Garlic - A Deep Dive

Raw garlic cloves and aged or fermented garlic can lower total cholesterol levels and LDL cholesterol in the blood while at the same time raising HDL cholesterol, what is commonly called the good cholesterol. (1,2) Blood-pressure-lowering properties have been observed in a number of studies in individuals with high blood pressure; a 2015 overview meta-analysis summarizes these observations as relevant but mild, and the researchers add that more research is needed in this area. (3)

There are several interesting in-vitro studies documenting how bioactive substances in garlic interact with cancer cells in a controlled environment (test tube). (4,5) There are also so-called prospective cohort studies in humans that have looked at the links between regular consumption of garlic over a longer period and the prevalence of prostate cancer and colon cancer, or precursors to colon cancer. (6-8) Taken together, there is a great deal of interesting research here to read and follow that points to garlic's effects.

Garlic has strong evidence behind it when it comes to supporting the immune system. (9) Today garlic extract is an approved herbal medicine in Sweden for relieving the symptoms of a cold. Studies on garlic extract have shown that, taken regularly, it both reduces the number of sick days per year and shortens the course of the illness itself. (10) The active substances in garlic also have a stimulating effect on glutathione, an antioxidant produced by the body that plays a leading role in its defense system against toxins and free radicals. (11)

Bioactive substances

What does garlic contain that makes it special? Garlic is nutrient-rich and contains a number of essential nutrients such as calcium, selenium, iron, zinc, vitamin C and vitamin B1. But it also contains other, non-essential substances, some unique to garlic and some not. One of the two most interesting examples is the sulfur compound alliin, a substance entirely unique to garlic. It is alliin that is indirectly the cause of much of garlic's healthy properties. Alliin can easily release a metabolite called allicin through a reaction that occurs when garlic is chewed, for example, or otherwise broken up in contact with oxygen. Much of the allicin formed is quickly converted into a range of other potent, bioactive sulfur compounds in the body, chiefly diallyl sulfides, ajoene and vinyldithiin. On top of that there is one more very important and unique substance in garlic, namely glutamyl-S-allylcysteine. When garlic is aged or fermented it is converted into the bioactive sulfur compound S-allylcysteine, or SAC. SAC too is usually credited with much of garlic's healthy effects, especially when it comes to garlic as an extract in capsules.

Garlic extract

In aged or fermented garlic extract, glutamyl-S-allylcysteine breaks down over time and loses its glutamyl group, leaving behind the potent substance SAC. For that reason, and because the aging process also results in other unique bioactive substances being formed, garlic extract has a more powerful antioxidant potential than fresh garlic. Aging and fermentation can also make garlic entirely odorless, which many people appreciate. Otherwise it is garlic's sulfur compounds that are normally volatile enough to give off molecules in the form of hydrogen sulfides. These hydrogen sulfides are the reason for garlic's unmistakable smell and taste, and its effect on the breath.

Dosing garlic, and how garlic should be taken for the best effect

If you already use garlic cloves frequently and regularly in your cooking, you have a very healthy routine going. For the best effect, garlic should not be boiled or heated in a microwave, as that affects the active substances negatively. It is better to eat it raw, with the cloves properly crushed or chewed. Frying and grilling also work as alternatives. If you want to make the most of garlic's positive properties without smelling of garlic for days on end, an aged or fermented garlic extract in a high concentration, taken daily, is recommended instead. Garlic extracts are also usually experienced as gentler on the stomach if you are sensitive to raw garlic.

- Most studies on garlic have used doses of between 500 and 1200 mg per day in the form of a highly concentrated garlic extract, always aged or fermented. If that means more than one capsule, the dose can usefully be spread out across the day.

- The lowest effective dose of fresh garlic is considered to be 1 crushed clove of normal size, 2 or possibly 3 times a day.

- Always take garlic with a meal.

Important to know about garlic

It is important to bear in mind that garlic contains many potent substances that interact with the body's physiology. It can thin the blood and lower blood pressure, for example. For that reason it can have unwanted effects if combined with certain medicines. Two examples, among several, are blood-pressure-lowering and blood-thinning medicines.

Garlic, like much else, is toxic if taken in extreme amounts or doses. So never overdo your intake — more is not always better. Examine.com suggests that the intake of fresh garlic should not exceed 5% of the total diet. That means fresh garlic should not exceed ~17 grams a day if you weigh around 68 kg, and not more than ~23 grams a day if you weigh around 90 kg.

All in all, you could say that garlic's greatest effect does not come from supplying the body with vitamins or minerals the way traditional supplements do. Instead it is about entirely unique substances that are admittedly not essential to life, but which are still prized for their broad and interesting health effects. Garlic can rightly be categorized as a typical “superfood”, a term that is otherwise often misused and that few foods actually deserve, since there should be plenty of well-conducted studies of various kinds that together can confirm both mechanisms of action and potency when consumed.

Sources (click the title to follow the link):

1. Warshafsky, S et al. “Effect of garlic on total serum cholesterol. A meta-analysis.” Annals of internal medicine vol. 119,7 Pt 1 (1993): 599-605.

2. Ried, Karin et al. “Effect of garlic on serum lipids: an updated meta-analysis.” Nutrition reviews vol. 71,5 (2013): 282-99.

3. Rohner, Andres et al. “A systematic review and metaanalysis on the effects of garlic preparations on blood pressure in individuals with hypertension.” American journal of hypertension vol. 28,3 (2015): 414-23.

4. Yi, Lan, and Qi Su. “Molecular mechanisms for the anti-cancer effects of diallyl disulfide.” Food and chemical toxicology : an international journal published for the British Industrial Biological Research Association vol. 57 (2013): 362-70.

5. Chung, J G et al. “Inhibition of N-acetyltransferase activity and gene expression in human colon cancer cell lines by diallyl sulfide.” Food and chemical toxicology : an international journal published for the British Industrial Biological Research Association vol. 42,2 (2004): 195-202.

6. Steinmetz, K A et al. “Vegetables, fruit, and colon cancer in the Iowa Women's Health Study.” American journal of epidemiology vol. 139,1 (1994): 1-15.

7. Zhou, Xiao-Feng et al. “Allium vegetables and risk of prostate cancer: evidence from 132,192 subjects.” Asian Pacific journal of cancer prevention : APJCP vol. 14,7 (2013): 4131-4.

8. Millen, Amy E et al. “Fruit and vegetable intake and prevalence of colorectal adenoma in a cancer screening trial.” The American journal of clinical nutrition vol. 86,6 (2007): 1754-64.

9. Percival, Susan S. “Aged Garlic Extract Modifies Human Immunity.” The Journal of nutrition vol. 146,2 (2016): 433S-436S.

10. Josling, P. “Preventing the common cold with a garlic supplement: a double-blind, placebo-controlled survey.” Advances in therapy vol. 18,4 (2001): 189-93.

11. Avci, Aslihan et al. “Effects of garlic consumption on plasma and erythrocyte antioxidant parameters in elderly subjects.” Gerontology vol. 54,3 (2008): 173-6.