Nutrition (19 page)
The LAST study (Lutein Antioxidant Supplementation Trial) showed that macular pigment optical density increased over time in those taking 10 mg lutein supplements daily for one year, and declined slightly without supplementation. Visual acuity also improved in those with ‘dry’ AMD by the equivalent of 5.4 letters on a Snellen chart with improved contrast sensitivity. No improvement occurred in those taking inactive placebo. In the follow-up LAST2 study, individuals with the lowest macular pigment optical density, and in greatest need of supplements, were most likely to benefit from either a 10 mg lutein supplement or a lutein plus antioxidant supplement. For those responding, macular pigment optical density continued to increase for at least 12 months, with the benefits still continuing when the trial ended. Diet should always come first, of course, but where people are unwilling, or unable, to eat more lutein-rich fruit and vegetables, there is a strong argument for proposing a lutein supplement as a safety net to augment the diet. The potential benefits for eye health greatly outweigh any risks, which are negligible.
Phytosterols
Phytosterols (also known as plant sterols) such as sitosterol, stigmasterol and campesterol are the third main category of phytochemicals as mentioned at the beginning of this chapter. They have a similar structure to the cholesterol found in animals and, as a result, can block cholesterol absorption from the intestines although they are not much absorbed themselves. As well as reducing uptake of the cholesterol you eat in animal-based foods, they also block absorption of the cholesterol made in your liver, which is squirted into your gut within bile. As the blocked cholesterol is voided from your bowels (along with most of the plant sterols), your circulating cholesterol levels are reduced if your diet contains these beneficial phytochemicals.
A large trial involving over 22,500 men and women living in Norfolk in the UK showed that people with the highest dietary intake of plant sterols have the lowest cholesterol levels. Another study showed that a sterol-rich diet can reduce levels of harmful LDL-cholesterol by around 15 per cent – enough to lower considerably your risk of a heart attack or stroke. Plant sterols work in a different way to the statin drugs prescribed to lower elevated cholesterol levels and which have a direct effect on the liver. The two can therefore be used together for a synergistic effect. Research from the Mayo Clinic demonstrates that adding plant sterols to statin medication has been shown to reduce cholesterol levels even more than by doubling the statin dose.
The richest sources of plant sterols are vegetable oils, wholegrains and nuts. The average diet provides around 400 mg sterols per day, although some vegetarians may obtain three times as much. For optimum cholesterol-lowering benefits, however, an intake of 2 g per day is ideal. Higher doses do not appear to enhance their effectiveness, and may reduce absorption of some dietary carotenoids (although this is easily overcome by eating an additional serving of a carotenoid-rich fruit or vegetable per day – something we should be aiming to do anyway!).
While diet should always come first, it is difficult to obtain optimum amounts of sterols from normal food sources alone, especially as the phytosterols within plants are naturally bound to fibre, which limits their action. Fortified products (yogurt, spreads, milk and dairy-free drinks) have therefore been developed to augment dietary intakes.
Sulphur-containing compounds
Sulphur-containing plant chemicals are the fourth main category of phytochemicals to consider. We mainly obtain these in the form of sulphides (from the onion or Allium family of plants, which includes garlic, shallots, leeks and chives) and glucosinolates (from the Brassica or cabbage family).
Sulphides
Garlic is a source of the powerful sulphur-containing antioxidant allicin, whose full chemical name is diallyl thiosulphinate. Allicin is formed from an odourless precursor, alliin, which is an amino acid unique to the garlic family. Alliin is stored within garlic cells, separated from the enzyme, alliinase, that breaks it down. It is only when garlic is crushed or cut that alliin and alliinase come together to produce allicin with its characteristic odour. Fresh garlic can contain up to 4 g of alliin per kilogram, but if garlic is cooked immediately after peeling, alliinase is inactivated and some of its benefits are lost. Thus the best way to preserve the maximum benefit from garlic cloves in your cooking is to add them right at the end, rather than at the beginning as is more usual. This will increase the pungency of any odours you give off afterwards, however – great for keeping away vampires, not so good when it comes to attracting new friends.
Garlic and cholesterol
Some, but not all studies have found that garlic tablets can lower circulating cholesterol levels with proposed mechanisms including reduced cholesterol production in the liver and increased excretion of fatty acids. Discrepancies may relate to differences in the garlic preparations used, the dose and the duration of the trial. A meta-analysis of data from 29 trials, published in 2009, found that garlic did reduce total cholesterol levels by 19 per cent and triglycerides by 11 per cent, although there were no relative changes in LDL- or HDL-cholesterol balance.
