No other herb has served as many culinary and medicinal roles in as many cultures as garlic (Allium sativum L). Indeed, over the past 20 years, there have been more than 2000 papers published on garlic chemistry, pharmacology and clinical applications (Koch HP and Lawson LD, Garlic: The Science and Therapeutic Application of Allium sativum L. and Related Species, Baltimore: Williams & Wilkins, 1996).
Garlic is rich in antioxidant phytochemicals such as organosulphur compounds and flavonoids, both capable of scavenging tissue-damaging free radicals (Cancer J, 1990; 3: 20-1). Although the mechanisms of all garlic components are not known, many of its heart-protective, anticancer, anti-inflammatory and antiageing effects are thought to be due to these antioxidant actions (Planta Med, 1994; 60: 417-20; Planta Med, 1992; 58: 468-9; J Pharm Pharmacol, 1993; 49: 908-11; Neurochem Int, 1996; 29: 135-43).
Science has focused particularly on the potential of garlic in treating atherosclerosis. Studies show that garlic thins the blood and lowers 'bad' LDL cholesterol and triglycerides (Atherosclerosis, 1999; 144: 237-49). Trials using different forms of garlic providing a daily dose of around 10 mg of alliin, or a total allicin potential of 4000 mcg, have found that total cholesterol was reduc-ed by 10-12 per cent, and LDL and triglycer-ides by around 15 per cent each. In addition, some studies found that 'good' HDL cholesterol increased by 10 per cent (Ann Intern Med, 1993; 119: 599-605; Am J Med, 1993; 94: 632-5; Arzneim Forsch, 1990; 40: 1111-6; Arzneim Forsch, 1992; 42: 1223-7; Br J Clin Pharmacol, 1989; 128: 535-44).
But not all garlic studies are positive (J R Coll Phys [Lond], 1996; 30: 329-34; Athero-sclerosis, 1995; 113: 219-25; Arch Intern Med, 1998; 158: 1189-94). One controversial study using steam-distilled garlic found no benefit at all (JAMA, 1998; 279: 1900-2). However, this trial confirms the importance of using the right preparation - steam-distilled garlic has virtually no beneficial sulphides and no allicin.
A report by the US Agency for Healthcare Research and Quality (AHRQ; see www.ahrq.gov/clinic/garlicsum for links to the full report) reviewed all randomised, controlled trials of more than four weeks, and concluded that garlic has a modest but statistically significant effect in lowering LDL and triglyceride, but no overall effect on total or HDL cholesterol.
Cancer prevention is another touted benefit of garlic, but the findings are mixed. However, it's probably fair to say that if garlic has any effect at all, it is not as a cure, but as a possible preventative for certain types of cancer. The organosulphur components of aged garlic can block carcinogens and prevent oesophagus, colon, lung, breast, prostate and stomach cancers - at least in the lab (Carcinogenesis, 1993; 14: 1627-31; Nutr Cancer, 1997; 27: 186-91; Nutr Rev, 1996; 54: S82-6).
Although specific trials are lacking, the AHRQ noted that population studies show that people who regularly eat lots of garlic have a lower risk of developing certain cancers - laryngeal, gastric, colorectal and endometrial - than those who eat little or no garlic. This has been borne out by observational studies and meta-analyses (J Nutr, 2001; 131: 1032S-40S; Am J Clin Nutr, 2000; 72: 1047-52).
In one Chinese study of a population with a high incidence of stomach cancer, those in the highest quarter of garlic consumption had 40 per cent less cancer risk than those in the lowest quarter (J Natl Cancer Inst, 1989; 81: 162-4).
A study in Iowa looked at the diets of over 40,000 women and found that those who consumed the most garlic had the lowest risk of colon cancer (Am J Epidemiol, 1994; 139: 1-15). However, a Dutch study of more than 120,000 men and women found no relationship between garlic supplementation and breast, colon, rectal or lung cancer prevention (Carcinogenesis, 1996; 17: 477-84), which seems to reinforce the view that it is fresh garlic that provides the most effective protection.
