Vitamin observational studies

Low levels of vitamin E have been associated with increased incidence of coronary artery disease. Observational studies have therefore suggested that supplemental a-tocopherol might have value in the treatment of cardiovascular disease. Clinical studies demonstrated contradicting results regarding the benefits of vitamin E in the prevention of cardiovascular disease. Four... 

Numerous observational studies have found that vitamin C may decrease LDL cholesterol and elevate HDL (50). Some studies found an inverse association between serum vitamin C concentration and coagulation factor or coagulation activation markers (51). 

In macromolecular crystallography, this process cannot be observed because hydrogen atoms are not located and dynamic processes cannot be seen . However, in the crystal structure of vitamin B12 studied by X-ray and neutron diffraction methods, an interpretation of the water movement could be derived [127, 880, 881]. 

Vitamin C positively influences coUagen production, and increases osteoblast formation and osteoclast formation and survival. Some, but not aU, observational studies showed that vitamin C supplementation (1000 mg or more) was associated with higher BMD than that of nonusers. Again, it is premature to suggest routine supplementation. 

Vol. 330, pgs. 1029-1035,1994. (Note This is interesting in that protective benefits of taking vitamin E supplements were not seen while beta carotene may have worsened the situation. The authors concluded that taking just supplements needs to be further evaluated and that possibly these vitamins are not the active cancer-inhibiting components of fruits and vegetables that are observed to protect in observational studies.)... 

In summary, trials of antioxidant supplementation of smokers and nonsmokers with vitamin C, a-tocopherol, or p-carotene did not generally find significant decreases in biomarkers of oxidative DNA damage, in contrast to observational studies, which suggest an inverse association between dietary antioxidant intake and oxidative DNA damage. 

Data about the preventive effect of vitamin E on cardiovascular diseases are corttradictory. Observational studies showed that the consumption of vitamin E reduces the risk, but large interventional studies involving thousands of people could not verify this. The latter test types are more rehable in general. Similarly ambiguous is the relationship between vitamin E and cancer. There are numerous positive interventional studies, but an especially significant test showed that high... 

However, the overall picture is by no means as dark. To demonstrate this clearly, the evidence should be interpreted correctly. It is a fact that a diet with high levels of carotene is advantageous (Table 3.8). But a very high-dose consumption of purified carotenoids (especially p-carotene and vitamin A) is associated with higher risk for humans exposed to increased oxidative stress. General statements about the effects of carotenoids are impossible. For example, several studies showed that lycopene (the red pigment in tomatoes) rednces the risk of prostate cancer, and observational studies of tomato consumers revealed no side effects. 

Thus, epidemiological cohort studies can provide associations between vitamin intake either from food or from fortified food or supplements and a specific disease, and the RCTs can provide a proof whether this association is causal or not. The major differences are that the cohort studies usually include healthy people at baseline, while the RCTs usually include patients who suffer from the disease (secondary prevention). Observational studies usually have longer follow-up periods and they assess food intake, from which vitamin intake is calculated from. It may also be mentioned that an observational study may either find an increased disease risk at low intake or low plasma levels of a nutrient (usually in the lowest quartile or quintile of the cohort), which is opposite to the finding of a reduced disease risk at high intake or high plasma levels. RCTs, however, aim to find a reduced disease risk at high intake levels that is achieved through the nutrient supplement used. This difference is discussed in more detail below. 

Reasons for the divergent results achieved by observational studies and randomized clinical trials are largely unknown. Among possible reasons, there may be substantial residual bias in the observational studies (Lawlor et al. 2004) it is important to consider lifelong vitamin intake instead of short-term vitamin supplementation, different ehemieal stmetures of vitamins in food V5. in supplements (most pronouneed for folie add in supplements vs. the various forms of folate in food), and the differenees in treating patients already suffering from a disease (as in the randomized elinieal trials) from observing healthy subjects (as in the observational studies). Additionally, in the majority of studies, most patients randomized were without overt vitamin delidency and the interactive effects of other nutrients have not usually been considered. 

B vitamin intake or vitamin blood concentrations have also been related to various cancers. The cancer types that have been best investigated with respeet to folic acid are colon cancer and colorectal cancer. Convincing evidence from observational studies led to the initiation of randomized controlled trials with folic acid in colorectal adenomas. However, similar to cardiovascular disease, there seems to be a discrepancy between the observational epidemiological studies that reported in the majority an inverse association of folate and cancer risk, and the effect of folic acid supplementation in the randomized controlled trials, which reported no effect of folic add on recurrence of colorectal adenoma risk. 

