Nutrition (6 page)
Essential fatty acids
You cannot make the short-chain omega-6, linoleic acid, or the short-chain omega-3, alpha-linolenic acid. These two fatty acids are therefore classed as ‘essential’, as they must come from your diet. Ideally, linoleic acid should supply at least 1 per cent of your energy intake, and linolenic acid at least 0.2 per cent. Once you consume these ‘parent’ polyunsaturates, however, your body can convert them on to longer-chain PUFAs (such as DHA and EPA) which are important for healthy brain and eye function. However, omega-6 fatty acids cannot be converted into omega-3 fatty acids in the body, or vice versa. Linoleic acid can only be converted into other omega-6 fatty acids, such as gamma-linolenic acid (GLA) and arachidonic acid (AA), while alpha-linolenic acid can only be converted on to other omega-3 fatty acids such as eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). The way your body makes these longer-chain polyunsaturated fatty acids involves two important metabolic pathways, as shown in
Diagram 2
.
Diagram 2
.
This method of lengthening the parent short-chain essential fatty acids (LA and ALA) as shown in
Diagram 2
is not that efficient, and the enzymes involved are readily blocked by a number of factors associated with an unhealthy diet, lifestyle and toxicity, including excess intakes of saturated fats, trans-fatty acids, sugar or alcohol; dietary lack of vitamins and minerals, especially vitamin B6, zinc and magnesium; crash-dieting; smoking cigarettes and exposure to pollution.
Diagram 2
is not that efficient, and the enzymes involved are readily blocked by a number of factors associated with an unhealthy diet, lifestyle and toxicity, including excess intakes of saturated fats, trans-fatty acids, sugar or alcohol; dietary lack of vitamins and minerals, especially vitamin B6, zinc and magnesium; crash-dieting; smoking cigarettes and exposure to pollution.
As a result, only around 5 per cent of dietary alpha-linolenic acid (ALA) is converted on to the important long-chain omega-3 polyunsaturated fatty acid EPA, and only 0.5 per cent is further converted on to form DHA. These long-chain fatty acids are therefore often classed as essential, too.
The long-chain polyunsaturated fatty acids are also important for good nutrition. They act as building blocks for making the membranes found in all body cells, including heart muscle cells, brain cells and artery lining cells. The long-chain omega-3s, DHA and EPA, are especially important for healthy brain and eye function.
A healthy adult needs to consume around 6 g to 10 g of essential fatty acids per day, in the right balance. Our ancestors evolved on a Stone Age diet supplying an omega-6 to omega-3 ratio of around 2:1. The average Western diet currently contains a ratio of omega-6 to omega-3 fats of 10:1 due to increased consumption of vegetable oils, vegetables spreads and convenience foods. We also tend to eat fewer omega-3 rich nuts and less oily fish than in the past. Most people would benefit from increasing their intake of anti-inflammatory omega-3s, and reducing their intake of inflammatory omega-6s, which have been linked with inflammatory diseases such as asthma, eczema and coronary heart disease (which is now recognized as an inflammatory process).
Good sources of omega-3 essential fatty acids include: oily fish such as mackerel, herring, salmon, trout, sardines, pilchards, fresh tuna (not tinned, see
Table 5
); wild game meat such as venison and buffalo; grass-fed beef; omega-3 enriched eggs; and omega-3 fish-oil supplements.
Table 5
); wild game meat such as venison and buffalo; grass-fed beef; omega-3 enriched eggs; and omega-3 fish-oil supplements.
Food | Portion size (grams) | Total long-chain omega-3s per portion (grams) |
Kippers | 150 g | 3.89 g |
Salmon | 150 g | 3.25 g |
Mackerel | 150 g | 2.89 g |
Pilchards (in tomato sauce) | 110 g | 2.86 g |
Herring | 150 g | 1.97 g |
Tuna (fresh) | 150 g | 1.95 g |
Trout | 150 g | 1.73 g |
Sardines (in tomato sauce) | 100 g | 1.67 g |
Salmon (canned in brine) | 100 g | 1.55 g |
Plaice | 150 g | 0.45 g |
Cod | 150 g | 0.38 g |
Haddock | 150 g | 0.24 g |
Tuna (in oil, drained) | 45 g | 0.17 g |
Tuna (in brine, drained) | 45 g | 0.08 g |
You can reduce your intake of inflammatory omega-6s by consuming fewer omega-6-rich vegetable oils such as safflower oil, grapeseed oil, sunflower oil, corn oil, cottonseed oil or soybean oil (replace with healthier oils such as rapeseed, olive or walnut oils); margarines based on omega-6 oils such as sunflower or safflower oil; convenience foods; fast foods; manufactured goods such as cakes, sweets and pastries.
Essential fatty-acid deficiency
When you do not get enough essential fatty acids (EFAs) in your diet, your body makes do with the next-best fatty acids available. It incorporates saturated fatty acids and even harmful trans-fatty acids into cell membranes. These fatty acids have a less flexible structure, however, and may affect the elasticity of your arteries, and the speed at which chemical and electrical messages are passed within your nervous system.
