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In separate studies, researchers linked the two nutrients -- or lack thereof -- to higher risks of certain lupus symptoms and complications.
In the other, people who ate more omega-3 fats -- mainly found in oily fish -- tended to have less severe symptom flare-ups and better sleep quality.
Neither study actually proves that the nutrients deserve the credit, said Dr. Stacy Ardoin, a member of the Lupus Foundation of America's Medical-Scientific Advisory Council.
To do that, she explained, researchers need to run trials where they actually test the effects of vitamin D or omega-3.
But it's "encouraging" to see such diet-focused studies, said Ardoin, who was not involved in the research.
"Questions about diet are some of the most common ones I get from patients with lupus," said Ardoin, an assistant professor at Ohio State University. "And it's an area where we have little evidence."
Lupus has several forms, with the most common called systemic lupus erythematosus. In SLE, the immune system mistakenly produces antibodies against the body's own tissue. The onslaught can have widespread effects, damaging the skin, joints, heart, lungs, kidneys and brain.
At least 1.5 million Americans have the disease, according to the Lupus Foundation of America. It mostly strikes women, usually starting in their 20s or 30s.
Despite some Internet claims, there is no such thing as a lupus-defeating diet, Ardoin said.
But the new studies looked into whether certain nutrients are related to the risks of certain symptoms and complications. Both were reported at the annual meeting of the American College of Rheumatology in San Diego this week.
The vitamin D study involved nearly 1,400 patients with SLE who had their blood levels of the nutrient measured in 2009-10. Slightly more than 27 percent had low levels -- defined as less than 20 nanograms per milliliter.
The researchers looked at the patients' rates of organ damage over the following years, and estimated their lifetime risks of those complications.
On average, they found, patients with low vitamin D had a 66 percent higher risk of severe kidney damage, versus those with normal levels. The nutrient was not linked to damage in other specific organs.
Dr. Michelle Petri, of Johns Hopkins University in Baltimore, led the study.
It's known, she said, that vitamin D helps reduce protein in the urine, which is a predictor of future kidney damage. So that might explain why low vitamin D was specifically tied to that risk.
While the study was not a clinical trial, Petri said, vitamin D can be safely prescribed to lupus patients who need it.
According to Ardoin, many rheumatologists already screen lupus patients for low vitamin D, because it is a common problem for people with the condition.
"If you haven't been screened," she said, "you can ask your doctor about it."
The omega-3 study involved 456 SLE patients who completed a detailed diet survey. In general, researchers found, the more omega-3 fats people consumed, the better their sleep quality.
And when they ate plenty of omega-3, and tempered their intake of omega-6 fats, they typically had lower scores on a measure of lupus "disease activity."
Omega-6 fats are largely found in vegetable oils like corn, sunflower and safflower oils. Omega-3s come mainly from fish like salmon, mackerel and tuna, plus certain seeds and nuts -- such as flaxseed, chia seeds and black walnuts.
The average U.S. diet is high in omega-6, according to the researchers, led by Prae Charoenwoodhipong, of the University of Michigan.
Other studies, they say, have suggested that diets heavy in omega-6 promote inflammation in the body. In contrast, a better balance of omega-6 and omega-3 might quiet inflammation.
Again, Ardoin said, the findings do not prove that omega-3 will ease any lupus symptoms. And even if there is a benefit, she added, it's not clear how much omega-3 is necessary to achieve it.
That said, Ardoin noted, paying attention to diet -- including getting enough healthy fats -- is generally a wise move.
Research presented at meetings is usually considered preliminary until published in a peer-reviewed medical journal.
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