Garlic and blood pressure
Sulphur compounds formed from the breakdown of allicin have also been found to react with red blood cells to produce hydrogen sulphide, which relaxes blood vessels. This can promote circulation through small arteries (arterioles) and small veins (venules) as well as lowering blood pressure in those with an elevated systolic blood pressure. A meta-analysis of data from ten trials found that, compared with inactive placebo, garlic extracts reduced blood pressure by 16.3/9.3 mmHg in people with hypertension, which is better than is achieved with many prescribed drugs! Interestingly, it did not markedly affect blood pressure in those without hypertension (which is important to reduce potential side effects such as faintness), and research is currently under way to work out why.
Garlic and cancer
The strongest evidence for benefit from garlic constituents is in the area of cancer protection. The sulphurous compounds found in garlic have been shown to suppress the formation of carcinogenic substances (nitrosamines) in the body, and to arrest the growth of human cancer cells either through effects on the cell life cycle or by a direct interaction with DNA. Although most of these effects were observed at concentrations higher than those encountered in the normal diet, a recent meta-analysis of 21 studies involving over 543,000 people suggested that those eating the largest amount of Allium vegetables were 46 per cent less likely to develop stomach cancer than those with the lowest intake. There was a definite dose-response effect, so that every 20 g of Allium vegetables consumed daily reduced the risk of gastric cancer by 9 per cent. However, other dietary factors not accounted for may have contributed to these effects and, for now, this remains an interesting finding that should encourage you to consume more French onion soup and to add as much garlic to your cooking as you can tolerate (but remember to put it in just at the end).
Glucosinolates
Glucosinolates are sulphur-containing compounds, derived from glucose and an amino acid, found mainly in brassica vegetables such as mustard, horseradish, Brussels sprouts, radish, turnip and cauliflower. When these plants are crushed, chopped or chewed, the glucosinolates are broken down by an enzyme (myrosinase) to release a number of hot or bitter products that discourage animals from eating them – though humans often find them tasty! These substances include isothiocyanates (mustard, horseradish), bitter sinigrin (Brussels sprouts, cauliflower), thiocyanates and indoles. A study of intakes in Germany found that average consumptions of glucosinolates were around 14.5 mg per day.
Microbiological studies show that these glucosinolates have a notable antibacterial action and might be useful in reducing our susceptibility to intestinal infections.
Their most useful action, however, is in protecting against cancer. Those with the highest intake of glucosinolates – especially sulforaphane found in broccoli – have been shown, in epidemiological studies, to have a lower risk of developing cancers of the bowel (colon and rectum).
Currently there are no recommended dietary intakes for most phytochemicals except sterols for lowering cholesterol levels. However, eating a wide variety of fruit and vegetables as part of your five-a-day (minimum, preferably eight to ten a day) servings will provide good intakes. These phytochemicals are one of the key contributors to findings that people who eat the most fruit and vegetables are the least likely to develop cancer. A review of over 200 clinical studies found a consistent protective effect of fruit and vegetables against cancers of the stomach, oesophagus, lung, mouth and throat, uterus, pancreas and colon. The greatest protection came from eating raw vegetables, onions, garlic, carrots, green vegetables, members of the cabbage family and tomatoes – all of which are rich sources of phytochemicals, as discussed above. Similarly, a meta-analysis of eight studies involving over 257,000 people, published in
The Lancet,
found that eating three to five servings of fruit and vegetables per day reduced the risk of stroke by 11 per cent, compared with those eating fewer than three a day. However, those eating more than five a day had a 26 per cent lower risk. Overall, it seems that each additional portion of fruit you eat per day reduces your risk of stroke by 11 per cent, while each additional portion of vegetables reduces your risk by 3 per cent. And when it comes to heart disease, the results from studies involving over 278,000 people found that eating three to five servings of fruit and vegetables per day reduced the risk of coronary heart disease by 7 per cent compared to eating fewer than three servings a day. Those who ate more than five servings per day, however, enjoyed a 17 per cent lower risk.
The Lancet,
found that eating three to five servings of fruit and vegetables per day reduced the risk of stroke by 11 per cent, compared with those eating fewer than three a day. However, those eating more than five a day had a 26 per cent lower risk. Overall, it seems that each additional portion of fruit you eat per day reduces your risk of stroke by 11 per cent, while each additional portion of vegetables reduces your risk by 3 per cent. And when it comes to heart disease, the results from studies involving over 278,000 people found that eating three to five servings of fruit and vegetables per day reduced the risk of coronary heart disease by 7 per cent compared to eating fewer than three servings a day. Those who ate more than five servings per day, however, enjoyed a 17 per cent lower risk.