Most of us have been taught to buy standardised garlic supplements with a guaranteed allicin yield, or allicin potential. But this doesn't necessarily mean that we're getting what we think we are.
The allicin yield quoted on many labels generally refers to the potential of the alliin in the product to be converted into allicin by the enzyme alliinase. This yield can only be guaranteed at the point of manufacture as allicin degrades quickly. By the time it reaches the consumer, it may be a very different product.
Also, too high a heat during processing and prolonged storage at high temperatures can destroy alliin, resulting in low allicin potential. Not surprisingly, research shows that most commercial products don't contain allicin (Planta Med, 1991; 57: 363-70).
One possible reason why older garlic studies were mostly positive while newer ones have shown less benefit was hinted at by a reanalysis of the garlic products used in all studies carried out between 1989 and 1997. According to the US researchers, the older lots were more resistant to stomach acid and therefore released three times as much allicin than the newer ones (Planta Med, 2001; 67: 13-8). So, could it be that in the race to produce newer and better products, pharmaceutical science has managed to take a giant step backward?
At any rate, the focus on allicin content of garlic supplements may be inappropriate since allicin is just one of the dozens of the herb's active components. Ajoene (formed by combining allicin and diallyl disulphide) has proven antiplatelet and antibiotic activity. A number of constituents - including 33 identified sulphur compounds, 17 amino acids, germanium, calcium, copper, iron, potassium, magnesium, selenium, zinc, and vitamins A, B1 and C - have known healthful actions in the body (Br J Clin Pharmacol, 1989; 28: 535-44; Phytother Res, 1988; 2: 196-7).
Allicin may be only indirectly involved in garlic's blood-thinning ability (Thromb Res, 1992; 65: 141-56). As allicin doesn't readily enter the blood circulation, it may be that, once ingested, its role consists of its being converted into other bioavailable compounds that produce the anticlotting effects (Thromb Res, 1992; 65: 141-56).
In addition, many of garlic's sulphur-containing compounds - vinyldithiins, ajoene and S-allyl cysteine - and other constituents are quickly absorbed from the digestive tract, then extensively and rapidly metabolised. With regular ingestion, these can be detected in the blood and tissues. Allicin, on the other hand, cannot be detected in the blood or urine after ingestion of either raw garlic or pure allicin (Planta Med, 1992; 58: 345-50). For this reason, many scientists argue that it cannot be the primary active ingredient in garlic.
Experts now believe there is no single constituent of garlic that is its main active ingredient. Rather, the range of constituents in garlic act in synergy to produce a variety of healthful benefits. It may therefore make more sense to seek out minimally processed supplements that include as many of the health-giving constituents of garlic as possible (see box above).
Some final thoughts
The effectiveness of garlic preparations can be altered in other ways. Like all herbs, the taste and smell of garlic is a kind of in-your-mouth quality control. Without them, it's likely to be an inferior product. Odourless garlic preparations are usually cooked; this inactivates alliinase, the enzyme that breaks down alliin to help make allicin. Or an odour-masking compound such as chlorophyll is added to the preparation, although such additions are rarely effective.
Less drastic is odour-controlled garlic, which is usually coated with a glaze or uses a sturdy capsule to make sure the garlic is released only in the gut, where it is unlikely to produce 'garlic breath' or reflux.
On the plus side, 200 mg daily - around 70 cloves - has no known side-effects. To be toxic to the stomach and liver, you'd have to eat 300-500 mashed cloves in one sitting. Many people 'overdose' on fresh garlic in the belief that it's good for them, but the most likely effect of eating four or more cloves in one go is nausea and vomiting.
Because of its anticlotting properties, garlic supplements should not be taken with drugs such as heparin, warfarin and coumarin derivatives, and not before surgery, as this could lead to uncontrollable bleeding. These small considerations aside, garlic used properly and for the right indications is still one of our most valuable herbs.
