Nutrition (28 page)
The omega-3 fatty acids found in fish oils are the long-chain docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA), which are derived from the microalgae on which the fish feed. These omega-3 fatty acids are used in the body to make substances (series 3 prostaglandins and series 5 leukotrienes) that have an anti-inflammatory action, and help to protect against inflammatory conditions such as arthritis, asthma, eczema and coronary heart disease which is linked with low-grade inflammation in the artery walls.
Fish oils and heart disease
Research has shown that omega-3 fish oils help to reduce blood pressure, to lower raised circulating triglyceride levels, to prevent abnormal blood clotting and protect against abnormal heart rhythms (especially in heart muscle receiving a poor blood supply). As a result, even modest increases in dietary intakes of oily fish can help to prevent a fatal heart attack. In those who have already had a heart attack, increasing consumption of fish significantly reduces the chance of a second heart attack and, if one does occur, the chances of dying from this second heart attack. An intake of at least 1 g omega-3 fish oils per day from eating oily fish twice a week or from pharmaceutical-grade supplements has consistently been shown to reduce the risk of sudden cardiac death by 40 to 45 per cent. As a result, both the American Heart Association and the European Society of Cardiology recommend a daily intake of 1 g long-chain, omega-3 fish oils as a preventive measure in those at risk of coronary heart disease. In fact, eating oily fish at least twice a week can lower your risk of a fatal heart attack more than following a low-fat, high-fibre diet. Although it is worth noting that the protective effects of oily fish are seen after only six months. After two years, those on a high fish diet are almost a third less likely to die from coronary heart disease than those eating the least amount of fish.
Fish oils and stroke
People who regularly eat oily fish are less likely to die from stroke than those who do not. A meta-analysis of data from six studies suggests that eating any fish on a weekly basis reduces the risk of stroke by 12 per cent, with each additional portion reducing the risk by another 2 per cent per serving per week.
Fish oils and arthritis
For those with arthritic joint pains, omega-3 fish oils lower the level of inflammatory chemicals and thus reduce pain in a similar way to that of non-steroidal anti-inflammatory drugs, as they inhibit inflammatory enzymes (COX-1 and COX-2) to reduce joint pain and swelling. Consequently, a number of studies show that taking fish-oil supplements can reduce the need for taking non-steroidal anti-inflammatory drug (NSAID) painkillers. High intakes of 3 g to 6 g EPA and DHA are needed per day, however, for a good anti-inflammatory effect in joints.
To obtain this level of intake from eating fish alone is an unrealistic target. The average adult following a Western-style diet only eats one-third of a portion of oily fish per week, and two out of three adults eat no oily fish at all. Average intakes of the long-chain omega-3s EPA and DHA therefore average just 1 g per week. In addition, some government agencies suggest limiting intakes of deep-sea fish to reduce exposure to marine pollutants such as mercury (linked to impaired neurological development), dioxins and PCBs, both of which are linked to reduced immunity, reduced fertility and some cancers.
LIMITING INTAKES
In the UK, there are now limits on the maximum recommended intake of oily fish, and some white fish and crabmeat.
Oily fish
Although you should eat at least one portion (140 g) of oily fish a week, boys, men and women past reproductive age should eat no more than four portions of oily fish per week to reduce exposure to marine pollutants such as mercury, dioxins and PCBs (polychlorinated biphenyls) that can build up in the body. Girls and women of childbearing age should eat no more than two portions of oily fish a week as these marine pollutants may affect the development of a baby in the future.
Although you should eat at least one portion (140 g) of oily fish a week, boys, men and women past reproductive age should eat no more than four portions of oily fish per week to reduce exposure to marine pollutants such as mercury, dioxins and PCBs (polychlorinated biphenyls) that can build up in the body. Girls and women of childbearing age should eat no more than two portions of oily fish a week as these marine pollutants may affect the development of a baby in the future.
Canned tuna
If you are trying for a baby, or are pregnant, do not have more than four cans of tuna (140 g drained weight each) a week because of its mercury content. There is no limit for breastfeeding. Canned tuna is not classed as an oily fish, so you can eat this in addition to your maximum two portions of oily fish per week (as long as you don’t have fresh tuna).
