Nov. 4, 2008 -- Eating at least two servings of fish each week seems to protect people with diabetes who also have kidney disease, according to a long-range study of more than 22,000 adults in England.
The study, published in the American Journal of Kidney Diseases, the official journal of the National Kidney Foundation, shows that fish consumption lowers abnormal levels of protein in the urine in people with diabetes.
Abnormal amounts of protein appear in the urine when the kidneys are damaged; it's a key indicator of kidney disease. Previous studies have shown that fish and fish oil consumption decrease protein in the urine, increase glucose tolerance, decrease fats in the blood, and lower blood pressure -- all benefits to people with diabetes.
Diabetes affects an estimated 23.6 million Americans and is the leading cause of end-stage kidney disease. While there is no cure for the disease, a balanced diet and a lifestyle that includes regular exercise and weight loss for those who are overweight or obese helps slow the progression of complications.
The British study was part of the European Prospective Investigation of Cancer (EPIC), a 10-country collaboration that investigated the link between diet and cancer. The EPIC-Norfolk Study, conducted from 1993 to 1997, involved 22,384 mostly white middle-aged and older men and women, 517 of whom had diabetes.
Urine tests and dietary-lifestyle questionnaires led to the finding that those with diabetes who on average ate less than one serving of fish each week were four times likelier to have macroalbuminuria (abnormally high levels of protein in the urine) than those who eat fish regularly.
For the people without diabetes in the study, eating fish showed no difference in urine-protein levels.
"Protein in the urine is one of the earliest signs of kidney disease," says co-investigator Amanda Adler, MD, PhD, of the Medical Research Council Epidemiology Unit at Addenbrooke's Hospital in Cambridge, England.
The participants in the study who had diabetes were 64 years old on average; the people who didn't have diabetes were 58.8 years old on average. Everyone underwent a medical examination, kept a food diary, and completed a food-frequency questionnaire. At the beginning and end of the study, their urine was taken to determine protein levels.
Fish consumption was defined as average weekly intake of fried fish, oily fish, white fish, and fish fingers. The study accounted for lifestyle factors, like alcohol and tobacco use, family medical history, socio-economic status, and ethnicity, but the researchers found they didn't have a significant impact on risk of macroalbuminuria.
Which Types of Fish Protect the Kidneys?
Adler says it's unclear whether it's the fish oil or the type of protein in fish that protects the kidneys. And the study makes no distinction between eating fried vs. unfried fish or warm-water vs. cold-water fish like mackerel and salmon. The study simply shows that eating more of it has a protective effect on kidney function in those with diabetes.
"We included all types of preparation [of fish] in this study. However, we did not find any difference in risk between oily fish or fried fish, such as fish and chips. There is a possibility that our study would have to be bigger to find differences between types of fish," Adler says.
Leslie Spry, MD, a kidney specialist in Lincoln, Neb., who serves as a National Kidney Foundation spokesman, says he typically doesn't tell patients to eat more fish but recommends fish oil supplements to control triglycerides (blood fats).
"This is the first study that has translated it into a dietary recommendation," Spry says, adding that he'd like to see a broad study of people with diabetes that attempts to find a link between fish and fish-oil consumption and reduced protein levels in the urine.
"The next study that ought to be done is to take a group of [people with diabetes] and randomize some of them to take high fish intake and some to take low fish intake and compare," he says. "I was kind of struck by their selection methods, that [participants] could eat any old kind of fish. This study suggested you could. I wouldn't tell my patients to go to a fast-food place and eat fish sticks, but this study seems to say it's OK."