Thank God for Garlic! This pungent, yet delicious herb has been cultivated for its unique healing properties for more than 5,000 years!!! (Adaki, Adaki, Shah, & Karagir, 2014; Majewski, 2014). Traditional historic uses ranged from fending off crocodiles, vampires, and demons, to treating heart disease and cancer (Majewski, 2014). Research is shedding light on the benefits of garlic and helping us separate fact from fiction. Read on to learn about the research-backed benefits of garlic!
Garlic and nutritional properties
Containing more than 2,000 bioactive compounds, garlic is rich in organosulur compounds, oils, flavonoids, natural sugars, saponins, phenolic compounds, and polysaccharides (Majewski, 2014; Shang, et al., 2019). Garlic has limited amounts of vitamins and minerals, such as:
- Vitamin C, vitamin B1, vitamin B2, and beta-carotene
- Potassium, magnesium, and calcium
- traces of iron , copper, nickel, cobalt, chromium, selenium, and germanium (Majewski, 2014)
Notably, the trace minerals selenium and germanium have been noted to have anti-cancer properties by blocking cell division, tumor growth antagonism, and direct toxicity to cancer cells (Majewski, 2014). Perhaps of greater interest in terms of unique health promoting properties are the organosulur compounds. These compounds of note include:
- Alliin and alliianase which combine to form allicin –a powerful antibiotic–when garlic is crushed, minced, or processed
- Bioactive allicin derivatives that naturally occur when allicin breaks down, such as ajoenes , vinyldithiins, and sulfides
- Unique sulfer oxides called garlic-nins (Majewski, 2014)
Garlic is a rich source of antioxidants whether cooked, raw, or as an aged-garlic extract (Shang et al., 2019). The combination of the above mentioned properties work independently and in synergy to produce a number of the health benefits discussed as follows.
Garlic has been found to promote cardiovascular health through various mechanisms of action (Shang et al., 2019). Intake of garlic powder has produced the following benefits:
- Reductions in blood pressure
- Reductions in total cholesterol levels as well as reductions in LDL cholesterol
- Reductions in platelet aggregation/ blood-clotting
- Reduction of plague size in arteries with fatty deposits
- Protection against cardiac cell damage (Adaki et al., 2014; Shang et al., 2019; Wan, Li, Du, Zhao, Yi, Xu, & Zhou, 2019).
Blood pressure reduction
Numerous studies on animal models and humans alike have shown garlic to have significant blood pressure reduction effects in hypertensive participants (Shang et al., 2019; Wan et al., 2019). Two meta-analysis studies (studies that analyze the combined results of multiple individual studies) involving 9-20 individual studies each, with an median of more than 500 human participants per individual study, showed modest but significant blood pressure reductions (Wan et al., 2019). Reductions of blood pressure had a weighted average as follows:
- Systolic blood pressure reduction of 8.35 mmHg in hypertensive patients
- Diastolic blood pressure reductions of 3.82 mmHg in hypertensive patients (Wan et al., 2019)
Garlic did not impact blood pressure significantly for persons with normal blood pressure. Doses of garlic used to achieve this reduction averaged between 600-900mg/ day. Garlic was taken in the forms of oil, powder, extract, and supplemental forms in the various studies. The median duration of each study was 12 weeks.
Aged-garlic extract was found to promote the production and release of nitric oxide, down regulate calcium-channel activity, as well as to block angiotensin converting enzyme, leading to dilation/ relaxation of blood vessels (Shang et al., 2019). The actions of garlic in relation to angiotensin converting enzyme and calcium-channel activity are interesting, as two classes of medications commonly used to treat high blood pressure include angiontensin converting enzyme (ACE) inhibitors, and calcium-channel blockers.
An umbrella review of six separate meta-analyses reviewing the impact of garlic on various metabolic measures found the following significant effects concerning cholesterol on persons with dyslipidemia (abnormal cholesterol levels):
- Total cholesterol reductions by an average of 16.87 mg/dl
- Reductions of LDL cholesterol by an average of 9.65 mg/ dl
- Improvements of HDL cholesterol by an average of 3.19 mg/ dl
- Reduction of triglyceride levels by an average of 12.44 (Wan et al., 2019)
The doses used in the studies ranged from 890–1,700 mg/day. Study duration ran from 6-12 weeks. Forms of garlic used included garlic cloves, oil, extracts, and supplemental forms. Garlic appears to reduce activity of the HMG–CoA reductase enzyme, the same enzyme that is inhibited by the popular statin cholesterol medications (Majewski, 2014).
