A User’s Guide to Garlic

By Katya Korol and Subhas Ganguli

Introduction

Common Names, Scientific Names, Types

Garlic (Allium sativum) is a vegetable belonging to the Allium genus. It is closely related to the onion, shallot, chive, leek , Chinese onion¹ and scallion² . Garlic is composed of a ‘bulb’ (also known as the head), containing several cloves and enveloped in a white, yellow, or purple coloured covering². 

Garlic is typically classified according to four types: hardneck garlic, characterized by a hard central stalk which is also called a ‘scape’; softneck garlic, the most common type, often found in ‘braided’ packaging at the grocery store; black garlic, created using heat treatment and sometimes referred to as ‘fermented garlic’; and Creole garlic, often entirely purple in colour and not very common². 

Brief History

Scholars trace the origins of garlic to somewhere in ‘Middle Asia’; however, the exact origin is contested3. Therapeutic effects of garlic are documented throughout history and some of the earliest accounts date back to 2600–2100 BC³. In ancient Chinese and Indian medicine, garlic was recommended for a variety of health conditions including respiratory and digestive problems, parasitic infections and leprosy⁴. It was also used for its believed health benefits in several other parts of the world including Ancient Egypt, Greece, Israel, Tibet, and Rome³. Garlic was introduced to Britain in 1548³ and the idea that garlic is beneficial to health was increasing in popularity in Europe during the Renaissance⁴. 

Overview of Health Benefits 

Today, garlic is recognized for its antibacterial, antiviral, and antifungal properties. It has been further studied for its potential health benefits for conditions including cardiovascular disease, cancer prevention, and blood sugar regulation. 

Please note that garlic should not be taken to prevent or treat a medical condition without first discussing with your doctor or qualified health practitioner.

Allicin is an organosulfur compound obtained from garlic and is especially associated with many of garlic’s therapeutic effects.  In certain environments, Allicin may release a compound that works along with nitric oxide (NO) to enhance the elasticity of blood vessels and thus this may explain its potential cardiovascular benefits. 

Further, given the rarity of high-sulfur foods, sulfur-compounds in garlic have been suggested to contribute to garlic’s health-food status. These sulfur compounds are recognized for their role in the body’s detoxification system and oxidative metabolism, as well as for contributing to the health of our bones and connective tissue². 

Side Effects, Interactions, and Warnings

Side Effects 

If you are on prescription medications or under the care of a physician, check with them before taking garlic.  Though it is commended for its health benefits, it is important to note several potential side effects of garlic. When taken orally, mild side effects of garlic (particularly raw garlic) may include heartburn, burning sensation in the mouth or stomach, gas, nausea, vomiting and diarrhea. Since garlic may cause irritation or burning to the skin it should not be applied topically. Garlic contains anti-clotting factors which may increase risk of bleeding and bruising after prolonged ingestion and increase the effects of anticoagulant and antiplatelet drugs so it should be avoided by individuals with bleeding disorder or who are on blood thinners such as warfarin (Coumadin) and related medications⁵ .  For the same reason, stop taking garlic at least two weeks before a scheduled surgery. Other more severe possible side effects of garlic include low blood pressure (hypotension), gastrointestinal irritation  and low blood sugar⁵.

 

Interactions 

Garlic may reduce the absorption of Isoniazid (Nydrazid, INH) and should not be taken by patients on this drug. Garlic may also increase the rate at which the body metabolizes some medications, which can compromise their effectiveness. Medications that may interact with garlic in this way include HIV/ AIDS medications (NNRTIs), and Saquinavir (Fortovase, Invirase). It is recommended that individuals on these medications avoid combining them with garlic⁵.

Other medications which may moderately interact with garlic in this way include contraceptive medication (‘birth control’) which may become less effective (therefore use additional forms of birth control such as a condom).  Garlic may increase the rate of breakdown of Cyclosporine (Neoral, Sandimmune) and should not be taken with these drugs⁵. It is also recommended to consult a physician if you are taking medications that are broken down in the liver, such as Cytochrome P450 2E1 (CYP2E1) substrates, and Cytochrome P450 3A4 (CYP3A4) substrates, as garlic intake can influence this process⁵.

