A4: There is a problem about taking too much vitamin D, just as there can be trouble if too little is taken. The usually recommended daily dose of vitamin D is 500 international units and the upper limit is 1,000 international units. Cod liver oil is usually treated so that both the content of the vitamin A and the vitamin D should it be necessary is reduced to comply with the standard RDA. Fish oil, as opposed to cod liver oil, doesn't carry the same risk of having too much vitamin A. The concern is that people shouldn't take too much vitamin A. There is no known maximum dose for omega3 but reputable companies always put the dose of omega3 on the back of their containers.

Q5: Please could you recommend a vegetarian source of omega3 oil? I would also like to know whether levels of omega3 can be measured to indicate any deficiency? Margaret Wragg, Cheshire.

A5: Linseed oil, wheat germ, walnuts, soya beans all contain reasonably high proportions of omega3. There are a host of other less good sources of plant omega3 that include sunflowers, avocados, olive oil, almonds and many cereal crops. In general for those people who are not vegetarian the easiest and best way to take omega3 is to eat oily fish. Tuna, salmon, mackerel, herring, trout and sardines are all ideal.

Q6: In your article (Fish oil on troubled waters), you do not mention cod liver oil specifically. As I have been taking a supermarket brand (ASDA) for many years, I would ask if this is as beneficial as those that you mention, for health in general? I am a pensioner. John Murdoch, Carlisle, Cumbria.

A6: It is necessary when taking supermarket own brand vitamin supplements to check the amount of vitamin A to make certain it is within the recommended daily allowance and to check the amount of omega3 present to ensure that it is adequate.

Q7: Omega3 has been recommended in a number of articles in recent months as having many benefits including preventing blood clots. Would it be inadvisable for someone to take it if he or she is having aspirin or warfarin as part of their medication, also are there any other reasons why omega3 should be avoided please? Name and address withheld.

A7: People who are attending an anticoagulant clinic and taking warfarin should discuss fish oil with the doctor at the clinic before they take it.

As a rule trouble arises with anti coagulation if someone changes the amount of any particular foodstuff they are taking. Above all they must take the same amount of alcohol whatever the circumstances. The anticoagulation system is exactly the same if the patient is intending to sit by the television set or attend a celebratory wedding or other party, the same quantity of alcohol must therefore be taken if someone remains quietly at home or attends the party of all times at The Savoy. The secret of maintaining steady results from the anticoagulation and therefore not to risk a bleed is to take a similar diet every day and exactly the same amount of alcohol.

There are a few substances that are notoriously upsetting to coagulation times, these include cranberries. Fish oils have an effect but usually this can be accommodated by altering the dose of anti coagulant provided that the clinic knows about it. Discussion with the concerned doctors is all important.

Q8: We are a group of dietetics students wondering whether you think declining oily fish stocks due to climate change will lead to lower EPA and DHA consumption in the UK, whether this will affect the nations health, and whether fortification of everyday foodstuffs is a viable/realistic option. You may have guessed that we're asking these questions to help us with a research project, but we think that they may be of interest to the general public too. Any ideas/opinions will be gratefully received! Sian Roberts, Kate Pennington, Eileen Rowlands, Colin Ferguson and Chris Conifer. University of Chester.

A8: As fish becomes a less common constituent of the average diet there is a danger that omega3 with its essential EPA and DHA consumption will be reduced. It is not only a question of reduced fish stocks, and hence increased prices, but also the changing nature of cooking and kitchens. Fish smells linger in well insulated kitchens and can easily spread throughout a small flat. Cooking fish in a microwave with Ventaxia ventilation helps but isn't a complete answer. Taking omega3 supplements makes up for any deficiency of essential fats because of the shortage of fish. I would recommend reading a copy of The Omega 3 Phenomenom published by Sidgwick Softbacks about 20 years ago, although a rather elderly work it includes a very comprehensive picture of omega3. Modern research has added clinical evidence to support many of the suggestions in the book.

Q9: I have three children aged 8, 5 and 2. I have researched the benefits of omega3 fatty oils for children and have found a lot of the claims are confusing or unproven. As my children are fussy eaters the chances are very slim that they would receive a healthy amount of fatty acids through oily fish. We are concerned about our eight-year-old's concentration levels and we want to know if supplements, in capsule or oil form, are likely to make a difference to his concentration? Are there any other health benefits? Is there a supplement and a specific dosage that would be suitable for all my children? Carolyn Rouse, address withheld.

A9: The capsule of fish oil recently introduced by Haliborange contains it in a form that can be added to food without the most sensitive child (or me) noticing it. There are health benefits over and above the fish oil's effect on concentration so that fish oil is to be recommended to people who have behavioural problems, including difficulty in concentration. You should check the vitamin A levels from any other possible supplementary source of vitamins and add it to that provided by the Haliborange so as to make certain that the daily allowance of vitamin A is not exceeded. I have always had a strong dislike for the taste of cod liver oil myself but to my surprise found that although the neat oil in the new capsules of Haliborange tastes and smells disgusting once added to food, or a baby's bottle, it is undetectable.