Of our nine randomly selected products, only about half provided good label information. Also, allicin content was listed in different ways so that those who don't know that 'mcg' and 'ug' mean the same thing might rightly feel confused.
Price: lb5.79 (30 capsules)
Solgar has used a maceration process for a product that is 'rich in allicin' and also contains the active byproducts of allicin conversion, such as ajoenes, vinyldithiins and polysulphides. Indeed, recent research suggests that maceration is the only process that leaves significant quantities of ajoenes and vinyldithiins.
The softgel capsule releases the macerate only in the gut, ensuring effective odour control. The label offers a good amount of useful information. However, at one capsule a day, this will cost you 19 p, although the product's apparent quality may tip the balance for most consumers.
Maximum Garlic 6500
Distributor: Holland & Barrett
Price: lb6.99 (60 tablets)
Each of these high-potency tablets contains 650 mg of standardised garlic powder with a guaranteed allicin yield of 6500 mcg. The label also lists the total thiosulphinate yield (6500 mcg/ug), alliin yield (14,500 mcg/ug) and gamma-glutamylcysteine (5200 mcg/ug), which seems impressive.
But the product also contains binders, fillers and colouring (titanium dioxide), and uses a shellac glaze (an insect byproduct, and so unsuitable for vegans or strict vegetarians). It is odour-controlled by the inclusion of anise oil, which may be just as hard on sensitive stomachs as garlic oil.
Although the highest-potency product we looked at, it lies in the middle of the price range, with each capsule costing 12 p.
One-a-Day Garlic Plus
Price: lb6.25 (30 capsules)
This offering contains 400 mg of garlic concentrate (plus 200 mg of biotin) in a vegetarian capsule. Other than that, the label provides no information on how the product was produced or what the potential allicin yield is. It would be reasonable for a consumer to be suspicious of any product with such vague labelling. For the record, these capsules contain a powdered concentrate.
At one capsule daily, they were the most expensive at 21 p per day.
Whole Garlic Bulb
Price: lb4.49 (60 capsules)
Each of these capsules contains 300 mg of freeze-dried garlic (equal to 1070 mg fresh). FSC claims that freeze-drying the whole clove ensures that all the active ingredients of the fresh herb are maintained. In reality, it's not much different from any other drying process, though these products may contain slightly more S-allyl cysteine.
This product is made from 100 per cent Chinese garlic - although the relevance of this is not clear - and doesn't use a deodorising process because, says the manufacturer, it reduces the potency of the active constituents (a claim borne out by research). Odour may not be a problem, however, since these capsules are likely to dissolve in the gut. The label contains plenty of information, some of it useful.
The recommended one or two tablets daily would cost you just 7 p each.
Distributor: Lichtwer Pharma
Price: lb3.99 (30 tablets)
The manufacturer tells us that Kwai contains all the constituents of fresh, organically grown Chinese garlic. Each tablet is coated 60 times for odour control. The active ingredients are 300 mg of dried standardised garlic extract, and each tablet has a guaranteed allicin yield of 1800 mcg.
Kwai has been used in many studies of the effectiveness of garlic, but not always with the consistently good results, as Lichtwer would have us believe. This product uses a huge number of excipients, including sugar, lactose, glucose syrup, titanium dioxide and polysorbate 20, and strict vegetarians and vegans should note that this product also uses a shellac glaze.
Taking the suggested one tablet a day would set you back 13 p - fairly pricey for what you're actually getting.
Price: lb4.95 (60 tablets)
The box boasts that Wassen garlic can help maintain a healthy heart. Tipped as highly odour-controlled and high in potency, these tablets contain the equivalent of 960 mg of fresh garlic (or 1600 mcg of allicin). They are heavily coated with a glaze similar to Kwai's and contain just as many excipients, including sucrose, iron oxides, acacia powder and titanium dioxide. Consumers may be justifiably suspicious of so many extras. One tablet a day would cost you 8 p a day.
