There has been a lot of interest in this vitamin recently because it is being researched intensively. It is vital to get enough, especially for children, because too little can lead to very young babies having fits and children developing rickets.

That may seem like a ridiculous idea in this day and age, yet paediatric dietitian Jacqui Lowdon of the British Dietetic Association reports that dietitians are seeing a rise in cases of this problem.

Most of the vitamin D we have in our bodies comes not from food, but through the action of sunlight on our skin. When the rays of the sun hit our skin they convert a type of pre-vitamin D into its active form. While this can happen all year round at the Equator, the sun's rays are strong enough for it to occur only between April and September in the UK, and only then between roughly 10am and 3pm if you are out for a total of about 20 minutes.

Because few foods are good sources of vitamin D, it is recommended that babies from six months old and children up to five years old who are growing at a fast pace are given a supplement containing 7 micrograms of vitamin D daily to ensure that they are getting enough. This means that yes, your four-year-old son would benefit from supplementation.

Healthy Start children's vitamin drops are ideal and should be available from your local NHS health clinic, in children's centres and some pharmacies.

Whether your ten-year-old daughter is getting enough sun in the summer to build up vitamin D stores for winter depends on many factors. For example, the farther north you live in the UK, the less vitamin D you make. The same is true if you live in an area where the air is polluted.

It also depends on whether you always keep her arms and legs covered when

she is outside. She needs 20 minutes outdoors each day in the summer, ideally with her limbs uncovered and with a sun cream on of less than SPF8. The darker her skin, the less vitamin D she will be able to make.

In addition to all these factors, the type of summer we have also counts. According the British Dietetic Association, we had 464 hours of sunshine this year, which is seven per cent less than average.

If you feel that your daughter is affected by any of the factors mentioned above, it will not do her any harm to take a supplement tailored to her age group. Cod-liver oil contains vitamin D and should be counted as a source. It is best just to stick with one source of vitamin D and not to go over 10 micrograms in total.

Vitamin D facts

Pregnant and breastfeeding women, as well as adults over 65, are advised to take a daily 10 microgram vitamin D supplement. So too are people with dark skin and those who cover up most of the time when out of doors

Vitamin D is necessary for the absorption of calcium for strong bones, but has also been linked to reducing the risk of heart disease and some cancers. In addition, there is evidence from population studies that too little vitamin D may increase the risk of multiple sclerosis.

Oily fish such as sardines, salmon, anchovies and mackerel are the only good foods for vitamin D (you get 12 micrograms per serving of grilled mackerel). Eggs provide 0.9 micrograms each and skimmed milk powder fortified with vitamin D gives you 0.8 micrograms per pint.

If you have nutrition questions for Amanda, you can e-mail her.