CHICAGO – Less than half of the U.S. stores where milk is sold offer lower-fat or skim varieties, with healthier options most scarce in poor and minority communities that tend to have higher obesity rates, according to research by the Institute for Health Research and Policy at the University of Illinois in Chicago.

Reuters reports that part of the problem is a lack of supermarkets in poor communities, or so-called “food deserts,” leaving shoppers to rely on “smaller convenience stores and drugstores, where any milk is more expensive and low-fat varieties are less often available,” say researchers.

“These findings indicate that it’s more challenging and more expensive for populations at high risk for obesity and other diet-related health problems to follow dietary guidelines, at least with respect to milk,” lead study author Leah Rimkus told Reuters.

Rimkus and colleagues researched the price and availability of lower fat and whole milk at 9,226 stores across 468 communities in 46 states during the spring and summer months of 2010, 2011 and 2012. The representative sample of stores was 9% supermarkets, 7% grocery stores and 84% “so-called limited-service stores like bodegas or mini-marts.”

Nationwide, the study found that whole milk was available in 81% of stores, roughly double the proportion of places that sold fat-free milk. Whole milk was stocked in 99.9% of supermarkets, 89% of grocery stores and 78% of limited-service stores. Fat-free milk was most plentiful in supermarkets, sold in 52% of grocery stores but just 36% of convenience stores.

Using U.S. Census data for ethnicity and income levels, the study found that, compared with white communities, the odds that stores sold any type of milk were 31% to 67% lower in predominantly black neighborhoods and 26% to 45% lower in places with large minority populations.

As to why stores in lower-income communities tend to leave skim milk off shelves, it’s because it doesn’t sell, Adam Drewnowski, director of the Center for Public Health Nutrition at the University of Washington in Seattle, told the news source. “Food stores in low-income neighborhoods will have a limited range of luxury goods like fresh fruit and vegetables or fish and poultry,” he said. “Yes, those have become luxury goods, like it or not.”

However, Tamara Dubowitz, a nutrition researcher at RAND Corporation in Pittsburgh, Pennsylvania, says skim milk may not be selling because it’s more expensive than whole milk. “Food store owners would likely say that the store inventory is based on consumer demand, and that prices are based on algorithms of store turnover,” she told Reuters, adding, “Yet from a food justice perspective, it is easy to see that differential availability and pricing would make for differential consumption.”