If you are trying for a baby, or are pregnant, do not have more than four cans of tuna (140 g drained weight each) a week because of its mercury content. There is no limit for breastfeeding. Canned tuna is not classed as an oily fish, so you can eat this in addition to your maximum two portions of oily fish per week (as long as you don’t have fresh tuna).
White fish
Children, pregnant women and women who are trying to become pregnant should also avoid shark, swordfish and marlin. Other adults should have no more than one portion of shark, swordfish or marlin per week. This is because they can contain more mercury than other fish.
Children, pregnant women and women who are trying to become pregnant should also avoid shark, swordfish and marlin. Other adults should have no more than one portion of shark, swordfish or marlin per week. This is because they can contain more mercury than other fish.
Other white fish that may contain similar levels of certain pollutants as oily fish are sea bream; sea bass; turbot; halibut; rock salmon (also known as dogfish, flake, huss, rigg or rock eel).
Anyone who regularly eats a lot of fish should avoid eating these five fish but may otherwise safely eat as many portions of white fish per week as they like, and ideally should aim for at least two.
Crab
Avoid eating the brown meat from crabs too often. There is no need to limit the amount of white crabmeat that you eat.
Avoid eating the brown meat from crabs too often. There is no need to limit the amount of white crabmeat that you eat.
Omega-3 fish-oil supplements are an effective way to increase your intakes of EPA and DHA as they are screened to ensure low levels of pollutants. If you eat no oily fish, six high-strength capsules per day are needed to provide 3 g omega-3s, the ideal amount for a noticeable anti-inflammatory effect in those with painful, osteoarthritic joints. If you eat two portions of an oily fish such as salmon per week, however, you would only need four high-strength capsules per day to obtain the same level of intake. If you do not have painful joints, but wish to obtain the cardioprotective effects of fish oils, then a 1 g high-strength capsule per day is sufficient.
FISH-OIL SUPPLEMENTS
Standard fish-oil capsules typically provide 180 mg EPA and 120 mg DHA per 1 g capsule (a ratio of 3:2).
High-strength oils contain around 310 mg EPA and 210 mg DHA.
Cod liver oil naturally contains 170 mg total EPA plus DHA per 1 g capsule.
Select a pharmaceutical-grade fish-oil supplement to ensure they are free from marine pollutants. Those offered in the triglyceride (TG) form are most easily absorbed and used in the body.
If choosing to take cod liver oil, those described as high- or extra-high strength provide the highest amount of omega-3 fatty acids. If taking a multivitamin as well, check that the total amounts of vitamin A and D do not exceed recommended doses – vitamin A is best limited to less than 1,500 mcg per day; vitamin D’s upper safe level is 25 mcg daily.
NB Women who are pregnant should avoid cod liver oil supplements (due to their high vitamin A content) and select DHA-rich oils especially designed for pregnancy.
Fish oils and depression
DHA plays an important structural role within brain-cell membranes, improving their fluidity so that messages are passed on more rapidly from one cell to another. EPA is involved in cell signalling and also improves communication between brain cells. Studies suggest that populations who eat fish infrequently have higher levels of depression compared with those who eat fish regularly, and adding fish oils (2 g per day) to the usual drug treatment for depression has been shown to improve symptoms within two weeks, compared with placebo.
A recent meta-analysis of 15 randomized placebo-controlled trials involving 916 people, published in the
Journal of Clinical Psychiatry,
found that EPA-rich fish-oil supplements were effective against primary depression.
Journal of Clinical Psychiatry,
found that EPA-rich fish-oil supplements were effective against primary depression.
There is some evidence that fish oils may help to improve painful periods, dyslexia and protect against age-related macular degeneration. In children with attention-deficit hyperactivity disorders, fish oils may help to improve cognitive scores and general behaviour, although this is controversial.
FORTIFIED OMEGA-3S
Healthy staple foods such as margarines, bread and even milk are often fortified with omega-3 oils to help boost intakes in those who do not eat much fish. However, few of these are fortified with the long-chain omega-3s (EPA and DHA). Many of these products contain only tiny amounts of added omega-3s, usually of the short-chain variety (ALA) derived from vegetable oils. Less than 5 per cent of these are converted on to EPA in the body, and less than 0.5 per cent are transformed into DHA.
