Module 28 Flashcards
How might tanning salons lead to birth defects?
- It is known that low folate levels in pregnant women is associated with an increase in the risk of birth defects such as spina bifida.
- Folate is photosensitive and is degraded by UV light. UVA light, the kind used in tanning salons, is lower energy than UVB, but it penetrates the skin better than UVB and has sufficient energy to degrade folate. Therefore, using a tanning salon is contraindicated in pregnant women.
What is the evidence that vitamins are effective in the prevention of cancer? What are the advantages of eating carrots, etc. rather than taking ß-carotene supplements or vitamin A?
- For some time there was a lot of hype about the benefits of taking vitamin supplements, especially those that acted as antioxidants.
- It was found that patients with lung cancer had, on average, lower blood levels of vitamin A. Therefore, a study was done in which smokers were given vitamin A; however, if anything, the smokers receiving vitamin A had an increased incidence of lung cancer.
In general there is no good evidence that taking vitamins decreases the incidence of cancer. There are hundreds of carotenoids in food, which is likely to be better than taking just ß-carotene.
- It was found that patients with lung cancer had, on average, lower blood levels of vitamin A. Therefore, a study was done in which smokers were given vitamin A; however, if anything, the smokers receiving vitamin A had an increased incidence of lung cancer.
Why must calcitriol be given to patients with severe kidney disease?
- Vitamin D is formed in the skin under the influence of UV light, and some is also obtained in the diet from things such as salmon.
- However, it must be hydroxylated in the liver to 25-hydroxy-vitamin D, and then further metabolized in the kidney to 1,25-dihydroxy-vitamin D, which is the active form.
- Many patients with severe kidney disease cannot make this active form of vitamin D, and therefore it must be administered. In theory, patients with liver disease would not be able to carry out the 25-hydroxylation step, but in practice, that is not usually a problem.
Should people take vitamin D supplements, and if so, what is the optimal dose?
- This is very controversial. Some “experts” think that everyone should take several thousand units/day, but the data just aren’t there. To some degree it depends on how far north you live, how much time you spend outside, and how dark your skin is.
Then a recent study showed that patients taking 10,000 units/day had a 3.5% decrease in bone density after 3 years, those taking 4,000 units/day had a decrease of 2.4%, and those taking 400 units/day had only a 1.2% decrease after 3 years. More is not always, or even usually, better
What diseases have been associated with vitamin D deficiency, and is the association causal?
multiple sclerosis. Persons being raised at higher latitudes are exposed to less UV light and are at significantly higher risk of developing multiple sclerosis. Exceptions are individuals living near a coast who eat lots of fish such as salmon, which are a source of vitamin D, and persons living at higher altitudes, where there is less filtering out of UV light. However, even with multiple sclerosis causality has not been definitively established. There have been studies in which patients with multiple sclerosis are treated with high doses of vitamin D, but the results to date are not impressive.
Why should thiamin be given to alcoholics who are brought to the ER in a coma?
Alcoholics often have very low levels of thiamin, both because of a poor diet and because thiamin is consumed during the metabolism of alcohol. Severe thiamin deficiency is a medical emergency because it can lead to brain damage. If a patient is brought into the ER in a coma and the cause of the coma is unknown, they are given IV glucose because one treatable cause of coma is hypoglycemia. However, this would further decrease levels of thiamin because the metabolism of glucose also utilizes thiamin. Therefore, along with glucose, thiamin is administered, especially if the patient is suspected of being an alcoholic.
What vitamin causes a severe peripheral neuropathy when given in high doses?
vitamin B6, high doses cause peripheral neuropathy
What vitamin interferes with a test for colon cancer? What other vitamin can interfere with lab tests?
- The common test for colon cancer detects the presence of blood because such tumors usually cause subclinical bleeding. The basis for the test is that the heme iron in blood catalyzes the peroxide-mediated oxidation of guaiac to a colored product.
○ Vitamin C, a strong reducing agent, inhibits this oxidation and can lead to a false negative test.- A newer test is an immunochemical test for the presence of the protein globin. It is not affected by reducing agents such as vitamin C. It is also not associated with false positive tests because of food peroxidases such as horseradish peroxidase, or by eating food that contains blood, i.e. rare meat, because the globin is digested in the upper GI tract.
- There is a recent FDA Safety Communication indicating that biotin (vitamin B7) can interfere with several lab tests. They were not very specific about which tests.
Both vitamin C and E are antioxidants. From their structures what differences would you expect in their site of action in this regard?
- Vitamin C is very water soluble and is an effective antioxidant in the cytosol, but its concentration in lipid membranes is low.
In contrast, vitamin E is very lipid soluble, and therefore it is more effective in preventing lipid peroxidation.
Vitamin E was used to try to prevent cardiovascular disease. What was the rationale, and what is the clinical evidence?
- Cardiovascular disease is associated with lipid peroxidation in the fatty deposits in the vascular endothelium. This leads to inflammation and plaque formation. Therefore, it was plausible that vitamin E would decrease the lipid peroxidation and be protective. However, when controlled studies were performed, low doses of vitamin E were not effective, and higher doses (>40 units/day) were associated with increased mortality.
Three B vitamins were used to try to prevent cardiovascular disease. What vitamins are they, what was the rationale for their use, and what is the evidence for their efficacy?
- It was observed that patients with high levels of homocysteine were at increased risk of cardiovascular disease. Homocysteine appears to be toxic to the vascular endothelium. Homocysteine is converted to methionine by methylation.
- Vitamins B12, folate, and B6 increase the methylation of homocysteine. Therefore, a prospective study was done in which patients with high homocysteine levels were treated with these vitamins. The result was that the vitamins significantly decreased homocysteine levels, but if anything, the incidence of cardiovascular events was higher in the treated group than the control group.
What is the evidence that vitamin C supplements can prevent/treat “colds”.
In general the studies have shown that vitamin C supplements do not prevent colds. The only possible exceptions are people with a very poor diet who are significantly vitamin C deficient and athletes who are involved in extremely vigorous activities, but even in these two possible exceptions the evidence is not conclusive.
Are antioxidants the fountain of youth? How, in theory, could antioxidants be harmful?
- They are not the fountain of youth. Antioxidants work because they are easily oxidized by more reactive oxygen species and other free radicals to relatively nontoxic products. But the word relatively nontoxic is key; they are easily oxidized, and the products are not totally nontoxic; therefore, very high levels of antioxidants can actually produce a prooxidant environment. It appears that the most effective antioxidants are actually prooxidants that induce the body to produce enzymes that degrade reactive oxygen species
What drugs may decrease vitamin B12 absorption?
- Metformin significantly decreases the absorption of vitamin B12, and the incidence of B12 deficiency of patients on long-term metformin therapy is almost 30%. The mechanism is not clear.
Proton pump inhibitors also can decrease the absorption of B12, however, the effect is usually not clinically significant. The combination of metformin and a proton pump inhibitor appears to decrease absorption more than metformin alone. As far as I know thiazide diuretics have no effect on B12 absorption
What vitamin analog can be used to treat cyanide poisoning?
- Hydroxycobalamine can be used to treat cyanide poisoning. Vitamin B12 is usually administered as cyanocobalamin. Hydroxycobalamin has a very high affinity for cyanide and is converted to cyanocobalamin. It is an effective antidote for cyanide poisoning although there are other effective treatments that are usually used instead.
