When the mayor of New York city decided to take on as powerful a lobby as the fizzy drinks industry with a proposed ban on oversized sugary sodas, the furore that ensued was entirely predictable. Coca-Cola and McDonald's, among several other usual suspects, accused Michael Bloomberg of overriding New Yorkers' freedom of choice – as they would.

But this week some new, and highly unexpected opponents have added their voice to a lawsuit seeking to squash Bloomberg's fizzy drink restriction before it comes into effect on 12 March. The New York conference of the NAACP and the Hispanic Federation have both given support to a legal challenge that came before the courts on Wednesday by lodging a joint amicus brief in which they criticise the proposals for "stripping New Yorkers of their democratic rights".

That the NAACP, America's oldest and most august civil rights organisation, and the federation, an umbrella of about 100 Hispanic groups in the American north-east, should jump into bed with soda and fast food multinationals is deeply puzzling. Obesity and diabetes, the double scourge that the Bloomberg measure is designed to combat, hits two ethnic groups hardest: African Americans and Latinos – precisely those communities served by the NAACP and Hispanic Federation.

The health arm of New York City points out that some predominantly black or Latino New York neighbourhoods now have rates of overweight and obesity approaching 70% of adults. "Black New Yorkers are three times more likely, and Hispanics twice as likely, as whites to die from diabetes," the city's experts say.

As the city points out, obesity and diabetes are not minor issues; they are killers. About 6,000 New Yorkers die every year from obesity-related illness, and it is against that backdrop that Bloomberg has proposed limiting the size of sugary drinks sold in restaurants, delis, cinemas and sports stadiums to 16oz.

New York City points out that you would have to walk three miles from Union Square in Manhattan across the East river to Brooklyn just to burn off the calories from one Coke in the soon-to-be banned 20oz size.

So what are the NAACP and the Hispanic Federation thinking? When the Guardian called up the New York chapter of the NAACP, the reporter was told to contact a Washington phone number and ask for the association's statement.

The phone number turned out to be that of a publicist for the American Beverage Association, the trade association of the fizzy drinks industry whose members include Coca-Cola, Nestle, Pepsi and 7 Up.

The ABA released to the Guardian a joint statement drawn up between the ABA, the NAACP and the Hispanic Federation.

Hazel Dukes, president of the NAACP's New York state conference, is quoted as saying: "The decision by the board of health to restrict the sale of sugar-sweetened beverages in packages or cups larger than 16oz is neither prudent nor helpful in the overall fight against obesity. The ban will be ineffective in that it does not get to the root of the problem of obesity in New York or in the African American community."

The Coca-Cola Foundation, the philanthropic arm of the company, last month awarded the NAACP $100,000 to support "a program promoting healthy eating, physical activity and healthy lifestyles in African American communities". The New York conference of the NAACP has also received $35,000 grants from Coca-Cola for "education initiatives".

The Guardian did reach Jose Calderon, the president of the Hispanic Federation, who is also quoted in the joint ABA statement. Asked why the federation was opposing a measure designed to combat a potentially lethal epidemic within the Latino population, he said: "The diabetes rates among Hispanics are astronomical. That's why we want all parties – academics, government and the voluntary sector – to come together and work towards a solution."

Calderon said he was opposed to the Bloomberg initiative because "it is a diversion. It doesn't in any serious way tackle the issue."

The website of the Hispanic Federation lists its funders. They include Goya Foods, makers of fruit-flavoured sparkling drinks geared to the Latino market, and the Coca-Cola Company.