Minnesota Vikings All-Pro defensive tackle Kevin Williams testified Tuesday that he was under pressure to lose weight and earn a $400,000 bonus when he took a weight-loss supplement two years ago — a supplement he didn’t know contained a substance banned by the NFL.

The testimony was the first from a player in the closely watched trial deciding whether the NFL’s use of its anti-doping policy broke Minnesota labor law.

Williams said he learned about the StarCaps supplement from a teammate and thought that the over-the-counter drug contained natural substances, which his attorney identified as “papaya and garlic.” After undergoing surgery on his left knee in 2006, Williams said he began taking StarCaps at the start of training camp in 2007 to “help me shed a couple extra pounds, for health reasons, keep the coach off my back” and to make his weight bonus.

Williams, whose playing weight is 310 pounds, said his weight will rise to 315 pounds in the offseason. When he’s not at his playing weight, the lineman said Vikings defensive line coach Karl Dunbar makes him “feel real bad.”

Williams and teammate Pat Williams have sued the NFL over its anti-doping policy, which resulted in positive tests for both of them during training camp in 2008. Both players tested positive for bumetanide, which can mask steroids, though neither player is accused of taking steroids.

The Williamses, who are not related, are suing the NFL for unspecified damages and attorneys’ fees. The players contend the NFL knew StarCaps contained the banned diuretic — which was not listed as an ingredient on the label — and had an obligation to tell them. Among the issues to be settled at trial is whether the NFL has to follow Minnesota labor law when it comes to drug testing.

At issue is whether the NFL notified both players of their positive tests within a required three days and whether the league leaked test results in violation of confidentiality requirements. The NFL counters that it complied with all Minnesota laws in suspending the Williamses and that there is no evidence the leak came from the league.

Kevin Williams, 29, said he was appealing his four-game suspension and waiting to tell his family about his positive test when “it came across the news.” He said he did not tell the media about his test results and was “a little upset” when he heard the report.

Williams said friends asked him “’Are you taking steroids? What’s the deal?’ All they see is the (words) banned substance flash across the screen.”

Speaking softly, Williams said he has never taken steroids, performance-enhancing drugs or substances to mask their use.

Earlier, the medical overseer of the NFL’s anti-steroid policy said other players tested positive for banned diuretics before the Williamses in 2008 but were not punished. Dr. John Lombardo testified that more than six players tested positive for diuretics from 2005 to late 2007.

Attorneys for the Williamses contend the NFL inconsistently applied its anti-doping policy. NFL attorney Dan Nash argued that those questions had already been settled by a federal judge, but Hennepin County Judge Gary Larson reversed himself Tuesday and allowed attorneys for the Williamses to pursue that line of questioning.

Kevin Williams is expected to be cross-examined by NFL attorneys Wednesday. Pat Williams has yet to testify.