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These days if you are reading about health studies, chances are they have something to do with vitamin D.

The latest study, released Thursday, showed that vitamin D cut the risk of several types of cancer by 60 per cent overall for older women.

"The findings ... are a breakthrough of great medical and public health importance," said prominent vitamin D researcher at the University of California-San Diego. "No other method to prevent cancer has been identified that has such a powerful impact."

That study was designed mainly to monitor how calcium and vitamin D improve bone health, and the number of cancer cases overall was small, showing up in just 50 patients.

Other research has suggested that cancers and other disorders in rich countries aren't caused mainly by pollutants but by vitamin D deficiency.

Some researchers are linking low vitamin D status to a host of other ailments, including multiple sclerosis, juvenile diabetes, influenza, osteoporosis and bone fractures among the elderly.

But not everyone is willing to jump on the vitamin D bandwagon, especially since smoking and some pollutants, such as benzene and asbestos, irrefutably cause many cancers.

So, what are we to think after seeing these studies? Should consumers run out to buy vitamin D supplements, or spend more time in the sun? Is vitamin D everything the research seems to suggest?

Dr. Reinhold Vieth has worked in the vitamin D field for over 30 years. He is a Professor in the Department of Nutritional Sciences, and Department of Laboratory Medicine, University of Toronto. He is member of the Canadian Society of Clinical Chemists and Director of the Bone and Mineral Laboratory, Mount Sinai Hospital, Toronto, which provides specialized laboratory services for vitamin D and osteoporosis.

He is a scientific adviser to Osteoporosis Canada, and to several pharmaceutical firms. His research studies into vitamin D nutrition are funded by the National Cancer Institute of Canada, The Multiple Sclerosis Society of Canada, The Dairy Farmers of Canada, and by the Direct-MS Charity.

This is not one of our usual hour-long live discussions. Rather, this is an online question-and-answer session. Your questions and Dr. Vieth's answers will appear at the bottom of this page after 1 p.m. EDT today.

We will follow the same policy for this Q&A as we do for our normal hour-long discussions: globeandmail.com editors will read and allow or reject each question/comment. Comments/questions may be edited for length or clarity. HTML is not allowed. We will not publish questions/comments that include personal attacks on participants in these discussions, that make false or unsubstantiated allegations, that purport to quote people or reports where the purported quote or fact cannot be easily verified, or questions/comments that include vulgar language or libellous statements. Preference will be given to readers who submit questions/comments using their full name and home town, rather than a pseudonym.

Christine Diemert, globeandmail.com: Thanks to Dr. Vieth for agreeing to answer reader queries on vitamin D. Judging by the number of questions, there is a lot of interest in the latest research and need to understand what it all means to us.

Dr. Vieth: Before I begin to answer questions, I want to offer you, the reader, a way to see the summary abstracts of some of the research I will refer to, as I answer your questions.

Biomedical researchers from around the world use a website called PubMed, (http://www.pubmedcentral.nih.gov/) which is maintained by the National Library of Medicine in the United States. PubMed is a library resource that has a high standard of scientific quality, it focuses on peer-reviewed journals.

To find articles in PubMed, just type in the last name of the scientist or the topic, like you would in Google, or you enter a code number for an article. If you want to find the article I cite below, just type or cut/paste the words or numbers from inside the brackets below.

Robert Rose from Montreal Canada writes: Could a mild form of (common and recurrent) psoriasis (legs), around and after age 60, be linked to vitamin D deficiency?

I suspected this could be the case some time, despite my very healthy diet, after my dermatologist had told me that sun exposure on naked skin (more than the cream she had prescribed) helped greatly get over an episode of such psoriasis. My own initiative to take vitamin D3 supplements has seemed to help in reducing the number and length of episodes in the year... More particularly that as an ex-lifeguard I have developed in the past, and needed to have removed surgically, some small skin cancers (basocellular), which means I have to be careful with sun exposure on naked skin... Thank you very much indeed.

