Q: What evidence is there that the diet can affect the health?
A: The evidence is often based on epidemiological research conducted by public health scientists. They study trends across populations to try to identify what factors affect health and illness. For example a particular ethnic group may encounter fewer health problems while living in their own country but then consistently encounter greater health problems if they emigrate and follow the lifestyle and diet of the country they have moved to. If so, public health researchers will often try to identify what the main factors appear to be.
It is this kind of research, for example that has suggested the value of the Mediterranean diet in reducing the risk of heart disease and cancer.
Q: Is there anything we might be surprised by?
A: Breakfast is the most important meal of the day. People who have breakfast tend to be thinner, have a higher nutritional intake and find it easier to concentrate.
Modest intakes of alcohol have been shown to increase protective HDL cholesterol levels.
Contrary to popular belief, carbohydrates are not fattening and in general are either fat free or very low in fat. Recent research has shown that a higher cereal fibre intake in adults aged 65 years and over is associated with lower risk of CVD - with the lowest risk appearing to be related to intake of dark breads, such as whole wheat, rye and pumpernickel. Meals should therefore be based around carbohydrate foods. This would include high fibre breakfast cereals especially porridge, brown breads, potatoes, pasta, rice, noodles and pitta bread.
Q: So what is a balanced diet?
A: A well balanced diet is one that provides the body with enough energy, and all the nutrients required in their correct amounts to prevent illness and disease. People who consume a varied diet tend to have a wider nutritional profile, and therefore are more likely to achieve recommended nutrient intakes.
Conversely, a low fibre diet is associated with cancer of the bowel, constipation and other bowel disorders, while low intake of fruit and vegetables is associated with higher incidence of cancer and heart disease.
Q: What are three useful dietary tips to reduce the risk of a heart attack?
A: Try to include three or more portions of oily fish a week. Herrings, kippers, salmon, sardines, mackerel, trout, tuna and swordfish all contain much higher levels of Omega-3 than white fish. Omega-3 fatty acids are beneficial for blood clotting and platelet aggregation, having an anti-thrombotic and anti-inflammatory effect. Omega-3 fatty acids also reduce plasma triglyceride levels, but have minimal effects on LDL and HDL cholesterol levels.
Eat plenty of fruit and vegetables. They contain many important nutrients. Potassium, for instance from bananas, orange juice and potatoes, can help reduce blood pressure. Folic acid limits homocysteine formation and soluble fibre helps reduce LDL cholesterol and improves HDL cholesterol. In addition, fruit and vegetables contain anti-oxidants as well as bioactive compounds with anti-oxidant activity such as carotenoids and flavonoids.
Eat less fat, in particular saturated fat and trans fatty acids. These are linked to excess weight, obesity and raised blood cholesterol levels especially LDL cholesterol. So limit how much cream, butter, mayonnaise, cheese, chocolate and biscuits you eat. Products like biscuits are often produced using cheap vegetable oils, which change their chemical composition when cooked, creating trans fatty acids, which are particularly damaging.
Q: What dietary changes can help reduce the risk of cancer?
A: The association between diet and cancer is still only in its infancy. Most studies are epidemiological in origin and research trials are sometimes hard to replicate.
However, consuming diets high in fruit and vegetables seems to be the most significant thing we can do, partly through the anti-oxidants they provide, which result in a decreased risk for many cancers including colorectal and lung. For example, resveratrol , a polyphenolic phytoalexin found in grapes is an anti-oxidant that has been shown to exhibit strong anti-inflammatory, cell growth-modulatory and anti carcinogenic effects so can be helpful if there is genetic predisposition. Diets rich in cruciferous vegetables like broccoli, cabbage, cauliflower, horseradish and mustard seed appear to reduce the risk of cancer of the colon and rectum. Allium vegetables like garlic and onions appear to be preventative against stomach cancer. In addition vitamin C has been associated with a decreased risk of many types of cancers especially stomach cancer.
The evidence is currently suggesting that isoflavones, which are phytoestrogens present in high quantities in soy products, may play a preventative role in breast cancer.