Lack of omega-3 essential fatty acids during infancy and childhood has been linked to an increased risk of allergies such as eczema, asthma and hay fever. It has also been associated with learning difficulties such as dyslexia. Symptoms that may be due to an essential fatty-acid deficiency are:
• excessive thirst
• frequent urination
• dry, scaly, itchy skin
• keratosis pilaris (pimply ‘goosebump’ skin on the upper arms and legs)
• dull, straw-like hair and soft or brittle nails
• impaired immunity, with frequent infections
• atopic conditions (eczema, hayfever, asthma)
• visual difficulties (poor night vision, sensitivity to bright light, visual disturbances when reading)
• learning difficulties (distractibility, poor concentration, poor working memory)
• emotional sensitivity (depression, excessive mood swings, undue anxiety)
• sleep problems (difficulty settling at night, early waking).
Trans-fatty acids
Polyunsaturated fatty acids are oils at room temperature. They can be converted into semi-solid margarines and spreads by incorporating extra hydrogen atoms into their chemical structure to convert some double bonds to single bonds. Some of these artificially produced, partially hydrogenated fats have a twisted structure and are known as trans-fatty acids.
The body processes trans-fats in a way that is harmful to health. They increase the rigidity of your cell membranes, raise blood levels of ‘bad’ LDL-cholesterol, lower ‘good’ HDL cholesterol, increase inflammation and may also affect glucose control and promote weight gain. They may also block the metabolism of essential fatty acids so that their beneficial effects are not fully realized. As if this weren’t enough, trans-fats were recently also linked with the development of certain cancers, especially of the breast and prostate gland. As a result, margarines and low-fat spreads are now being reformulated to reduce their trans-fat content. Some countries have introduced guidelines aimed at reducing intakes of trans-fatty acids to no more than 2 per cent of your total energy intake. Always check labels and select foods with the lowest content of trans-fats or partially hydrogenated polyunsaturated fats.
NATURAL VS ARTIFICIAL
Natural trans-fatty acids are produced in the rumen of cattle, sheep and goats. Small amounts of trans-fats (2 to 4 per cent) are therefore found in milk, cheese, butter and meat. These naturally occurring trans-fats are structurally different from those produced commercially during the hydrogenation of fats, however, and have not been implicated in increasing health risks such as coronary heart disease. This has turned the butter-versus-margarine controversy on its head, as some scientists now believe it is healthier to eat butter than margarine or low-fat spreads. The simplest advice is to eat as wide a variety of foods as possible, including a little of everything (butter and margarine, if you wish) and to eat nothing to excess.
Cholesterol
Cholesterol is a fatty substance made in animal livers. Your own liver makes around 800 mg cholesterol every day from certain fats in your diet. You also obtain some – around 300 mg per day – from animal-based foods such as meat, egg yolks and prawns (see
Table 6
).
Table 6
).
Food | Cholesterol content per 100 g |
Pig’s liver | 700 mg |
Lamb’s kidney | 610 mg |
Caviar | 588 mg |
Lamb’s liver | 400 mg |
Chicken liver | 350 mg |
Calf’s liver | 330 mg |
Prawns | 280 mg |
Pheasant meat | 220 mg |
Butter | 213 mg |
Squid | 200 mg |
Duck meat | 110 mg |
Lobster meat | 110 mg |
Chicken meat (dark) | 105 mg |
Hard cheese | 100 mg |
Lean beef | 58 mg |
Chicken meat (white) | 70 mg |
Although cholesterol has a bad name, a certain amount is vital for health. Like other fats, it acts as an important building block to make healthy cell membranes, steroid hormones (e.g. cortisol, oestrogen, testosterone, progesterone), vitamin D, bile acids and coenzyme Q10 – a vitamin-like substance essential for processing oxygen and generating energy within cells.
If you make or eat too much cholesterol, however, your risk of coronary heart disease may increase through a process known as atherosclerosis.
ATHEROSCLEROSIS
If you lack dietary antioxidants (found in fruit and vegetables) the cholesterol in your circulation undergoes a chemical reaction called oxidation. Immune scavenger cells (macrophages) do not recognize the oxidized cholesterol and absorb it in an attempt to remove ‘foreign’ material from your circulation. If circulating levels are high, they soon form bloated ‘foam’ cells. When they try to leave your bloodstream by squeezing through cells in your artery linings, they become trapped and contribute to hardening and furring-up of the arteries (atherosclerosis).
As cholesterol is insoluble, your liver packages it with special proteins to make it soluble before releasing it into your circulation. These proteins surround the cholesterol to form spherical particles known as lipoproteins. Insoluble cholesterol stays on the inside, and the proteins, which have an affinity for both fat and water, remain on the outside.
Low-density lipoprotein (LDL) cholesterol forms small, light particles that can seep into your artery walls. LDL-cholesterol is also prone to oxidation and is readily engulfed by scavenger cells. The higher your level of LDL-cholesterol, the higher your risk of atherosclerosis and cardiovascular disease.
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