Dr. Vieth: Hello Robert: It has been known for years that ultraviolet light improves psoriasis. More recently scientists have discovered that cells within the skin possess both of the enzymes needed to convert some of the skin's vitamin D into the molecule (1,25-dihydroxyvitamin D, or calcitriol) that generates the vitamin D-related signals within cells.

However, nobody has ever tried to find out whether putting simple vitamin D onto the skin might improve psoriasis. Instead, chemical derivatives of the calcitriol have been approved for clinical use in the treatment of psoriasis.

Wendy Lomnes from Calgary, Alberta: I am a great believer in the benefits of Vitamin D. At present, I take 2000 I.U./day in summer and 4000 I.U. in winter. Since I prefer to get Vitamin D by being outdoors rather than taking a supplement, I wonder how much time in the sun is equivalent to 1000 I.U.?

Does the skin receive Vitamin D uniformly or do some places (face, for instance) have more effective receptors than others?

Thirdly, does one still intake Vitamin D on a cloudy day, or is sunshine necessary?

Dr. Vieth: Hello Wendy: Here are my short answers to your 3 questions: a. How much for 1000 IU? This depends on the intensity of the sunshine, and the amount of skin exposed. If 100 per cent of skin surface area is exposed, eg in your bathing suit, lying down, then I would estimate that a white person would make 1,000 IU in less than 5 minutes in summer sunshine (UV index about 7) per side exposed. You could double the amount of vitamin D produced by turning over for another 5 min. A white person's skin produces no more vitamin D after about 20 minutes, approximately the point at which skin starts to turn red.

Are some places on skin more effective than others? An area of skin that has a sun tan will be somewhat less effective than an untanned area. Melanin acts like a natural sunscreen, and it can increase the amount of time required to make vitamin D (or to burn) by severalfold.

A person with very black skin can require as much has six times as long as a white person to make the same amount of vitamin D.

c. Cloud effect? If it is cloudy, I doubt the UV index gets over 3.

The UV index needs to be at least 3, to make vitamin D. If it is sunny, your shadow needs to be shorter than your height before sunlight delivers enough ultraviolet light B to make vitamin D in the skin of a white person.

robert mccaw from newmarket Canada writes: Will the daily use of Vitamin D be of any help to patients already diagnosed with Multiple Myeloma? Would this also work instead of or in conjunction with Pamidronate.?

Dr. Vieth: Hello Robert. At this time, I'm not aware of any research publications that could substantiate any answer I might give to your question.

Certainly, there will be no harm in taking 1000 IU/day of vitamin D.

Love for Canada from Canada writes: 5 Years ago, I was diagnosed with prostate cancer. I was treated with radiation only. So far so good.

Things look Ok ,for now, thank God! I would like to know if Vitamin D or any other supplement would be beneficial to my case.

Dr. Vieth: Hello Tony. A few years ago, colleagues at Sunnybrook Hospital Cancer Centre in Toronto studied the rate of rise of prostate specific antigen (PSA) in men whose prostate cancer had been treated with radiation, like yours. The rate of rise of PSA slowed down substantially once they started on 2000 IU/day of vitamin D. If you type the following into Google or PubMed: "choo, woo, psa". Your first hit will be to the summary of that research paper.

Roy Barker from Ajax Canada writes: Are there any problems associated with taking too much of the supplement?

Dr. Vieth: Hello Roy. For years, vitamin D was regarded as the most toxic of vitamins, and many health professionals still think that. Safety for drugs or for nutrients relates to the difference between the amount that is effective compared to the amount that might cause harm.

I have studied and produced research publications on this topic for years, and a recent review (PubMed: Hathcock, vitamin D) concludes that the upper limit of vitamin D should be set at 10,000 IU/day — per day, on a long-term, multi week basis.

Another way to put this, if you wanted to do harm to yourself, you would have to take at least 40,000 IU/day repeatedly.

C. T. from Canada writes: As I have osteo, I was prescribed both Calcium 1500mg and Vitimin D 800mg but told to be careful not to exceed 800 mg of Vitimin D per day.

