Older adults with relatively low intakes of vitamins B6 and B12 may have a higher risk of developing depression than those who get more of the nutrients, a new study suggests.

Researchers found that among 3,500 older adults they followed for up to a dozen years, the risk of developing depression symptoms declined by 2 percent for every 10-milligram (mg) increase in daily vitamin B6 from food and supplements.

The same was true for every 10-microgram (mcg) increase in vitamin B12 intake.

The findings, published in the American Journal of Clinical Nutrition, do not prove that the B vitamins themselves protect against depression. But the results do echo those of some previous studies tying the vitamins -- as well as folate, another B vitamin -- to depression risk.

The prime shortcoming of most of those previous studies, however, was that they studied people at one point in time, rather than following them over years -- making it unclear whether the lower B-vitamin intake came before or after the depression.

The main strength of the current study is that it followed a large group of people over 12 years, measuring depression symptoms at several time points, lead researcher Dr. Kimberly Skarupski, an associate professor at Rush University Medical Center in Chicago, told Reuters Health in an email.

That bolsters the case that relatively low intakes of the B vitamins might contribute to depression in some people. Still, Skarupski and her colleagues say their results should be interpreted with caution, because study participants' consumption of the vitamins might be a proxy for other factors, like a generally healthy diet.

The study included 3,500 Chicago-area adults who were at least 65 years old and depression-free at the outset. Participants' consumption of folate and vitamins B6 and B12 was estimated based on their responses to a detailed dietary questionnaire. They were then assessed for depression periodically over the next dozen years.

The researchers found that anywhere from 11 percent to 14 percent of participants had symptoms indicative of clinical depression at some point during the follow-up.

When they looked at the relationship between B vitamins and depression, they found that the risk generally dipped as consumption of B6 or B12 increased. The link held when Skarupski's team accounted for a number of other factors -- such as race, education and income, antidepressant use and lifestyle habits like smoking and drinking.

While the results do not prove cause-and-effect, Skarupski said they do emphasize the importance of older adults' being mindful of their diets.

Older adults should eat well-balanced diets and talk with their healthcare providers about their nutritional intake and nutrient status, she advised.

Vitamin B6 is found in a range of foods, including beans, potatoes, bananas, meat, chicken, peanut butter and certain fish, like salmon and tuna. The recommended intake for men older than 50 is 1.7 mg per day, while women are advised to get 1.5 mg; the upper limit is set at 100 mg daily, as too much B6 can cause nerve damage.

Foods naturally rich in B12 include beef, certain fish, like salmon and trout, and milk products; B12 is also added to some foods, such as fortified breakfast cereals.

The recommended dietary allowance for the vitamin in adults is 2.4 micrograms per day; there is no established upper limit for B12 because of its low potential for harmful effects. Because older adults have difficulty absorbing the B12 naturally found in food, experts generally recommend that they try to get their daily allotment through fortified food or a multivitamin.

People in the current study consumed anywhere from 0.6 mg to 200 mg of B6 each day, while B12 intakes ranged from 0.3 to 266 mcg per day.

According to Skarupski's team, it is biologically plausible that the two vitamins would affect depression risk.

Both B6 and B12 are involved in healthy nervous system function, and overt B12 deficiency causes a neurological syndrome that includes problems with thinking and memory, as well as depression symptoms.

For its part, vitamin B6 is involved in synthesizing chemical messengers in the brain, including serotonin. Serotonin dysfunction is believed to play a key role in depression.

Much remains to be learned about B vitamins and depression, however. There is little evidence, for example, that taking the vitamins can help treat depression -- though preliminary findings from one recent study suggested that folate may enhance the effects of antidepressant medication.

In this study, folate intake was not linked to depression risk, as it has been in some studies from other countries. A potential explanation, Skarupski and her colleagues speculate, is that outright folate deficiency is rare in the U.S., where grain products are routinely fortified with the vitamin.

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