However, the presence of allicin does not mean that consuming garlic produces the same effects as administering purified allicin in a laboratory experiment. Its instability, absorption, metabolism, and interaction with other substances in the body complicate the translation of experimental findings into clinical recommendations.
A scientific review of garlic-derived sulfur compounds describes how crushing and processing conditions influence the formation of allicin and its transformation into other compounds (2025 review, available through the National Library of Medicine).
2.2. Diallyl Disulfide and Other Organosulfur Compounds
Diallyl disulfide is another important sulfur-containing compound associated with garlic. It is formed through chemical transformations involving allicin and related substances.
Laboratory investigations have demonstrated that diallyl disulfide possesses antimicrobial activity against certain microorganisms. One particularly interesting study published in the Journal of Antimicrobial Chemotherapy examined its effects on Campylobacter jejuni, a bacterium associated with foodborne illness.
Researchers found that diallyl sulfide eliminated bacterial cells and disrupted bacterial biofilms under experimental conditions at concentrations substantially lower than those required for two tested antibiotics. The findings highlighted the potential antimicrobial activity of garlic-derived compounds, although they did not establish garlic as a substitute for antibiotics in human treatment (Lu et al., 2012).
Other organosulfur compounds, including ajoene and diallyl trisulfide, have attracted attention because of their potential antioxidant, antimicrobial, and cardiovascular effects.
2.3. S-Allyl Cysteine in Aged Garlic Extract
Aged garlic extract is produced by storing garlic in a liquid solution over an extended period. During this process, unstable compounds are transformed into more stable substances, including S-allyl cysteine (SAC).
S-allyl cysteine has attracted scientific interest because of its antioxidant properties and its potential influence on cardiovascular and neurological health.
Unlike freshly crushed garlic, aged garlic extract generally contains relatively stable water-soluble sulfur compounds and has a milder odor.
Some clinical investigations have used aged garlic extract to evaluate changes in blood pressure, inflammatory markers, and immune-cell activity. Nevertheless, evidence relating to one preparation should not automatically be generalized to all garlic supplements.
2.4. Flavonoids and Phenolic Compounds
Garlic also contains various phenolic compounds and flavonoids, which are plant-derived substances with potential antioxidant properties.
Antioxidants help protect cells against oxidative stress, a biological condition involving an imbalance between reactive oxygen species and the body's antioxidant defense systems.
Oxidative stress has been associated with several chronic conditions, including cardiovascular disease, metabolic disorders, and certain forms of cellular damage.
The antioxidant effects of garlic are therefore an important subject of scientific research. However, antioxidant activity measured in laboratory experiments does not necessarily translate into a measurable improvement in long-term health outcomes.
2.5. Essential Fatty Acids
Garlic contains small quantities of polyunsaturated fatty acids, including omega-3 and omega-6 fatty acids.
These fatty acids are involved in cell membrane structure, normal neurological function, and the regulation of various physiological processes.
Nevertheless, the quantities obtained from ordinary garlic consumption are too small for garlic to be considered a meaningful dietary source of essential fatty acids. Foods such as oily fish, walnuts, flaxseeds, and certain vegetable oils provide substantially larger amounts.
3. Garlic and Immune System Function
One of the most frequently discussed health benefits of garlic is its potential influence on immune function.
The immune system consists of a complex network of cells, tissues, organs, and signaling molecules that protect the body against infectious organisms and help maintain normal physiological balance.
Garlic contains several biologically active compounds that may interact with immune pathways. Experimental studies have investigated whether these substances influence the activity of natural killer cells, T lymphocytes, macrophages, and other immune cells.
Some small human studies have reported changes in immune-cell activity following garlic supplementation.
For example, a clinical trial involving 120 healthy adults investigated the effects of aged garlic extract consumed over approximately three months. Researchers observed changes in the proliferation of certain immune-cell populations. However, the results did not establish a clear reduction in the overall number of cold and influenza illnesses.
This distinction is important because changes in immune-cell measurements do not necessarily mean that someone becomes more resistant to infections.
The immune system must function in a coordinated manner. Increasing the activity of one type of immune cell does not automatically improve overall immune protection.
Garlic may nevertheless contribute indirectly to a healthy immune system because it is a nutritious plant food containing vitamin C, vitamin B6, manganese, and other compounds involved in normal physiological processes.
Its potential antimicrobial properties are another reason why researchers continue investigating its relationship with infectious diseases.
However, there is currently insufficient high-quality clinical evidence to conclude that garlic reliably prevents infections or substantially strengthens immunity in otherwise healthy people.
For this reason, garlic should be considered part of a balanced diet rather than a replacement for established preventive healthcare measures.
4. Can Garlic Help Prevent or Relieve Colds and Flu?
Garlic has traditionally been used in remedies for coughs, colds, and other respiratory complaints. Its reputation as a natural remedy is partly related to its strong aroma and the antimicrobial properties demonstrated by certain garlic-derived compounds in laboratory studies.
Some research has investigated whether garlic supplements can reduce the frequency, severity, or duration of common respiratory infections.
One frequently cited randomized clinical trial examined 146 participants who received either an allicin-containing garlic supplement or a placebo during a 12-week winter period.
The researchers reported fewer common-cold episodes in the garlic group than in the placebo group. Participants receiving the supplement experienced 24 reported cold episodes compared with 65 in the placebo group.
Although these findings were encouraging, the study was relatively small and relied on reported symptoms and illnesses.
A subsequent Cochrane systematic review evaluated the available evidence and concluded that only one trial met its inclusion criteria. The authors emphasized that the available evidence was insufficient to establish whether garlic could reliably prevent or treat the common cold (Lissiman et al., 2014).
The National Institutes of Health Office of Dietary Supplements also notes that only a limited number of clinical trials have investigated garlic supplements for colds and influenza, with inconclusive findings.
These results suggest that garlic's potential immune-related effects deserve further investigation, but they do not justify presenting garlic as a proven treatment for influenza, COVID-19, or other respiratory infections.
Garlic can still be included in ordinary meals, soups, and other dishes during cold and flu season. Its nutritional properties make it a useful component of a healthy diet, but people who develop significant respiratory symptoms should rely on appropriate medical advice rather than attempting to treat infections with garlic alone.
5. Garlic and Cardiovascular Health
Cardiovascular health is one of the areas in which garlic has received considerable scientific attention.
