26. Garlic and cardiovascular disease Dr HACollin
26.1. Introduction
The official wording from the Food and Nutrition Board of the National Academy, USA, defines functional foods as any food or ingredient that may provide a health benefit beyond the traditional nutrients it contains (Thomas and Earl 1994). The European view states that a food can be said to be functional if it contains a component (whether or not a nutrient) that benefits one of a limited number of functions in the body in way that is relevant to either the state of well being and health or the reduction of the risk of a disease or it has a physiological effect (Clydesdale1997; Bellisle et al 1998). Garlic clearly falls into the functional food category since it is normally eaten in small amounts either raw, cooked or as a salt. It is also sold as a supplement in the form of powdered tablets, capsules, steam distilled oil, or plant extract all of which can be taken on a daily basis and guaranteed not to produce the strong smell associated with consumption of the raw garlic.
While the garlic may be taken as a pharmaceutical now, it has been used as a flavouring and food additive for many centuries as described in detail by Rivlin, (2001). The earliest reference is from 2600-2100 BC on Sumerian clay tablets then subsequently its use was reported by the Egyptians and the Greeks to improve stamina generally, but it was the Chinese who prescribed it for specific illnesses and in this case it was for an aid to digestion and respiration and more particularly for diarrhoea and worm infestation. In India early texts described its use in the treatment of heart disorders. In Europe it was of value in the treatment of a wide variety of ailments, which ranged from digestive disorders to worm infestation, kidney problems, toothache, constipation, dropsy and plaque (Rivlin, 2001) If the modern herbalists books are consulted fresh garlic is still prescribed for the treatment of many diseases including the prevention of heart and circulatory problems (Grieve, 1998).
Garlic has received renewed prominence recently because of the popularity of the Mediterranean diet, which includes the extensive use of fresh and cooked garlic. The Mediterranean diet is thought to reduce the risk of cardiovascular disease through a reduction in cholesterol and its consequent reduction in blood pressure. There is extensive Web page documentation in for example and of the benefits of the diet and in supporting clinical trials (de Lorgeril, 1996; Laino 2003) but the exact cardioprotective mechanism of the diet is unknown. Since the Mediterranean diet includes a wide array of vegetables fruit, fish and oil, the protective effect is likely to be multifactorial. Garlic is an essential component of the diet and is thought to make an important contribution to its medical benefits.
The current enthusiasm for herbalists remedies and healthy diets has led to detailed studies of the chemical composition of garlic and the medical importance of the chemicals it contains. The commercial direction is now away from the fresh plant and into a variety of garlic extracts. The long term aim of the supplement industry is either to identify an active single compound in the extract which can then be used as a new pharmaceutical or to use the complete plant extract in concentrated form as a food supplement. It is assumed that since garlic has been prescribed for many centuries, there is no or less need for the supplements to undergo the rigorous (and very expensive) testing that is required for the introduction and acceptance of a new pharmaceutical. Garlic supplements and extracts are advertised extensively on the Web where the accompanying text claims that regular consumption of the supplement could delay the onset of cardiovascular disease. However the supplement industry is now under increasing scrutiny from external regulators to justify their claims, and is looking towards the scientists to provide medical evidence that garlic can in fact prevent the early onset of specific diseases. There have been a large number of studies recently in which the effect of garlic on specific stages of cardiovascular disease has been explored. As an indication of the interest in the subject there has also been a number of reviews (Ernst,1987; Alexander et al.,1996; Bannerjee and Maulik, 2002; Rahman,2003) describing the relationship between garlic and cardiovascular disease.
26.2. Composition of raw and cooked garlic
Raw garlic is eaten in small amounts largely as a flavouring, and while popular especially in the Mediterranean diet, it does lead to a strong odour on the breath. The flavour and pungency is a result of a high concentration of sulphur related compounds in the cloves which amount to 1.0% of their dry weight. In the undamaged clove over 70% of the sulphur compounds exists as (+)-S-allyl-L-cysteine sulphoxide or alliin, (+)-S-trans-1-propenyl-L-cysteine sulphoxide or isoalliin and S–methylcysteine sulphoxide or methiin and γ glutamyl peptides such as γ glutamyl-S-allylcysteine and γ-glutamyl-S-trans-1-propenyl cysteine. The sulphur is divided approximately 50% between the cysteine sulphoxides and the γ glutamyl peptides (Mütsch-Eckner et al., 1992). In the plant the function of the cysteine sulphoxides is probably one of protection since their breakdown products appear to have antibiotic and fungicidal propertie (which also contributes to their medicinal value), while the γ glutamyl peptides appear to have a storage function for N and S. The major alkylcysteine sulphoxide in garlic is alliin (85%), with isoalliin (5%) and methin (10%) occupying much more minor roles (Lawson, 1996).