Fish oils and diabetes
In the early days of fish-oil research, it was suggested that they may increase blood sugar levels in people with Type 2 diabetes. However, several large-scale analyses have now concluded that taking a fish-oil supplement has no real effect on glucose control in this group. On the contrary, studies now show that omega-3 fish oils can boost glucose tolerance as well as improving other heart-disease risk factors associated with diabetes. In fact, some studies suggest that increased dietary intake of omega-3 fish oils with reduced intake of saturated fat may lower the risk of someone with impaired glucose tolerance progressing to Type 2 diabetes. If you have diabetes, and choose to take any food supplements, including fish oils, always monitor your glucose levels closely.
In summary
Most people would benefit from increasing their intake of long-chain omega-3s and cutting back on their intake of omega-6s to obtain a better balance of polyunsaturated fats.
Aim to eat more omega-3 rich oily fish (two to four portions per week) such as mackerel, herring, salmon, trout, sardines, pilchards, fresh tuna (not tinned), wild game meat (such as venison and buffalo), grass-fed beef and omega-3 enriched eggs. At the same time, cut out excess omega-6s by consuming less omega-6 vegetable oils (safflower oil, grapeseed oil, sunflower oil, corn oil, cottonseed oil or soybean oil) and replace with healthier oils such as rapeseed, olive, walnut or macadamia oils. Other omega-6 rich foods to avoid (or at least consume only in moderation) are margarines based on omega-6 oils such as sunflower or safflower oil, convenience foods, fast foods and manufactured goods such as cakes, sweets and pastries.
Meat
Meat is an important source of protein as it supplies all the essential and non-essential amino acids. It is also a good source of iron. As explained in
Chapter 2
, although meat fat is usually classified as ‘bad’ saturated fat, it is also an excellent source of healthy monounsaturated fat, which it often supplies more than any other source.
Table 19
shows the relative amounts of saturated and monounsaturated fat found in lean, raw cuts of various meats. Meat is also a source of dietary, preformed cholesterol.
Chapter 2
, although meat fat is usually classified as ‘bad’ saturated fat, it is also an excellent source of healthy monounsaturated fat, which it often supplies more than any other source.
Table 19
shows the relative amounts of saturated and monounsaturated fat found in lean, raw cuts of various meats. Meat is also a source of dietary, preformed cholesterol.
Meat (raw, lean) | Total fat per 100 g | % saturated | % Mono-unsaturated | Omega-3s per 100 g | Cholesterol |
Beef | 4.3 g | 39% | 44% | 20 mg | 58 mg |
Pork | 4 g | 35% | 38% | 20 mg | 63 mg |
Lamb | 8 g | 44% | 39% | 78 mg | 74 mg |
Venison | 7.1 g | 48% | 18% | 104 mg | 80 mg |
Rabbit | 5.5 g | 38% | 24% | 90 mg | 53 mg |
Duck | 6.5 g | 31% | 49% | 80 mg | 110 mg |
Chicken white meat | 1.1 g | 27% | 45% | 40 mg | 70 mg |
Chicken dark meat | 2.8 g | 29% | 46% | 110 mg | 105 mg |
Turkey light meat | 0.8 g | 38% | 38% | 40 mg | 57 mg |
Turkey dark meat | 2.5 g | 32% | 40% | 90 mg | 86 mg |
White meats such as chicken and turkey tend to have a lower fat content than red meats, but sizeable amounts of fat (and calories) can be avoided by trimming meats of all visible fat before cooking. As most of the fat in poultry is in the skin, this is also easily removed either before or after cooking.
Meat is also an important source of micronutrients, especially iron, zinc, B vitamins, vitamin D and selenium, as well as supplying useful amounts of magnesium, copper, cobalt, phosphorus and chromium.
There are concerns that cooking meat in a way that crisps it (frying, chargrilling and barbecuing), and consuming processed meats, may increase your exposure to chemicals (N-nitroso-compounds) that are linked with bowel cancer. However, studies are inconclusive and these findings may have more to do with high-fat diets and the cooking methods (overcooking or charring meat) rather than the meat itself. Even so, it is generally recommended that you limit your intake of red meat to no more than 500 g cooked red meat per week (equivalent to 70 g, or half a small beefsteak every day). Average intakes in the UK are around 96 g red meat per day for men, and 57 g red meat for women.
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