Aged-garlic extracts may be superior to fresh garlic (Shang et al., 2019). While these reductions are not earth-shattering, the addition of garlic to compliment healthy lifestyles and dietary patterns can help move cholesterol in the right direction.
Reduction in inflammation levels
One common marker of inflammation measured is c-reactive protein (CRP). CRP is a protein released by the liver due to inflammatory processes occurring in the body (United States Library of Medicine, 2020). CRP levels may be increased in cases of vascular inflammation, and levels over 10mg/ dl have been associated with increases in fatal cardiovascular disease over a 10 year-period (Cozlea, Farcas, Nagy, Keresztesi, Tifrea, Cozlea, & Carașca, 2013).
A meta-analysis measuring the impact of garlic as an intervention for elevated CRP levels in nine separate studies found a significant reduction of CRP by an average of 0.8 mg/ dl.
In a randomized, double-blinded placebo-controlled trial of 80 overweight or obese women ages 50-75 years with mild to moderate knee osteoarthritis, the women were given either 500mg garlic tablets twice daily (1,000mg/ daily) or a placebo for 12 weeks. The women receiving the garlic has significantly lower levels of knee pain (from nearly 6.8 down to a 5.2 on a 10-point scale on average) and significantly lower levels of the inflammatory hormone resistin (6.41 ng/dl to 5.56 ng/dl on average).
Additional research supports anti-inflammatory effects of garlic. A randomized-controlled trial in 40 dialysis patients analyzed the effects of 400mg garlic extract twice daily (800mg total per day) on interleukin-6 (IL-6, a pro-inflammatory cytokine) levels, CRP levels, and erythrocyte sedimentation rate (ESR, a general measure of inflammation) (Zare, Alirezaei, Bakhtiyari, & Mansouri, 2019). Nineteen patients received the garlic capsules over an 8-week period, while 21 patients received a placebo.
Results noted a significant decrease in all three inflammatory markers (Zare et al., 2019). Other anti-inflammatory effects include reductions in gingivitis and gum bleeding ( Zini, Mann, Mazor, & Vered, 2018 as cited in Wan et al., 2019).
Garlic and anti-cancer properties
An umbrella study of seven meta-analysis studies, each averaging an analysis of six individual studies, evaluated the impact of garlic and similar allium-rich vegetables (such as onions and chives) on cancer (Wan et al., 2019). Significant findings noted consumption of garlic, garlic powders, or allium-rich vegetables reduced the risk of gastric cancers by anywhere from 22% to 57%!!! Other interesting findings included:
- 12% reduction in colorectal adenomatous polyps
- 23% – 50% reduction in prostate cancer
- An inverse relation between garlic and esophageal cancer, meaning as garlic consumption increased, esophageal cancer rates decreased (Majewski, 2014; Wan et al., 2019)
Additional research suggests garlic compounds may additionally have protective effects against lung and bladder cancer (Shang et al., 2019). How does garlic protect against cancer?