Garlic cloves have a slight smell when intact but on cutting or maceration a strong smell is released rapidly. The volatile is only released on damage because there are different parts to the flavour mechanism, a degradative enzyme, alliinase, and its substrates, the alkylcysteine sulphoxides, which are stored in separate compartments in the plant. Unlike the onion where there is an intracellular separation of the alliinase and the alkylcysteine sulphoxides (Lancaster and Collin, 1981), in the garlic there is a spacial separation based on location within different tissues of the clove (Ellmore and Feldberg, 1994; Wen et al.,1995). The amounts of alliinase in the garlic are unusually large amounting to at least 10% of the total protein in the clove and would explain why there is such a rapid release of flavour and complete breakdown of the flavour precursors upon cutting the tissue.
The chemistry of the Alliums is interesting since from a small number of original flavour precursors a large number of compounds are produced many of which have medicinal properties (Block, 1985, Lawson 1996). In the alliinase catalysed reaction the first products of the reaction are the sulphenic acids which have a very short half life before being condensed to form thiosulphinates. From the three original sulphoxides, eight or nine possible thiosulphinates are formed. The major one derived from alliin is diallyl thiosulphinate, or allicin, which represents about 70% of the total thiosulphinates. Isoalliin does not form the lachrymatoury compound found in onion but is rapidly converted into allyl 1-propenyl thiosulphinate and the methiin finally degrades to largely allyl methanethiosulphinate. The thiosulphinates are colourless liquids with a pungent smell reminiscent of fresh cut garlic of which allicin is the main contributor to the odour. The allicin yield is important since it is used as a measure of garlic quality in commercial preparations. It appears to be relatively stable especially in water but in homogenates it is less stable which is a major consideration when assessing the effectiveness of different garlic preparations.
The thiosulphinates are described as unstable and reactive and convert readily to more stable compounds which contain the thioallyl (S-allyl) or thiomethyl group. In crushed garlic at room temperature the reactive thiosulphinates are transformed to diallyl sulphide, diallyl disulphide and allyl methyl trisulphide, which is also the reaction that occurs in the mouth. Invitro studies of the thiosulphinates, allicin and methyl and propyl thiosulphinates show that they are capable of combining with the sulphydryl group of the amino acid cysteine to form S-allyl mercaptocysteine, S-methyl mercaptocysteine and S-propyl mercaptocysteine thereby inhibiting the activity of enzymes which contain cysteine as the active site. This ability to react with the sulphydryl group of acetyl–CoA SH the building block of cholesterol and triglyceride synthesis may help to explain its biological effect (Wills, 1956). These reactions may have some significance in the intestinal tract during digestion as they may determine the form in which the breakdown products of garlic enter the blood stream. There is no conclusive data that clearly identifies the main metabolites in the blood stream after garlic consumption (Amagase et al., 2001). Evidence has shown that allicin, which is an unstable product of alliin breakdown and could be present in the digestive system after consumption of garlic or garlic supplements, is rapidly converted to diallyl disulphide in the blood ( Freeman and Kodera, 1995). This compound was found in micromolar concentrations in the rat plasma and liver tissues after oral administration of diallyl disulphide implicating it as a major garlic derived metabolite in the body (Germain, 2002) but it does not seem to remain in the body for long after consumption of raw garlic (Lawson et al 1992). The water soluble compound S-allylcysteine itself a breakdown product of garlic does seem to be a stable residue in the blood following oral consumption (Nagae et al., 1994).
Garlic is more normally cooked so the composition will differ from the raw garlic. Heat inactivates the alliinase and therefore inhibits the formation of allicin and other thiosulphinates but some breakdown of the alliin occurs leading to the accumulation of small amounts of diallyl trisulphide, diallyltrisulphide, di and tetrasulphides. In crushed garlic the cysteine sulphoxides are largely converted to the thiosulphinates (Lawson, 1996). After boiling in a closed container there was complete conversion of the thiosulphinates to sulphides, whereas in an open container over 90% of the sulphides were lost. After cooking in hot fat, most of the alliicin was lost but a much higher proportion of the sulphides remained. In an analysis of the volatiles released after standard periods of cooking such as oil frying, baking and microwaving the dominant volatile was diallyl disulphide (Yu, 1993)
26.3. Commercial sources of garlic supplements
The commercial forms of garlic that are sold as a supplements include powdered dried garlic cloves, oils produced upon treating chopped garlic with steam, vegetable oil or ether, and aged extracts of chopped garlic in ethanol or water as well as garlic cloves pickled in vinegar. An important aspect of the supplements is that the composition of the product will vary according to variety, the year of harvest and on the processing method. The preparation and composition of the major commercial products are described below.
26.3.1.Garlic powder
Garlic powder such as Kwai is prepared by peeling the cloves, oven drying at 50-60o C then grinding them to a powder. Some conversion of the flavour precursors by alliinase does occur on cutting but the loss is minimised by reducing the amount of cutting before drying. Alliinase retains its activity since the powder is very stable and will only lose about 10% of its allicin yield after 5 years of storage. However it is important that the powder is stored at no more than 4-6% water content to prevent the alliinase being activated. The powder can be stored as tablets when again the alliicin potential is also very stable but with an average loss of 36% alliicin over 5 years is more variable than the powder. Garlic salt products generally have a lower yield of alliicin than quality garlic, which probably reflect the greater degree of chopping prior to dehydration (Lawson and Hughes, 1992).