The Dr. said it's one of the Vitamins that will harm you if you have too much. Is that true or have newer studies proven otherwise? I have always been very careful not to exceed the prescribed amount for that reason.

Please tell me what the maximum allowable per day is if the studies have proven different so I don't take too much? Thanks

Dr. Vieth: Hello C.T. Your Dr's concern about an intake over your 800 IU/day (note, its IU, not mg) for vitamin D is not based on any evidence in the published literature.

I have gone into this in great detail in an article published in 1999, which you should find easily available over the internet, if you'd like to pass it along to your Dr. (on PubMed enter "10232622" to find the article). As for your asking for max allowable, see the response to Roy above.

The Prince of the Pontiac Tory Blue from Ottawa Canada writes: I take an omega-3 pill every day. Has it the same properties as vitamin D? And, there is controversy about how good one brand of vitamins is over others. What brand/brands do you recommend and how would a consumer know how good a brand is?

Dr. Vieth: Hello Prince. Omega-3 is good stuff but if the bottle does not say anything about vitamin D, you can be sure there's very little or none in the oil. While on the topic of fish oil, I want to emphasize that cod liver oil is NOT a good way to get vitamin D because it contains too much vitamin A (Enter this number into PubMed's search box: 11754708).

High levels of vitamin A lead to higher rates of bone fracture in women. Therefore, it is cheaper, more pleasant, and healthier to take your vitamin D supplement in the plain and simple way. That is, take your 1000 IU/day as a simple pill or one drop of vitamin D oil.

We are quite lucky in Canada, because Health Canada forces all of our vitamin products to pass very strict quality guidelines, and so far as I can tell all of the major brands of vitamin D in Canadian stores are vitamin D3, which is the form used in all clinical trials showing benefit with vitamin D. Unfortunately, products from the United States often contain vitamin D2, and if the American products do not have the term "USP" on the label they are not necessarily at Canadian quality standards.

Sara K from Montreal Canada writes: I have two questions. The first is whether it is safe for pregnant women to be taking such high dosages of vitamin D (1000 IU or more). The second is whether vitamin D is a water soluble vitamin - i.e. does your body get rid of any excess in your system as it does with folic acid, or is it possible to overdose on vitamin D, especially if you are eating healthy and also taking supplements?

Dr. Vieth: Hello Sara K. It is certainly safe for a pregnant woman to be taking 1000 IU/day of vitamin D3. That is definitely NOT a high dose. Bear in mind that all breast-fed infants should now be given 400 IU/day of vitamin D from birth onward, so 1000 IU for the mother who is ten times the size of the infant is modest.

1000 IU/day is certainly at the low end of what my wife would be doing if she were pregnant. There is a recent free article available that offers a good overview of this topic (Enter this number into PubMed's search box: 16636329). I would also highly recommend that you read the material under autism at the following website http://www.vitamindcouncil.com/9.

joanne wright from toronto writes: Thanks for taking our questions.

Now we have some direction on the vitamin, could you also provide additional direction as we stand in front of the vitamin D bottles on the shelves of our local drug store. What should we be looking for? that is, are there different types of vitamin D. Is a generic drug store brand going to give you the same 'quality' of vitamin D as a designer brand of vitamin?

Dr. Vieth: Hello Joanne. As I mentioned above, we are quite lucky in Canada that our vitamin manufacturers must pass strict quality standards. Look closely at the bottle to make sure that it possesses a number that starts with NPN or DIN. This number indicates that the manufacturer complied with Health Canada quality standards. If it has such a number, a generic drug store brand will boost your blood vitamin D concentration as well as a brand name product would.

leafs fan from Toronto Canada writes: Can I absorb enough vitamin D in my 25 minute (one way) daily walk to work? And does the body store any 'unused' dosage from day to day?

Dr. Vieth: Hello Leafs Fan: The answer depends on how much skin you are exposing, and what the UV index is at the time. I would say that if you are wearing shorts and a shirt with short sleeves, and the sun is so high that your shadow is shorter than you are tall, then you will be producing between 2000 and 3000 IU of vitamin D during your walk.

