Garlic (Allium sativum) is a vegetable that can be eaten raw or cooked. It is also sold as a dietary supplement. Garlic contains several sulfur-containing phytochemicals that are metabolized when eaten and can affect cardiovascular health and inflammation. These chemicals include allicin, diallyl disulfide (DADS), diallyl trisulfide (DATS), and S-allylmercaptocysteine (SAMC).
Supplementation with garlic lowers circulating markers of oxidative stress (e.g., malondialdehyde) and inflammation (e.g., C-reactive protein and TNF-α). Supplementation with garlic can also reduce total cholesterol levels, particularly in people with cardiovascular disease, and improve other measures of cardiovascular health (coronary artery calcium, carotid intima-media thickness, etc.). However, its direct effects on cardiovascular morbidity and mortality are currently unclear.
Observational studies also show that garlic consumption, through food or as a supplement, is associated with a lower risk of gastric and colorectal cancers.
Garlic consumption can cause “garlic breath” and body odor, which are typically most pronounced after eating raw garlic. These side effects are also frequent-reported “adverse events” in clinical trials. Some supplement formulations (e.g., aged garlic extract) are designed to minimize such odors.
In rare cases, garlic may cause an allergic reaction.
Metabolizing garlic’s sulfur-containing chemicals (i.e., allicin, diallyl disulfide, diallyl trisulfide, and S-allylmercaptocysteine) eventually produces hydrogen sulfide (H2S). H2S is not only responsible for the characteristic garlic odor, but is also a signaling molecule with direct effects on vascular tissue, nerve synapses, inflammatory processes, and more. However, the sulfur-containing chemicals in garlic also have their own direct effects, activating several signaling pathways involved in anti-oxidant, anti-thrombotic, and anti-inflammatory mechanisms, which explains garlic's role in cardiovascular health. These chemicals also likely regulate the cell cycle in cancer cells, causing apoptosis (cell death), which explains the potential role of garlic in lowering gastric and colorectal cancer risk.
Most studies on garlic use a dosage range of 600-1,200mg a day, usually divided into multiple doses. The minimum effective dose for raw garlic is a single segment of a garlic bulb (called a clove), eaten with meals two or three times a day.
Aged garlic is a popular form of garlic to use for supplementation, since it does not have a fresh garlic scent. Garlic supplementation can also be done through food alone, though side-effects will include strong garlic-scented breath.
Microwaving garlic will partially destroy the beneficial components of the vegetable, but grilling and roasting will not damage the bioactives, provided the garlic is sliced or crushed beforehand. Garlic can be toxic if consumed in very high doses, so supplementation should never go beyond the following maximum dosages:
- 17.0 grams for a 150lb person
- 22.7 grams for a 200lb person
- 28.4 grams for a 250lb person