The evidence is strong that, although eating five servings of fruit and vegetables per day is good, eating more than five is considerably better.
Probiotics
The term
probiotic
literally means ‘for life’ and is used to describe the ‘friendly’ bacteria and yeasts found in some fermented foods such as live ‘bio’ yogurts, fermented milk drinks and sauerkraut. According to the World Health Organization definition, probiotics are ‘live microorganisms which, when administered in adequate amounts, confer a health benefit on the host’.
probiotic
literally means ‘for life’ and is used to describe the ‘friendly’ bacteria and yeasts found in some fermented foods such as live ‘bio’ yogurts, fermented milk drinks and sauerkraut. According to the World Health Organization definition, probiotics are ‘live microorganisms which, when administered in adequate amounts, confer a health benefit on the host’.
The most common probiotics are lactic-acid bacteria such as
Lactobacillus acidophilus, Bifidobacterium longum
and related strains.
Because these are acid-tolerant, a considerable number survive passage through the stomach and small intestines to reach your large bowel. As you open your bowels regularly, the most beneficial probiotic bacteria are those that attach to landing sites on the intestinal wall where they set up colonies to replenish their numbers.
Lactobacillus acidophilus, Bifidobacterium longum
and related strains.
Because these are acid-tolerant, a considerable number survive passage through the stomach and small intestines to reach your large bowel. As you open your bowels regularly, the most beneficial probiotic bacteria are those that attach to landing sites on the intestinal wall where they set up colonies to replenish their numbers.
Altogether, your gut contains around 11 trillion bacteria – more than the number of human cells in your body. Together, these bacteria weigh an astonishing 1.5 kg and provide a major contribution to the bulk of bowel motions. Every gram of fibre you eat, for example, increases the weight of faeces by 5 g as fibre fuels the growth of these bowel bacteria.
Ideally, at least 70 per cent of ‘your’ bacteria should be healthy probiotic bacteria, and only 30 per cent should be other bowel commensals (bacteria that live happily on or in another organism without causing harm) such as
E. coli.
In practice, however, the balance is usually the other way round.
E. coli.
In practice, however, the balance is usually the other way round.
Probiotic bacteria play an important role in maintaining digestive health by:
• producing lactic and acetic acids to discourage growth of other potentially harmful organisms
• secreting natural antibiotics known as bacteriocins to help discourage overgrowth of other less beneficial intestinal bacteria
• competing with harmful bacteria and yeasts for available nutrients
• competing with other bacteria for attachment sites on intestinal cell walls – literally crowding them out so they pass through the intestines without gaining a foothold
• stimulating your production of natural antiviral substances such as interferons
• helping to stimulate your immune defences against bacteria in general. A large part of your immune defences are present within the wall of the ileum in the small intestines, where they form pale areas known as Peyer’s patches (see
Chapter 1
).
Chapter 1
).
As discussed above, probiotic bacteria also metabolize some of the phytoestrogens in your diet to release the more active plant oestrogen called equol, which has an oestrogen-like action. In addition, the probiotic bacteria (which produce lactic acid) produce short-chain fatty acids that act as a major energy source for your intestinal lining cells. These beneficial short-chain fatty acids are also absorbed from the colon and transported to your liver, where they have a positive effect on your cholesterol metabolism.
Lactic-acid bacteria can inhibit the growth of harmful bacteria that cause gastroenteritis such as
Bacillus cereus, Salmonella typhi, Shigella dysenteriae, Escherichia coli, Staphylococcus aureus
and
Clostridium difficile.
They can also reduce diarrhoea caused by taking antibiotics that lower the level of natural lactic-acid bacteria within the intestines. And, as alteration in normal bowel bacterial balance has been linked with irritable bowel syndrome (IBS), repopulating the bowel with lactic-acid bacteria can improve associated symptoms such as bloating, discomfort, constipation and diarrhoea. This is thought to result from their ability to reduce the presence of gas-producing enterobacteria that are associated with IBS symptoms.
Bacillus cereus, Salmonella typhi, Shigella dysenteriae, Escherichia coli, Staphylococcus aureus
and
Clostridium difficile.
They can also reduce diarrhoea caused by taking antibiotics that lower the level of natural lactic-acid bacteria within the intestines. And, as alteration in normal bowel bacterial balance has been linked with irritable bowel syndrome (IBS), repopulating the bowel with lactic-acid bacteria can improve associated symptoms such as bloating, discomfort, constipation and diarrhoea. This is thought to result from their ability to reduce the presence of gas-producing enterobacteria that are associated with IBS symptoms.
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