(However, I'd bet that when you walk to work before 9 am and come home after 5 pm your shadow will be taller than your height, and then you will be making practically no vitamin D). On the other hand, if you are a Caucasian nudist lying in the noon-day sun for 15 min on your front, and another 15 minutes on your back, then you will be producing about 10,000 IU of vitamin D. If you have dark skin it will take you more than 15 minutes to do the same.

As for vitamin D storage in the body, bear in mind that the vitamin D people make during the summer sustained them through the winter.

There's nothing wrong with storage of vitamin D in the body. In fact, it's a good thing.

Mark F. from Vacouver Canada writes: Does vitamin D need to be taken combination with any thing else to properly absorb it into the body and why? What are the downsides of taking too much vitamin D? Is the vitamin D from a supplement the same or better than naturally forming vit D from sunshine?

Dr. Vieth: Hello Mark. Vitamin D like other nutrients is absorbed best when taken with food. You need bile to absorb fat soluble compounds like vitamin D, and food stimulates release of bile into the intestine. If you take a vitamin D pill with water at night before going to bed, then you will not be absorbing it very well.

I don't want to go into the toxicity of vitamin D other than to mention that if you wanted intentionally to do your self harm, you would need to consume at least 40,000 IU/day of vitamin D for several weeks — that means you would have to buy a bottle of vitamin D pills every second day.

Unfortunately, vitamin D supplements that we can buy are limited to 1000 IU per pill. In contrast, that day at the beach in your bathing suit will be equivalent to the intake of about 10,000 IU for an adult. That 10,000 IU is a natural and safe amount for adults.

(Children are smaller and after sun exposure they will produce less vitamin D, as appropriate for them).

Joan Forsey from Toronto Canada writes: The news that Vitamin D cuts the risk of some types of cancer for older women is good news. However, a recent article by Reuters (in the National Post on May 3, 2007) was headed 'Higher vitamin D intake in older adults may lead to more brain lesions, study says.'

The (very brief) article reported on research conducted at Duke University in North Carolina. It said, 'Researchers have observed that elderly people who reported higher calcium and vitamin D intake were much more likely to have greater volumes of brain lesions — regions of damage that can increase risk of cognitive impairment.'

Two important questions were not answered: how high is high? and how old is elderly? Answers to those questions would help 'older' women balance the risk of cancer with the risk of cognitive impairment! Perhaps Dr. Vieth can provide the answers.

Dr. Vieth: Hello Joan Forsey: I am quite familiar with the article you are mentioning, and think the authors were irresponsible in publicizing it.

Payne et al was a small, questionnaire-based study that did not measure vitamin D in the blood. People with brain lesions are more likely to be in an institution. Institutionalized people are given more calcium and vitamin D. The amount of vitamin D they reported would barely have made a blip on the vitamin D blood tests I do in my laboratory, so I regard the relationship as implausible because it was not enough to make a difference.

The news report that is alarming you is a situation where the authors confused cause and effect. If people have brain lesions they are more likely to be in an institution, and in that institution they are fed a diet which includes vitamin D. This association does not mean that vitamin D caused the brain lesions. A small study by publicity hungry researchers is not something to get excited about.

This is also an example of why I think a professional scientist's assessment of news reports can be invaluable.

Kevin Stewart from Fort Frances Canada writes: Confused by D2 vs D3 - study I believe used D3 supplement? Many readily available supplements just say 'Vitamin D'? Do I need to look for D3?

Dr. Vieth: Hello Kevin. The problem about vitamin D2 relates to American products. Avoid products that use vitamin D2, unless you are a strict vegetarian, in which case I would suggest you double the dose compared to the vitamin D3 that others are taking. So far as I am aware, all Canadian vitamin D supplements at 1000 IU are vitamin D3, the same kind used in the research studies that show the benefit of using vitamin D.

Shawn Goldwater from Montreal Canada writes: What about whether or not to wear sunglasses? Does sun in the eyes have any role on D3 production, in the same way that it's thought to combat Seasonal Affective Disorder?