Dosage 

Please note that garlic should not be taken to prevent or treat a medical condition without first discussing with your doctor or qualified health practitioner.

In studies, doses of garlic and the method of administration varies depending on the condition for which garlic is being used . The following doses and methods of administration have been used in studies: 

Diabetes: 600-1500 mg daily or 300 mg garlic tablet (Allicor, INAT-Farma) taken two to three times daily (in addition to other prescriptions for the condition)⁵.

Cholesterol  or Hyperlipidemia: 1000-7200 mg of Kyolic, Wakanug (aged garlic extract ) daily, taken in divided doses⁵, or a  dose of 300-900 mg of garlic powder tablet (Kwai, Lichtwer Pharma) taken daily in divided doses⁵ or 300 – 1,200 mg of garlic powder taken daily in divided doses (sometimes in addition to other prescriptions)⁵.

High Blood Pressure: 300-1500 mg of garlic tablets taken in divided doses daily⁵ or 2400 mg of garlic powder tablets taken daily in divided doses or  960-7200 mg of aged garlic extract pills taken daily⁵ in divided doses. Or 500 mg of garlic oil daily (in addition to other prescriptions)⁵ .

Prostate Cancer: 1 mg/kg of a water-soluble garlic extract daily⁵.

Selecting and Storing

 It is always recommended to buy fresh garlic in its natural form (whole bulbs), rather than pre-prepared garlic products such as garlic powder or garlic paste, which are less flavorful for cooking and provide fewer nutritional benefits. When selecting a garlic bulb at the grocery store or farmers market, select garlic that feels firm, has unbroken skin, and which is dry rather than damp in texture. Refrain from picking garlic that is  moldy, squishy, shriveled, or which has begun to grow green sprouts. The size of garlic has not been reported to bear on its quality. It is recommended to buy organic garlic in order to limit exposure to pesticides and heavy metals which may be present in non-organic garlic. Local organic growers may sell organic garlic, and garlic will be labelled as organic at larger supermarkets. 

Store garlic uncovered somewhere cool and away from sunlight. If optimally stored, garlic can keep for approximately one month. Garlic does not need to be refrigerated and freezing garlic cloves will change their flavor. Leaving the garlic bulb intact and not separated into cloves will preserve it for longer and it is important to monitor the state of garlic cloves and dispose of those that are moldy².

Cooking

To prepare garlic, separate the individual cloves by cutting the bulb in half with a knife or by applying pressure to the bulb on an angle with your hands. Once separated, peel the individual cloves either by making a small incision in the skin of the clove and peeling it off with your hands or by using the flat part of the knife to apply pressure on the clove, allowing it to separate from the skin. It is advised to remove the green stem in the middle (found in hardneck garlic) as it is hard to digest.

Once peeled and separated into individual cloves, it is advised to crush the garlic cloves to facilitate the production of the aforementioned allicin compound which occurs when something called alliin (another sulfur compound) is broken down. Crushing garlic can be accomplished using a garlic press. Garlic can also be enjoyed raw (garlic cloves can be chewed in their raw state in order to achieve the same reaction required to release allicin,  mentioned above) or it can be cooked; however, it is believed that garlic should be cooked on lower heats in order to ensure that it retains its various nutritional benefits (around 150-250 Fahrenheit or 65-121 Celsius)². 

Nutritional Profile

 Garlic is composed of macronutrients (fat, carbohydrates and proteins), micronutrients (such as vitamins), minerals (including potassium, phosphorus and calcium) and other compounds such as ‘ash’ ². 

Sulfur compounds (including thiosulfinates, sulfoxides, sulfides, diallyl sulfides and polysulfides, vinyldithiins, ajoenes, and sulfur-containing amino acids and peptides) are likely what gives garlic its health benefits. In addition to these sulfur compounds, garlic contains several other nutrients. Listed in order of their nutrient density (highest to lowest), these include manganese, vitamin B6,  vitamin C , copper, selenium, phosphorus, vitamin B1 and calcium².