- Neutralizing effects against carcinogens
- Prevents nitrosamine formation that normally occurs during cooking and food storage
- Protection against nitrosamine-induced DNA alkylation, a precursor to DNA mutation and carcinogenesis
- Anti-proliferative (anti-growth) effects demonstrated in lab studies on human cancer cell lines against breast, colon, prostate, liver cancer cells
- Sulfur compounds found within garlic inhibited cell reproduction for ovarian and pancreatic cancer cells
- Raw and crushed garlic caused activation of pro-apoptosis (cell-suicide) gene expression
- Aged-garlic extract appears to inhibit tumor angiogenesis (blood vessel growth within tumors) in animal models
- Augmentation of current cancer therapies, such as potential to reduce occurrence of febrile neutropenia
- Both aged-garlic and fresh garlic appear to have toxicity against cancer cells
- Induces cell suicide (apoptosis) of gastric cancer cells
- Action against H. Pylori bacteria, a bacteria that can cause stomach ulcers and increase risk of gastric cancer
- Prevention of metastasis in animal models (Adaki et al., 2014; Shang et al., 2019; Wan et al., 2019)
Anti-diabetic properties: reductions in blood sugar levels
Diabetic patients taking garlic supplements for 12-24 weeks experienced significant reductions in their hemoglobin A1C levels and fasting blood sugar levels (Wan et al., 2019). The weighted mean difference in A1C levels was a reduction of 0.6mg/ dl. For fasting blood glucose levels, reductions averaged 10.9 mg/dl. The greatest effects were observed at 24 weeks.
Serum fructosamine levels were significantly reduced after just 1-4 weeks of garlic supplementation. Blood sugar levels in non-diabetics were not affected. Similar to the cardiac measures, the reductions here are modest. However, garlic is one more low-risk tool in the tool box for promoting healthy glucose levels.
Anti-bacterial, anti-viral, anti-fungal properties
Lab analysis shows garlic oil from fresh garlic to have action against the following bacteria:
- Helicobacter pylori
- Proteus, Bacillus
- Clostridium and Mycobacterium tuberculosis
- Staphylococcus aureus,
- Pseudomonas aeruginosa
- Escherichia coli (Majewski, 2014)
Research suggests garlic may be good for the urinary tract against harmful bacteria (Majewski, 2014). Lab analysis also demonstrates anti-viral properties against the following viruses:
- Cytomegalovirus (a virus that permanently infects more than 50% of adults, and can reactivate throughout life)
- Influenza B virus (a strain of seasonal flu)
- Herpes simplex virus type 1 (oral) and type 2 (genital)
- Parainfluenza virus type 3 (a flu-like virus that can cause pneumonia)
- Human rhinovirus type 2 (the common cold virus) (Majewski, 2014)
Anti-fungal effects of garlic have been noted against tinea pedis (athletes foot), tinea cruris (jock itch), and onychomycosis (fungal infected fingernails and toenails (Majewski, 2014). Other effects of garlic include anti-parasitic effects against worms.
It must be emphasized that lab analysis does not always translate to effects once a substance is inside the human body. Human biology is complex, and bio-active compounds are broken down and metabolized, impacting their function and use in humans in sometimes unpredictable ways versus a lab setting. More research in humans is necessary in exploring the use of garlic for anti-infection purposes.
Risks and side effects
Garlic is consumed widely as a food and is generally recognized as safe (Wan et al., 2019). Side effects can include:
- Odor (though less apparent with aged-garlic extract)
- Gastric effects such as bloating, belching, reflux, and flatulence
- Allergic reactions are rare but possible
- Potential drug-interactions due to effects on liver metabolic pathways (CYP 3A4)
- Bleeding due to anti-platelet aggregation effects (a significant risk if you are going for a surgical procedure, epidural, or “spinal tap” procedure–garlic should be avoided for 1-2 weeks before procedures) (Majewski, 2014; Wan et al., 2019)
Garlic in large quantities should be avoided in women who are pregnant or breast feeding, as well as children up to 10 months of age due to potential effects on early neuronal development (Majewski, 2014).
Garlic preparations used in research
Garlic preparations used in research vary from fresh cloves to oils, aged-garlic extracts, dried power preparations, among others. According to Majewski (2014):
- Fresh garlic contains only alliin and not allicin (responsible for the anti-infective properties). Allicin results from breakdown of aliin when garlic is crushed or processed as noted above
- Garlic oil does not contain allicin and does not have an established safety data profile, it does contain some of the other sulfer compounds
- Dried garlic powder has 3 times the alliin versus fresh garlic, along with small amounts of sulfur compounds
- Aged-garlic extract has a well established safety profile, and has been used in more than 400 research studies. Does not contain allicin, but does contain a number of the bio-active compounds that result from the breakdown of allicin
Research quality is impacted by the size of the research sample, the use of human subjects versus animals or lab studies, the study methods (blinded? placebo controlled? randomized? etc.). In an analysis of the research quality, Wan et al. (2019) concluded that the evidence for the effects of garlic on total cholesterol, HDL, LDL and triglycerides as well as fasting blood glucose (discussed above) is moderate to strong. The evidence for the other effects described above is less established, ranking low to very low on the evidence scale. This does not mean there is not an effect, but rather, the studies reviewed suffered from limitations such as small sample size, lab or animal research, inconsistent findings, or lack of a comparison group.
