Raw garlic contains alliin, an odorless precursor compound. When garlic tissue is disrupted by chopping, crushing, or chewing, the enzyme alliinase converts alliin to allicin, responsible for garlic’s characteristic smell and most of its bioactivity. Allicin is unstable and converts to secondary compounds (diallyl sulfide, ajoene, vinyldithiins) that also carry biological activity. Garlic also contains fructooligosaccharides (FOS), a prebiotic fiber. Importantly, allowing chopped or crushed garlic to rest for 10 minutes before cooking preserves more allicin, as the alliinase reaction completes and the product becomes somewhat more heat-stable.
Multiple meta-analyses of randomized controlled trials show that garlic supplementation produces modest but statistically significant reductions in systolic blood pressure, typically 5–9 mmHg in hypertensive individuals, with smaller effects in normotensive people. Effects on LDL cholesterol are less consistent, with some meta-analyses finding small reductions (~10 mg/dL) and others finding no significant effect. The cardiovascular benefit appears most meaningful for people with elevated baseline blood pressure or cholesterol. NCCIH notes that while evidence is promising, study quality varies and standardization of garlic preparations makes comparison across trials difficult.
Allicin and related compounds exhibit broad-spectrum antimicrobial activity in laboratory settings, inhibiting growth of bacteria (including Staphylococcus aureus, Salmonella, E. coli, and Helicobacter pylori), fungi (including Candida species), and some viruses. The evidence in humans is more limited but supports modest antifungal and antibacterial effects, particularly against H. pylori, the bacterium associated with gastric ulcers. Concentrations required for antimicrobial effect may exceed those achievable through dietary intake alone, limiting clinical applicability.
Garlic contains fructooligosaccharides, which serve as a substrate for beneficial gut bacteria including Bifidobacterium and Lactobacillus species. Regular garlic consumption has been associated in small trials with increased abundance of these probiotic-associated genera and reduced pathogenic bacteria. The prebiotic effect provides a mechanistic pathway for garlic’s documented anti-inflammatory effects, since gut microbiome composition strongly influences systemic inflammatory tone.
Organosulfur compounds from garlic downregulate NF-κB signaling, a central pathway in inflammatory gene expression, and reduce production of pro-inflammatory cytokines (TNF-α, IL-6, IL-1β). Human trials have shown reductions in hs-CRP (a systemic inflammation marker) in supplementation studies, though effect sizes are modest. The anti-inflammatory effects appear to be most consistent in populations with elevated baseline inflammation.
A 2014 meta-analysis of epidemiological studies (PMID 25586902) found that high garlic and onion intake was associated with significantly reduced risk of colorectal cancer, with evidence also pointing to protective effects against breast and other cancers. The mechanism involves organosulfur compounds, particularly diallyl sulfide and S-allylcysteine, which induce phase II detoxification enzymes via the NRF2 pathway (the same pathway activated by sulforaphane in cruciferous vegetables), increase glutathione levels, inhibit the activation of carcinogenic compounds, and trigger apoptosis in cancer cells through caspase pathways. Garlic’s H. pylori-inhibiting activity is also relevant: H. pylori infection is the primary cause of gastric cancer, and reducing its colonization through regular garlic intake may lower long-term gastric cancer risk.
Fresh garlic (1–2 cloves per day) and aged garlic extract are the forms with the most evidence. Garlic powder supplements vary widely in allicin yield depending on enteric coating and manufacturing. Cooking garlic reduces allicin content, but secondary sulfur compounds with their own activity are formed. The distinctive odor is a common deterrent; odorless aged garlic extract products show similar blood pressure benefits in trials. Garlic is generally well-tolerated; gastrointestinal upset and heartburn are the most reported side effects at higher intakes.