Regarding macronutrients, per 6 cloves or 18g (grams) of garlic, 0.18 g of these account for total sugar and 5.39 g for other carbohydrates, yielding a total of 5.57 g of carbohydrates. There are 0.04 g of unsaturated fat and 0.02 g of saturated fat. Dietary fiber accounts for 0.38 g and protein accounts for 1.14 g. 10.54 g of the 18 grams are water².

Possible Health Benefits

Garlic and Cardiovascular Health

In 12 trials involving 553 participants, systolic blood pressure was significantly lowered by 8.3 mmHg (P<0.00001; I2=43%, no significant heterogeneity)⁶. Garlic powder was used in 8 trials while aged garlic extract was used in 4 trials.  

In 8 studies involving 374 subjects, diastolic blood pressure was significantly lowered by 5.5 mmHg (P<0.00001;I2=59%, P = 0.02 for heterogeneity).  This decrease in blood pressure is associated with a 16-40% risk reduction of cardiovascular events such as heart attack and stroke.

The results for the effect of garlic are shown in the table below (orange background indicates statistical significance)⁸.  As can be seen, garlic had a greater effect in subjects with hyperlipidemia than in a mixed population (subjects with normal and abnormal blood lipids)⁸.  Interestingly, the changes in total cholesterol were only statistically significant in subjects with baseline total cholesterol levels of < 200 mg/dL (5.2 mmol/L) and in studies with a duration of greater than 8 weeks⁸.

In a meta-analysis of 9 studies (348 subjects), C reactive protein decreased significantly by 0.80 mg/L (7.6 nmol/L)⁸.  The median study duration was 12 weeks (range 2-48 weeks) and median garlic dose was 2,100 mg/day (range 250 – 3,600 mg/day)⁸.

There was a very low rate of reported side-effects from garlic.  The most important being garlic odor, breath or taste and mild gastrointestinal complaints such as discomfort, flatulence, bloating, reflux, and belching⁸ which was not associated with the type of garlic preparation or dosage.  A rare incidence of garlic allergic reaction was also reported⁸.

Garlic and Diabetes

In 13 studies with 557 participants with diabetes, fasting blood glucose significantly decreased by 10.9 mg/dL (equivalent to a change of 0.4% in HbA1c) and HbA1c decreased by 0.60 mg/dL; interestingly no significant associations were shown between garlic intake and the fasting blood glucose of healthy individuals⁸.

Duration of treatment with garlic was important with 24 weeks of intervention resulting in a greater drop in HbA1c than 12 weeks (drop of 13.25 vs 6.93)(8).  The median daily dose of garlic was 900 mg/day (range 890 – 1,770 mg/day). No significant associations were found between garlic consumption and serum lipoprotein (a) and apolipoprotein B levels⁸.

Garlic and Cancer

A meta-analysis⁹ of 11 studies (8 case control, 4 cohort, 12,558 cases) found that garlic consumption was associated with protection against colorectal cancers (RR = 0.80; P<0.05), however since significant results were only found in case-control studies, more research in this area is recommended. A single meta-analysis suggested that allium vegetables could reduce the risk of colon adenomatous polyps (RR 0.88; P<0.05)⁸.  Garlic consumption was also protective for prostate cancer (OR 0.77;P<0.05)⁸.

Gastric cancer risk was significantly decreased by garlic (OR 0.51;P<0.05) however this became nonsignificant if analysis was limited to prospective studies so more research is needed⁸.  Interestingly gastric cancer was also reduced by Chinese chives (RR 0.43;P<0.05), onions (RR 0.55;P<0.05), and allium vegetables (RR 0.78;P<0.05)⁸.