Dr. Vieth: Hello Shawn: I have heard people in the alternative medicine community talk about this idea about eyes and vitamin D. It is total nonsense and I have no idea where this strange idea originated. Feel free to wear sunglasses without affecting your vitamin D supplies at all.

As for seasonal affective disorder, there is some research to suggest that the lower moods people suffer in winter may be improved through the use of vitamin D (to read more about this I direct you to the following web page http://www.vitamindcouncil.com/depression.shtml).

Don Scott from Victoria Canada writes: Are there different types of Vitamin D? In the CTV clip, the doctor mentions that the study used Vitamin D3. Is that the normal form used as a supplement in milk etc. and readily available in tablet form, or does one have to specifically request (or find) D3?

I understand Vitamin D is critical to the absorption of calcium. Is that why the study used a Vit D with Calcium supplement? In nursing homes, it is not unusual to give a calcium supplement, but rare to provide Vit D. Why not Vit D also when it is so inexpensive and needed for the body to process calcium?

What is the chance that this study will be reversed in the future, just as the use of multi-vitamins was questioned in research released a week or so ago?

Apparently, it illustrated that use of multi vitamins increased the rate of prostate and other cancers, whereas previous evidence suggested the opposite? It is another case of too much of a good thing? How much do we really know (for sure) about the role vitamins play in human health?

Dr. Vieth: Hello Don: I appreciate your long list of skeptical questions very much. As a researcher in the field of vitamin D, I have come to expect the public to say, "we've heard this before, only to be let down."

All I can say is that the science progresses slowly, we look at the evidence and make conclusions. My shortest answer is that perhaps you should take a look at the webcast lecture I presented some time ago. Vitamin D is different from other nutrients because it does have so many different potential effects on health, so with it the odds are very high that a higher intake of vitamin D will be a no-lose situation.

As for your question about calcium, recognize that the purpose of the calcium and vitamin D research was and is to prevent osteoporosis, and for that, calcium kind of makes sense, although calcium by itself has not prevented fractures.

It is time to look more closely at vitamin D itself, but we can only start to understand the things that the research has uncovered for us. You ask great questions, but the research hasn't been done yet.

Vitamin D research is not something like a new drug that a private company would sponsor. This is the kind of research government needs to support. For the time being, I feel very comfortable in saying that the evidence behind vitamin D is far stronger than for multivitamins which generally contain very modest doses of nutrients as insurance for having things that people are usually getting food anyway.

Vitamin D is different because there is practically none of it in food, and so far, multivitamin amounts of vitamin D were designed for infants, not adults.

jamie messel from Kelowna writes: What are natural sources: fruit, vegetables and foods supplying Vitamin D? How can one person get sufficient Vitamin D?

Dr. Vieth: Hello Jamie. The problem with vitamin D is that it comes from the systems in the skin that are producing cholesterol, which only occurs in animals, not plants. Even the vitamin D2 derived from plant extracts is not normally present in foods, probably because plants are in the sun where the vitamin D2 becomes broken down about as fast as it is made.

Humans are a species designed to be living in tropical climates where sunshine is producing vitamin D all year long. In the North, or in modern society we shelter ourselves from sunshine and require vitamin D in foods. Certain ocean fish contain meaningful amounts of vitamin D3.

The vitamin D in milk and vitamin D supplements was produced by exposing the oil from sheep and lamb wool to ultraviolet light, then purifying the vitamin D3 from that.

Peter Martaus from Toronto Canada writes: Are the same vitamin D generating effects experienced when exposed to ultraviolet light in tanning salons, or is it only the UV from sunlight?

Dr. Vieth: Hello Peter. Most tanning salons select their light sources so that they match up to the range of colours produced by sunshine. That includes the ultraviolet light that makes vitamin D. The effect of tanning on vitamin D has been well-characterized, and one tanning session is equivalent to the oral intake of 10,000 IU of vitamin D3.