Some evidence has shown a protective effect of tea on the incidence of stomach and rectal cancers.
Conversely, high alcohol intakes are associated with cancers of the mouth, pharynx, larynx, oesophagus and liver.
Also excess intakes of salt, alcohol, heavily cooked meats and hydrogenated fats should be avoided. To achieve this cut down on barbequed food and processed meat, for instance.
Q: Is there anything diet can do to help boost our immune system?
A: Unknown to many people is the role that vitamin A plays in our immune system. Mild vitamin A deficiency leads to an increased susceptibility to a variety of infectious diseases. However, because it is a fat-soluble vitamin that can be stored in the body, supplements should be used with caution. It is far better to improve the sources in the diet, such as fortified milk with vitamins A and D added to it.
Liver is such a rich source of vitamin A that pregnant women should actually avoid it. Many margarines have vitamin A added to them. Carotenes, which are precursors of vitamin A are found in green, yellow and red fruits and vegetables, and these are also a great source of antioxidants.
Selenium (a mineral) and also an antioxidant plays a major role in immunity. Selenium deficiency is accompanied by loss of immunocompetence. Covert suboptimal selenium status may be widespread in human populations. However, there is a narrow margin between beneficial and harmful intakes of selenium so be careful of supplements. Brazil nuts are a rich source of selenium. Fish, shellfish, liver and kidney as well as milk and eggs are also good sources.
Zinc deficiency causes increased susceptibility to infections. Meat, (beef and lamb being higher than pork and chicken) seafood (especially oysters and crab) and liver are the main sources of bioavailable zinc.
Q: Do dietary supplements have any value or is the main benefit from what we eat and drink?
A: The aim should be to get all the nutrients that your body needs from your diet. For example anti-oxidant supplements in whatever form are not recommended to replace fruit and vegetables.
There is a role for supplements but in special situations (see below) not for the general population.
Women who do not eat dairy products should take a calcium supplement. Vegetarians should take an Iron and vitamin B12 supplement. Vitamin D requirements increase in the elderly so a vitamin D supplement could be necessary here. If someone has been ill then a short course of a vitamin supplement would be useful. Most important of course is folic acid for women who are planning a pregnancy, to help prevent neural tube defects. But for the rest of us, they really are not necessary.
One other point to bear in mind is the value of the sun, within reason, as a source of vitamin D. We are now hearing of an increase in rickets in children, as a result of very high levels of sunscreen use.
Q: The word diet sometimes conjures up the idea of weight loss. What is the best advice be for someone looking to lose weight?
A: There's no mystery to dieting. It is simply a question of mathematics. Bigger portions contain more calories, which can contribute to weight gain. So to lose weight, or maintain body weight, reduce portion sizes. Don't skip breakfast. Have three regular meals a day - just keep the portion sizes down. If you're eating in, a smaller plate is an easy way of achieving this.
Q: Food allergies seem to be an increasing problem. Is there anything we can do, through choice of diet, to reduce the risk of developing food allergies?
A: Food allergy is a reaction by the body to a dietary protein like milk, egg or nut. This allergic response produces a lot of IgE antibodies and will make you ill immediately after eating the food. 1 - 2% of the population is believed to have a food allergy.
Food intolerance causes symptoms hours/days after eating the offending food. This response may or may not involve the immune system. Foods like soya, milk, eggs, and wheat; chocolate, caffeine and wine can all be listed as offenders. There is no scientific test to prove food intolerance. Advice is usually to avoid the offending food. If you think you have a food intolerance seek advice from a registered dietitian.
Q: What would you recommend to people interested in finding out more about the health implications of diet?
A: The American Dietetic Association's site is very good, for both health professionals and laymen.
Other useful sites include:
The information above is based on an interview with Dr Mary McCreery, a consultant clinical nutritionist and dietitian based at the Blackrock Clinic in Dublin. She has been consultant nutritionist to both the Irish Olympic team and the Irish rugby team and is chair of the scientific committee of the Nutrition and Health Foundation.