To assess the relationship of Hey concentrations with vascular disease risk, a meta-analysis of observational studies was carried out, showing that elevated Hey is at most a modest independent predictor of ischemic heart disease and stroke risk in healthy populations. Studies of the impact on disease risk of genetic variants that affect blood Hey concentrations will help determine whether Hey is causally related to vascular disease, as may large randomized trials of the effects on ischemic heart disease and stroke of vitamin supplementation to lower blood Hey concentrations (Homocysteine Studies Collaboration 2002). 

Epidemiological studies. Epidemiology is the study of the distribution and determinants of disease frequency in human populations and the application of this study to control health problems. Therefore, epidemiological studies harvest valid and precise information about the causes, prevention and treatments for disease. Types of epidemiological studies are experimental studies and observational studies (cohort and case-control studies). Homocysteine. Homocysteine is a sulfur-containing amino acid that occurs naturally in all humans. Elevated plasma homocysteine concentration is linked to an increased risk of ischemic stroke. Lowering homocysteine plasma levels is linked to increasing the intake of folic acid and vitamins Bg and Bi2-... 

A number of clinical trials have attempted to support the observational study findings of a preventive effect of vitamin 65 against vascular diseases. Most of these clinical trials aimed to determine the long-term effect of homocysteinelowering treatment and accordingly involved the administration of folic acid and vitamin B12 in addition to vitamin Bg. Most findings indicated no significant effect of treatment. 

Recent evidence from observational studies indicates that vitamin Bg status, particularly blood level, is highly likely to be associated with reduced vascular disease. However, clinical trials has not supported those observational findings, possibly indicating that low blood levels of vitamin Bg may be a consequence of low-grade inflammation or smoking rather than the cause of cardiovascular disease. 

Recent evidence from observational studies indicates a possible association between vitamin Bg status and cancer, particularly colorectal cancer, albeit that the results from trials are still limited. 

Recent evidence from observational studies indicates that vitamin Bg status is highly likely to be associated with vascular diseases, albeit that the findings from clinical trials were inconsistent. 

The homocysteine hypothesis of vascular disease has attracted considerable interest since homocysteine levels are readily lowered by daily dietary supplementation with folic acid, vitamin Bg and vitamin B12 (Homocysteine Lowering Trialists Collaboration 2005), raising the prospect that dietary supplementation with these B vitamins could prevent vascular disease. Indeed, dietary supplementation with B vitamins to lower homocysteine levels of affected individuals is remarkably effective for the prevention of cardiovascular disease and other complications of homocystinuria (Yap et al. 2001). This review examines the evidence from the observational studies of homocysteine and vascular disease and from the randomized trials of B vitamin supplementation for the prevention of vascular disease. 

Although the observational studies suggested modest associations of homocysteine with risk of vascular disease that were biologically plausible, such studies could not establish if these associations were causal. The randomized trials assessed the effectiveness of dietary supplementation with B vitamins to lower homocysteine levels on risk of cardiovascular morbidity and mortality. The initial trials were designed in the mid-1990s before the results of the Homocysteine Studies Collaboration meta-analysis (Homocysteine Studies Collaboration 2002) were reported in 2002. Consequently, few of the individual trials had sufficient statistical power to confirm or refute the 10% difference in... 

It has been shown that regular use of vitamin C supplements increases blood concentrations by 0.36 to 0.46 mg/dl (Dickinson et aL, 1994). There also seems to be no differences in bioavailability between dietary and supplemental vitamin C (Mangels et al., 1993). Therefore, theoretically, results from epidemiological investigations regarding the effect of vitamin C supplements on risk in observational studies should be considered a significant contribution to the evidence of a causal relationship since the pure substance is applied. 

Another observational study, the first National Health and Nutrition Examination Survey, examined vitamin C intake using both food frequency and 24-hr recall questionnaires (Enstrom et ai, 1992). The 11,349 men and women in the study, ages 25-74, were followed for a median period of ten years. CVD mortality rates were 34% lower (relative risk = 0.66 95% Cl = 0.53-0.82) than expected among participants with the highest vitamin C intake defined as 50 mg or more from the diet plus regular supplements (Table IV). One potential limitation of this study was the inability to examine and control for the possible correlation of vitamin C with other vitamin supplements, particularly vitamin E. 

Henze, A., Raila, J., Kempf, C. et al. 2011. Vitamin A metabolism is changed in donors after living kidney transplantation An observational study. Lipids Health Dis 10 231. 

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