This Independence Day, competitors at Nathan’s Famous hot dog eating contest may gulp down as many as 76 wieners in about the time it takes to read this article. Like ultramarathoners hoofing 50 miles or football players shaking off big hits, “speed-eating” entails natural resilience, dedicated training, and serious risks to health. Some may see the annual Coney Island, N.Y., contest as an act of defiance, capturing the holiday spirit.