In order to standardise the various dried products, the quality is measured by the level of allicin. The values of some brands are 4mg/g which compares favourably with the levels in fresh garlic. An alternative measure is the ability to form allicin from alliin (called the alliicin potential), which depends on the presence of active alliinase in the tablet. Alliinase is inactivated by the acid conditions of the stomach. In order to provide the body with the pharmacologically active forms in the garlic, the activity of the alliinase must be maintained until the pills reach the non-acid environment of the intestine. The pills are protected from the stomach acid by being coated with cellulose esters which require the presence of intestinal enzymes for the coating to be removed. This has given rise to another measure, the effective allicin yield, which is obtained by using simulated gastro intestinal conditions ie 1 hour in simulated gastric conditions and two hours in simulated intestinal conditions (Lawson and Hughes, 1992). Those brands with an effective enteric coating were those with the largest release after one hour. Other factors determining the amount of alliicin released will depend on whether the tablets are consumed with or without food. A meal with a high protein content will see the pH of the stomach rise from 1.5 to 4.0 which will be less inhibitory to any alliinase activity. It is recommended that the tablets are taken with or just after a meal when the pH is higher and the alliinase inactivating ability is lowered (Blania and Spangenberg, 1991).
The other factor determining the release of allicin is the ability of the alliinase to convert all the alliin to alliicin. This is measured by the ratio of alliicin to alliin. With a 100% conversion this should appear as 0.46 whereas the range normally measures from 0.32-0.42 because allyl methyl and allyl 1-proenyl thiosulphinates are also formed. Ratios below this figure indicate a damaged alliinase. The usual reason for a reduced figure is due to excessive drying of the cloves at a high temperature which inactivates the alliinase. Significant amounts of γ glutamylcysteines are also present in the powders and tablets (Block, 1995).
26.3.2. Oil of steam distilled garlic
Steam distilled garlic contains exclusively about 98% allylmethyl and 1-propenyl mono- and polysulphides. The commercial product is diluted by vegetable oil so that the final composition of sulphides represents about the same amount of allicin and other thiosulphinates present in a similar weight of crushed garlic. This dilution stabilizes the polysulphides and decreases the extremely strong odour of the undiluted oil. The major compounds are the diallyl di, tri and tetra sulphides and the allyl methyl di ,tri and tetra sulphides. The total content of different brands does vary as a result of the different levels of dilution whereas the percentage composition does not. The composition is also relatively stable over time as a 5 year study showed virtually no change in the amount of the main allyl sulphides (Lawson, 1996).
26.3.3. Oil of macerated garlic
Garlic is macerated in a vegetable oil then the oil filtered or the crushed garlic is left suspended in the oil. The product contains compounds not found in the fresh or powdered garlic ie the ring structured vinyldithiins and the oxygenated ajoenes. The composition seems not to vary but the total amount of sulphur compounds is dependent on the quality of the original garlic and the amount of dilution by the oil. If the garlic bulb is homogenized with an equal weight of oil, the highest amount of transformation products is about 3-5mg/g. The yields rarely reach this value but can be increased by crushing the cloves first to produce the thiosulphinates more efficiently before addition of the oil. The oil based compounds appear to be stable since vinyldithiins were stable over a 5 year period but the ajoenes much less so even when enclosed in a gelatine capsule. It is recommended that oil macerate products should be kept refrigerated to retain their ajoene concentration for more than 18 months (Iberl 1990
26.3.4. Garlic aged in dilute ethanol
Chopped garlic is incubated in 15-20% ethanol for up to 20 months at ambient temperatures as in AGE (Lawson and Wang, 1995). The incubation medium is then filtered and evaporated to dryness and sold in dry form as tablets or powder or liquid forms. Analysis showed that allicin and other thiosulphinates were nearly depleted by 90 days having been transformed into diallyl and allyl methyl tri-,di- and tetrasulphides most of which was then lost to the atmosphere. There are considerable amounts of alliin in the extract, which had diffused out whereas the alliinase had remained in the cells. Under these condititions the γ-glutamyl-S-allylcysteine and the γ-glutamyl-S-1-propenylcysteine were converted to S-allylcysteinne and S-1-propenyl cysteine under the action of an enzyme γ-glutamyl peptidase. The quantity of these products will depend on the content of the original γ-glutamyl cysteine sulphoxides. The range in different sources varied from 1.6-6.8 mg/g fresh weight which in turn produces an amount of S-allylcysteine content of 2.7-11.3 mg/g dry weight which represents the approximate amount that should be found in commercial products. The amount of S-allylcysteine that is found in commercial products is much smaller than this figure. The alliin content of commercial aged extracts varies from 0.02-0.32 mg/g dry or fresh weight and only trace levels of the allyl sulphides have been found in commercial extracts. The total sulphur content of the liquid form of the commercial aged extract such as AGE was found to be 0.091% in contrast to the 0.35% typically found in garlic cloves.