The study, published last week in the American Journal of Clinical Nutrition, found that high phosphorus intake was associated with increased mortality in a nationally representative, healthy US population.
“Because of prevalence of high phosphorus intake in healthy adults [more than one-third of Americans reported phosphorus consumption that exceeded 1,400 mg per day, the study found] and the widespread use of inorganic phosphorus additives in processed food, our findings may have far-reaching public health implications,” wrote the authors.
For the study, exposure to dietary phosphorus was assessed between 1988 and 1994 in a nationally representative sample of 9,686 healthy US adults aged 20 to 80 years with normal kidney function.
All-cause and cardiovascular mortality was assessed through Dec. 31, 2006. The study found that high phosphorus consumption was associated with increased all-cause mortality at amounts exceeding 1400 mg per day, which is twice the US Recommended Daily Allowance for adults. (There was no significant association between absolute phosphate intake and mortality below this threshold.)
Since the study was observational, “we cannot prove that reducing phosphorus intake would reduce risk of death,” co-author Alex R Chang, MD, MS, clinical investigator at Geisinger Health System, told FoodNavigator-USA.
“More research is needed to understand the health effects of phosphorus intake. We also need more studies understanding the contribution of phosphorus in food additives to our diets.”
Dietary intake, not just blood levels of phosphate, linked to adverse outcomes
While phosphorus is an essential nutrient for cell structure and function found naturally in some foods such as dairy, meat, fish, eggs, nuts and legumes; intakes have increased in recent years as more inorganic phosphates are added to packaged foods in the form of phosphorus additives used as anti-caking agents, to help preserve moisture or color, as stabilizers, leavening agents and acidifiers.
And a growing body of research has linked elevated serum phosphate levels to higher mortality in people with chronic renal failure and increased risk of cardiovascular disease in healthy people.
"This is a very important publication with some potential implications for public health policy," Geoffrey Block, MD, director of clinical research at Denver Nephrologists, told FoodNavigator-USA.
"While we have known for some time that elevated blood levels of phosphate were associated with increased mortality and increased likelihood of developing kidney disease, and we've known that the amount of phosphate in the diet is likely to contribute to these outcomes, one of the critical pieces of information that has been missing has been solid evidence that dietary intake of phosphate per se, and not just blood levels of phosphate, are clearly linked to adverse outcomes."
“We have long seen this correlation in the population of persons with kidney disease, but only recently has this been recognized as an issue applicable to the general population,” added Janeen Leon, MS, RD, LD, a dietician and researcher at MetroHealth Medical Center in Cleveland, OH.
“[The Chang et al] findings are worrisome because of the broad use of phosphorus-containing food additives in a large proportion of foods sold in the United States.”
Not difficult to eat a diet high in phosphorus, even if we think we are eating healthy
Leon co-authored a recent study in the Journal of Renal Nutrition, which found that nearly half (44%) of the best-selling grocery products in the US contain phosphorus additives, based on a sample of product labels from 2,394 top-selling branded grocery items in northeast Ohio.
The additives were especially common in prepared frozen foods (72%), dry food mixes (70%), packaged meat (65%), bread and baked goods (57%), soup (54%), and yogurt (51%) categories.
“It is not difficult to consume a diet high in phosphorus, even if we think we are eating ‘healthy’,” Leon said, adding that even some beverages contain “surprising” levels of phosphorus additives.
"It is widely known that the reference database used in [the Chang et al] study and in many others underestimates dietary phosphate intake probably by at least 25-30% due to the use of food additives which are not accounted for," Dr. Block noted. "For some food products it is probably 100% of an underestimation."
Dr. Chang added that the source of phosphorus is particularly important, "as bioavailability is higher in animal-based proteins than many vegetable-based proteins, which often contain phytate-bound phosphorus that is absorbed less. Phosphorus-based additives are commonly used for multiple purposes including improving taste, texture, and reducing cooking time and may be absorbed even more readily than naturally occurring organic sources of phosphorus.”
A wake-up call for the food industry
Despite that, more research is needed to know the contribution of phosphorus-based additives to the average American’s diet, Dr. Chang says that transparency from food manufacturers offers a step in the right direction.
“Phosphorus-based additives are important for the food industry for many purposes. However, I think it would be helpful to provide information about phosphorus content in products, particularly for patients with kidney disease who have problems excreting phosphorus,” he noted.
Leon added that food manufacturers should also try to decrease their use of phosphorus additives, though this may not be a desirable option, given its moisture retention, flavoring and leavening properties.
“If manufacturers do not wish to reduce their use, then they should voluntarily quantify phosphorus content on the food label so that consumers can make purchasing decisions with knowledge,” she said. “A few manufacturers are starting to list this information on their websites and/or directly on the food label and that is appreciated. Ideally, technologies will fully develop to allow the differentiation between inorganic phosphorus from additives from phosphorus naturally present in the food.”
Dr. Block added: "I’m afraid this won’t be enough to move us fully towards trying to get phosphate content on food labels, but it should be a tremendous wake-up call to the food industry that we are headed that direction and we are continuing to generate the scientific data supportive of the need for full disclosure of phosphorus content in our food."
Source: American Journal of Clinical Nutrition
Nov. 13, 2013 doi: 10.3945/ajcn.113.073148
“High dietary phosphorus intake is associated with all-cause mortality: results from NHANES III”
Authors: Alex R Chang, Mariana Lazo, Lawrence J Appel, Orlando M. Gutierrez and Morgan E Grams