicin is a thiosulfinate (R-S(=O)-S-R′) that readily decomposes into diallyl sulfides (DAS, DADS, DATS) and S-allyl-cysteine (C₃H₇NO₂S).
• Garlic also contains flavonoids (quercetin, kaempferol), saponins, and trace minerals (selenium, manganese).
• Sulfide lipophilicity facilitates membrane permeation; allicin's S-O bond is highly electrophilic, reacting with thiol groups in proteins, modulating enzyme activity and signaling pathways.
• Allicin's instability leads to supplements often using aged garlic extract (AGE), where allicin converts to stable, water-soluble compounds (SAC, S-allyl-mercaptocysteine) retaining biological activity.

Sources & Quality

• Commercial garlic is primarily sourced from China, India, Spain, and the United States. High-quality cloves are cultivated in Spain's Mediterranean climate and high-altitude regions of China (e.g., Yunnan, Sichuan).
• Extraction methods:
  • Aged Garlic Extract (AGE): Raw cloves are sliced, submerged in ethanol or water, and aged for 20–30 days, converting allicin to stable S-allyl-cysteine.
  • Oil-based extracts: Cold-pressed garlic oil, rich in diallyl sulfides.
  • Powdered garlic: Freeze-drying and milling preserve alliin.
• Quality considerations: Standardized allicin content (≥0.3% allicin or ≥1.3% allicin equivalence), low heavy-metal contamination (≤0.5 ppm lead, ≤0.1 ppm cadmium), and absence of pesticide residues. Certified organic or GMP-certified products offer reliable purity and potency.