B-Vitamins & Diabetes: Protective or Causative?

In a very interesting study, scientists from China and Japan (Zhou. 2010) found that "long-term exposure to high level of the B vitamins may be involved in the increased prevalence of obesity and diabetes in the US in the past 50 years". At first this appears to be counterintuitive, since we have been told over and over that B-Vitamins are not only good for our health, but that we could not even "overdose" them. While the latter has been questioned for years and certainly is not the case for e.g. B6 and niacin, even the former seems questionable, if you read the results from the above mentioned study:

The prevalences of diabetes and adult obesity were highly correlated with per capita consumption of niacin, thiamin and riboflavin with a 26- and 10-year lag, respectively (R2 = 0.952, 0.917 and 0.83 for diabetes, respectively, and R2  = 0.964, 0.975 and 0.935 for obesity, respectively). [...] The relationships between the diabetes or obesity prevalence and per capita niacin consumption were´similar both in different age groups and in male and female populations. The prevalence of adult obesity and diabetes was highly correlated with the grain contribution to niacin (R2 = 0.925 and  0.901, respectively), with a 10- and 26-year lag, respectively.

These results (especially those referring to the detrimental effect of niacin) confirm test-tube studies conducted by a group of scientists from South Korea earlier this year (Choi. 2010), who found that

NA [nicotinic acid] alters gene expression in insulin-sensitive tissues by various mechanisms. Some of the NA-induced changes in gene expression are discussed as potential mechanisms underlying wanted and unwanted effects of NA treatment.

Just anecdotal: My personal perspective on B-vitamins has changed since my overall energy and well being, as well as my physique have largely improved after stopping to take those B-vitamin (over-)loaden mulit-vitamin preparations like Now ADAM, CL Orange Triad, Animal Pak, ON Opti-Men etc. But remember: it is mere speculation that this could in fact be related to their high B-vitamin contents - could be any other constituent, as well.