The use of garlic to moderately improve cholesterol levels and blood sugar levels is supported by research. While more research is needed, protective effects against various cancers, inflammation, as well as cardiovascular disease and high blood pressure are other potential benefits. Finally, can garlic support the immune system against various bacteria, viruses, and fungal infections? Its too early to tell.
However, given the high safety profile of garlic and aged-garlic extract, supplementation may make sense as part your health and fitness protocol. Kyolic aged-garlic extract (not an affiliate!) has been independently tested by consumerlab.com and has received their endorsement for integrity and purity, and has been utilized in numerous peer-reviewed research trials. It is the brand I currently use.
While the claim is that Kyolic is odorless, a mild odor can be noted at times on the breath. The odor is not near as powerful as fresh garlic. As such, aged-garlic extract is an option for those who want to take garlic on a regular basis for the health benefits without developing a reputation! 😉 In summary, garlic is a low-risk intervention you can incorporate into your daily supplement protocol as part of your health promoting regimen! Live well!
Adaki, S., Adaki, R., Shah, K., & Karagir, A. (2014). Garlic: Review of literature. Indian Journal of Cancer, 51(4), 577–581. https://doi-org.lopesalum.idm.oclc.org/10.4103/0019-509X.175383
CDC.gov. (2020). Cytomegalovirus (CMV) and Congenital CMV Infection. Retrieved from https://www.cdc.gov/cmv/index.html
Cozlea, D. L., Farcas, D. M., Nagy, A., Keresztesi, A. A., Tifrea, R., Cozlea, L., & Carașca, E. (2013). The impact of C reactive protein on global cardiovascular risk on patients with coronary artery disease. Current health sciences journal, 39(4), 225–231. Retrieved from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3945266/
Dehghani, S., Alipoor, E., Salimzadeh, A., Yaseri, M., Hosseini, M., Feinle-Bisset, C., & Hosseinzadeh-Attar, M. J. (2018). The effect of a garlic supplement on the pro-inflammatory adipocytokines, resistin and tumor necrosis factor-alpha, and on pain severity, in overweight or obese women with knee osteoarthritis. Phytomedicine : international journal of phytotherapy and phytopharmacology, 48, 70–75. https://doi.org/10.1016/j.phymed.2018.04.060 Format:
Majewski M. (2014). Allium sativum: facts and myths regarding human health. Roczniki Panstwowego Zakladu Higieny, 65(1), 1–8. Retrieved from https://pubmed.ncbi.nlm.nih.gov/24964572/
Shang, A., Cao, S. Y., Xu, X. Y., Gan, R. Y., Tang, G. Y., Corke, H., Mavumengwana, V., & Li, H. B. (2019). Bioactive Compounds and Biological Functions of Garlic (Allium sativum L.). Foods (Basel, Switzerland), 8(7), 246. https://doi.org/10.3390/foods8070246
United States Library of Medicine. (2020). C-reactive protein. Retrieved from https://medlineplus.gov/lab-tests/c-reactive-protein-crp-test/
Wan, Q., Li, N., Du, L., Zhao, R., Yi, M., Xu, Q., & Zhou, Y. (2019). Allium vegetable consumption and health: An umbrella review of meta-analyses of multiple health outcomes. Food science & nutrition, 7(8), 2451–2470. https://doi.org/10.1002/fsn3.1117
Zare, E., Alirezaei, A., Bakhtiyari, & Mansouri, A. (2019). Evaluating the effect of garlic extract on serum inflammatory markers of peritoneal dialysis patients: a randomized double-blind clinical trial study. BMC Nephrol, 20(26). https://doi.org/10.1186/s12882-019-1204-6