Most tanning salons appear to be quite responsible in providing a carefully measured dose of light. Certainly, it is no worse than just going outside on a sunny day and guessing at whatever might be a correct amount of sun exposure.

Deepa Shah from mississauga Canada: Hello, can you tell me the recommended supplement level for dark skinned individuals? One article says toxicity did not occur until levels near one million IU but in the interview with Dr. Rhonda Low she says 2000 IU is the safe upper limit.

Please clarify. Can you also tell me the safe level of supplementation for dark skinned children (age 2 and up). Should supplements be decreased in the summer months and increased in the winter months?

Dr. Vieth: Hello Deepa: You are posing an excellent question. Unfortunately the "official" upper level for vitamin D consumption is 2000 IU/day.

Years ago, this was stated as an intake that most adults could consume without a doctor's supervision and be reasonably assured safety. In my view, telling people that 2000 IU/day is the safety limit is like telling someone who is standing on a beach with the water at their ankles that their safety can no longer be guaranteed if they take one more step. I was an author of the article about the men poisoned with one million IU/day of vitamin D, and they are still alive.

People with dark skin make less vitamin D and as a group certainly have low blood levels of vitamin D. People with dark skin who emigrated to Canada seemed to suffer higher rates of several diseases related to vitamin D. Therefore, it is prudent for all people with darker skin to take that the minimum 1000 IU/day of vitamin D.

Unfortunately, there are zero publications about the health effects of higher doses of vitamin D — except in the Middle East, where women in Saudi Arabia have been given 5000 IU/day of vitamin D3, and is effectively cured the back pain most of them suffered (enter into PubMed the number 12544936).

Furthermore, I have been involved in a clinical trial in Lebanon where schoolchildren were given doses of vitamin D once a week to make it easier to take it. This was given as an oil solution of vitamin D, 14,000 IU once per week is the same as getting 2000 IU daily, and it was quite safe, and it increased bone density compared to placebo (enter into PubMed the number 16278262).

joe hill from Calgary Canada writes: Dear Dr. Vieth, Please comment on the form of Vitamin D that is most efficacious. I gather that in recent studies D3 is deemed most effective. Please comment. Are there Canadian Vitamin D manufacturers (or brands) that are more effective than others (e.g. which might have more rigorous quality control during manufacture)? Thank you.

Dr. Vieth: Hello Joe. As I wrote above we are fortunate that in Canada our government does impose strict quality regulation over our vitamin and mineral products. I trust all brands that possess NPN or DIN numbers.

There are differences in the size of the pill or the liquid the vitamin D comes in, that may make it easier or more pleasant to take the vitamin D. I think ease of use is an important factor in the purchase of vitamin D, because it affects how often you actually take the dose.

Lastly, just to repeat, do not use cod-liver oil as a way to supplement with vitamin D.

Anu de Monterice from Cotati, CA United States: A 2004 case-control study from Finland reported that high vitamin D blood levels — as well as low levels — were associated with higher prostate cancer risk. This increased risk started at a 25-hydroxy vitamin D level at 80 nanomoles per liter. Yet it's my understanding that many experts say that for many health benefits we should aim for such a level. Please comment.

Dr. Vieth: Hello Anu. Things are never simple. You ask how can both a low and a high level of vitamin D be harmful (PubMed article number 14618623). I wrote an editorial letter about that report Tuohimaa of the U-shaped risk curve for prostate cancer, and the authors of the report from Finland agree with it. My point was that vitamin D rises and falls in everybody in the north, and the higher the summer rise in blood vitamin D, the bigger the fall in blood vitamin D into winter.

It is that process of falling vitamin D levels that messes up the signalling systems by which vitamin D normally prevents disease. Better would be to keep vitamin D levels steady and high (with a supplement), than to have them cycle up and down every year the way they do in the far north.

Steve Samis from North Vancouver writes: Dear Dr Vieth: If it is assumed that Health Canada will recommend a daily dosage of 1000 IU of vitamin D to reduce cancer risk, 1) How might that dosage change on a day when a person gets plenty of sun?