Significant associations were also found for the following foods and cancers: a) Onion intake and esophagus cancer (RR 0.66;P<0.05)(8), b) Onion intake and larynx cancer (RR 0.72;P<0.05)⁸

Garlic and Immunity

An English study randomized 146 volunteers  to take 3 months of a garlic supplement or placebo and reported the occurrence of symptoms consistent with the common cold.  Results showed that the garlic supplement group had a significantly lower rate of common colds (33% vs 89%;P<0.01) as well a significantly shorter duration of symptoms (1.5 vs 5.0 days; P<0.001).  Overall completion rate was over 97%¹².

Another randomized controlled trial looked at the effect of 2.56 g of aged garlic extract (AGE) vs placebo for 90 days in 120 healthy subjects in terms of immune function and symptoms of the cold or flu¹³. Overall there was no significant difference between the experimental and control groups in the number of people who became sick (cold flu symptoms)(45% vs 55%;P=NS) or in the number of days of symptoms in those who got sick.  However, the AGE group had a significantly lower total number of symptoms (584 vs 737;P<0.001), and a trend towards a lower average number of symptoms per illness incident (11.9 vs 14.0;P=NS).  In addition, the AGE group had a lower number of days during which they had a reduced desire or ability to carry out their normal routine (15 vs 38;P<0.001) as well as fewer number of work days missed (8 vs 19;P=0.035)¹³.  Lastly, after 45 days, two types of T cells (gamma-delta and NK cells) were shown to have significantly higher rates of proliferation in vitro for the AGE group compared to the control group.

References

1. Wikipedia. (accessed 2020, March 25) Garlic. https://en.wikipedia.org/wiki/Garlic
2. The world’s healthiest foods. (n.d.) Garlic. www.whfoods.org/genpage.php?tname=foodspice&dbid=60#descr 
3. Petrovska, B. B., & Cekovska, S. Extracts from the history and medical properties of garlic. Pharmacognosy reviews, 2010:4(7), 106. 
4. Bayan, L., Koulivand, P. H., & Gorji, A. Garlic: a review of potential therapeutic effects. Avicenna journal of phytomedicine, 2014:4(1), 1.
5. www.webmd.com/vitamins/ai/ingredientmono-300/garlic
6. Ried, K. Garlic lowers blood pressure in hypertensive subjects, improves arterial stiffness and gut microbiota: A review and meta-analysis. Experimental and Therapeutic Medicine, 2020:19, 1472-1478
7. Macan, H., Uykimpang, R., Alconcel, M., Takasu, J., Razon, R., Amagase, H., & Niihara, Y. Aged garlic extract may be safe for patients on warfarin therapy. The Journal of nutrition, 2006:136(3), 793S-795S
8. Wan, Q., Li, N., Du, L., Zhao, R., Yi, M., Xu, Q., & Zhou, Y. Allium vegetable consumption and health: An umbrella review of meta‐analyses of multiple health outcomes. Food science & nutrition, 2019:7(8), 2451-2470
9. Zhou, X., Qian, H., Zhang, D., & Zeng, L. Garlic intake and the risk of colorectal cancer: A meta-analysis. Medicine, 99(1), e18575. Medicine, 2020:99(1), e18575
10. Taghizadeh, M., Hamedifard, Z., & Jafarnejad, S. Effect of garlic supplementation on serum C‐reactive protein level: A systematic review and meta‐analysis of randomized controlled trials. Phytotherapy research, 2019:33(2), 243-252
11. Danesh, J., Wheeler, J. G., et al. C-reactive protein and other circulating markers of inflammation in the prediction of coronary heart disease. New England Journal of Medicine, 2004:350(14), 1387-1397
12. Josling, P. Preventing the common cold with a garlic supplement: a double-blind, placebo-controlled survey. Advances in therapy, 2001:18(4), 189-193
13. Nantz, M. P., Rowe, C. A., Muller, C. E., Creasy, R. A., Stanilka, J. M., & Percival, S. S. Supplementation with aged garlic extract improves both NK and γδ-T cell function and reduces the severity of cold and flu symptoms: a randomized, double-blind, placebo-controlled nutrition intervention. Clinical Nutrition, 2012:31(3), 337-344