Distributor: Natural Brand
Price: lb4.29 (90 capsules)
Each gelatine capsule contains 3 mg of garlic oil - equivalent, says the manufacturer, to 15 mg of fresh garlic. Otherwise, the label provides little else upon which to make a decision. The product has the strong garlicky smell characteristic of garlic oil, which some may find unpleasant.
The recommended intake is one capsule daily, costing 5 p - reasonable until you consider that, as a garlic oil supplement, it's likely to contain only minute quantities of oil-soluble diallyl sulphide and diallyl disulphide, and no allicin.
Concentrated Garlic One a Day
Price: lb4.90 (30 tablets)
Boots' special offer of three-for-the-price-of-two makes this seem like good value - until you read the label. These low-dose tablets contain 300 mg of garlic powder and have a guaranteed allicin yield of 1800 mcg.
The label also boasts that these tablets can help maintain healthy cholesterol levels and a healthy heart - although, at one a day, this dose is far below the 4000 mcg found to be effective in scientific studies. This also has loads of excipients, including the synthetic binder polyvinyl pyrrolidone.
Even at the special price offer, these tablets cost 11 p a day, more than some of the better mid-range products we reviewed.
Price: lb2.59 (30 capsules)
The beneficial effects of garlic consumption - a healthy heart, good circulation and a healthy digestive system - are listed on the box, but each capsule contains only 2 mg of odourless garlic, equivalent to 200 mg of fresh garlic. It uses no preservatives, artificial colours or flavours, though the gelatine capsule is unsuitable for vegetarians.
Not surprisingly, there's no mention of allicin yield - probably because odourless garlic is devoid of the allinase necessary to make allicin. In addition, this product uses odourless garlic powder in sunflowerseed oil. For 9 p a capsule, this gives you a low dose of something not very effective.
garlic supplements, allicin, alliinase, antioxidants, atherosclerosis, lipid-lowering, cancer prevention, blood thinning, side effects, benefits
Road testChoosing garlic wiselyDifferent ways of producing garlic supplements produce markedly different products (J Nutr, 2001; 131: 955S-62S; Minnesota Pharm, 1999; 53: 13-8, 24-6; WHO Monographs on Selected Medicinal Plants, vol 1, Geneva: World Health Organization, 1999).
Type Preparation Active ingredients Useful for
Fresh None; allowing Alliin, allyl sulphides May prevent cancer
10 minutes including S-allyl (e.g. bowel, colon);
between peeling cysteine, plus a range may also reduce
and cutting/cooking of useful minerals cholesterol, boost
preserves beneficial and other nutrients immunity and aid
constituents cardiovascular health
Dehydrated Cloves are sliced or Similar to fresh, but Stimulates immunity,
powder crushed, dried and slightly more alliin and free-radical scavenger,
pulverised; when S-allyl cysteine; some improves cholesterol,
oven-dried at low oil-soluble sulphur aids cardiovascular
temperatures, may compounds; enteric health
have slightly more coating protects alliinase
active constituents from stomach acid
Dried Whole or sliced Mostly water-soluble May lower cholesterol,
alcohol cloves soaked in sulphur compounds may prevent cancer;
extract extracting solution (S-allyl cysteine, S-allyl gentler on stomach,
(kyolic or (e.g. water, alcohol) mercaptocysteine); less odour; water-
aged garlic) for variable time small amounts of soluble compounds
(up to 20 months oil-soluble compounds; less cytotoxic than
for kyolic) no alliicin or allyl oil-soluble compounds
Oil Whole cloves ground During processing, Acts as a free-radical
macerated into vegetable oil at some alliin converted scavenger, improves
low temperatures to allicin,which converts cholesterol levels,
into other beneficial aids heart health,
compounds (dithiins, boosts immunity
Steam Whole cloves ground Minute quantities May be useful
distillation in water, heat (steam)- of oil-soluble sulphides topically, but few
distilled or extracted (e.g. diallyl sulphide, benefits when
in alcohol; usually diallyl disulphide), ingested; toxic in
contains 1% garlic oil but no allicin very large amounts
in vegetable-oil base