2) Can lots of sun exposure plus a daily dose of 1000 IU of vitamin D cause side effects (vitamin D toxicity)?

3) If a person takes 20 TUMS (calcium-based antacids for GERD) daily, should that individual increase the vitamin D intake above 1000 IU?

4) Should a small adult (110-125 lb) take less than 1000 IU of vitamin D daily (that is how close to the toxic level is 1000 IU)?

5) Should vitamin D only be taken in fall and winter in Canada (or is the risk of side effects low)? Thanks for offering to respond to questions!

Dr. Vieth: Hello Steve: Health Canada will almost certainly not change anything about vitamin D until the Food and Nutrition Board in the US undertakes a very formal examination some day... eventually.

I was very pleasantly surprised that the Cancer Society of Canada made the 1000 IU recommendation — the society deserves a lot of credit for this!

As for your questions: a) You are microanalyzing the vitamin D story too much! Vitamin D builds up slowly over months, and whatever happens sunshine-wise on a few days, frankly has nothing to do with intake considerations. Just keep taking the vitamin D because a steady intake is the way it was done in all clinical trials.

b) If you intentionally wanted to harm yourself with vitamin D, you would need to taken more than 40,000 IU/day for many days. Sunshine is natural and on a person with a bathing suit produces 10,000 IU/day of vitamin D. Adding even another 10,000 IU/day of vitamin D would not do any harm based on everything I have ever seen in the literature about this topic.

c) If someone takes Tums or a lot of calcium, this will have no bearing on the need for vitamin D. Vitamin D does help with calcium absorption when blood vitamin D levels are low, but when there is enough vitamin D it does not help further.

d) For a small person for a child, 1000 IU is extremely safe and probably quite healthy. From my perspective, to be worrying about vitamin D in this dose range is like telling a thirsty person you are worried about them drowning when they are drinking glass of water.

e) Again, you are worrying and trying to analyze something very simple.

Just plain take vitamin D every day. You could even take all seven of your doses per week once weekly. The effect would be the same as taking it daily.

Linda Furney from Victoria Canada writes: Hello: 3 questions: I have MS, diagnosed 1997, entering a progressive phase slowly. Will Vit D help and how much daily?

2) My 16 year old daughter should be taking how much?

3) I have had abdominal surgery that inhibits food absorption. How easily/fast is Vit D absorbed, when taken as a supplement? Thank you.

Dr. Vieth: Hello Linda. a) There's much interest in the Multiple sclerosis (MS) community about vitamin D, and the Direct-MS website contains a wealth of information about vitamin D and MS. Please take a look at it and decide for yourself about your dose (4000 IU/day is not unreasonable).

b) Your daughter is at a statistically higher risk of multiple sclerosis, and I know there are experts who would like to carry out what is called a primary prevention clinical trial using vitamin D (probably at 2000 IU/day) to minimize the risk that the children of adults with MS will develop the disease (the research trial will probably never be done, because it is too difficult to fund). There is a very interesting recent publication indicating that higher levels of vitamin D in the blood earlier in life lower the odds of eventually developing MS (You can find this if you search for the name "Munger" at the www.direct-ms.org website).

c) Abdominal surgery may or may not affect your vitamin D absorption, but you can't tell till you test for it. If you had abdominal surgery, you should have a 25-hydroxyvitamin D blood test done, and want the result to come back as over 75 nmol/L. If it is not at least that high, then take more vitamin D (testing every 3 months) until the blood test is up.

d) You didn't ask, but I think teenagers may have a problem taking a vitamin D supplement over the long term, because this usually means taking a pill for the rest of your life. One alternative to pills that I think might help for a child or teen is a supplement called Ddrops. That contains 1000 IU in each drop of an oil that can be put onto food.

Christine Diemert, globeandmail.com Thanks Dr. Vieth for so generously, thoroughly and clearly answering the reader questions we sent. Although there were some queries that came after our deadline, we hope the detailed answers above will give you the information you wanted. There is obviously a great interest in the subject and hopefully Dr. Vieth will agree to join us online again in the future.