Tiara Brown is running in circles around Headbangers Gym in south-east Washington. She looks sunny and trim in her tracksuit, dreads tucked beneath a pink bandana. Nothing has changed since last year, when she became the female amateur featherweight champion of the world.

She still gets to Headbangers when it's empty and trains until it's empty. She still polishes her jab, because the jab is the most important punch in boxing, and her body shots, because body shots can break a girl down.

She still prays every day. She still buys one-way tickets for round trips. She's still broke.

At 25, Brown is only the third US woman in history to win a gold medal at the International Boxing Association (AIBA) World Championships.

Her dominating performance in May of last year in Qinhuangdao, China, had the whole stadium cheering, but when she returned home, the silence was deafening.

After 5km of running, Brown heads inside to skip rope. She breezes through 10 rounds, bouncing on the balls of her feet in the red-white-and-blue boots she will wear at the Police Athletic League national championships. She hopes there will be some decent opposition there.

"Maybe Ginny Fuchs will move up from flyweight," she says.

She puts on her gloves and moves to one of the heavy bags that stretch in bright blue rows across the gym. She will do 10 rounds here, too. Her bag work is of a refinement rare in the women's game. Her stance is upright, her legs taut. The punches come in a jazzy rhythm, lots of one-twos to the body, the occasional five-piece. Her head moves just enough to evade imaginary counters.

In the raised ring beside her, IBF light welterweight titleholder Lamont Peterson, Headbangers' most famous champion, is gently parrying the jabs of a young welterweight called Reemo. The house style is in evidence. Reemo is trying to hit Peterson the same way Brown is hitting the bag. Like artists who paint representationally until they find abstraction, the Headbangers are perfectly capable of classically beautiful boxing. They all have excellent jabs and slick moves on the outside. Yet at some point during every fight – generally when the opponent shows weakness or unwillingness to exchange – they bring out their vicious brand of DC. minimalism. Walk 'em down and work the body. Right uppercut to the solar plexus, left hook to the liver. Once, Brown broke a girl's rib.

Coaches Barry Hunter and Boogie Harris are away in a meeting today, but none of their fighters are slacking. Headbangers train hard so the fight will be easy. Everyone is proud of their new space in an annex of the Bald Eagle Recreation Centre, a $7m city-funded renovation. It is white-walled and airy with a full weight room, clean showers,and open doors that look out on to the grass. Hunter recruited her after he saw her fight in 2009. Now she splits her time between her mom's place in Florida and various people's couches in Washington.

When he's done sparring, I ask Peterson who is the best female boxer he has ever seen. He closes his eyes, smiles, and points a glove at Brown.

"That girl Claressa, too," he adds.

Seventeen-year-old Claressa Shields won the gold in women's middleweight in the 2012 Olympics. She gets a $2,000 training stipend every month from USA Boxing, to add to her $25,000 Olympic medal bonus. Brown gets nothing, because she does not compete in a weight class contested in the Olympic Games. Male boxers have 10 weight classes in the Olympics, ranging from 49kg to super heavyweight. Female boxers must squeeze into just three: 51, 60 or 75kg.

Brown tried very hard to move up to 60kg. She ate more baked chicken and pumped more iron, but her power didn't quite translate. When the International Olympic Committee, in association with AIBA, recently ruled that no additional weights would be added for the Rio Games, it was a blow to Brown's career. "They said they were going to do it," she says. "I can't believe it. I'm devastated."

Sometimes she gets a little bitter when she thinks about the Dr Pepper commercial featuring Mikaela Mayer, an excellent boxer who also happens to be a tall, pretty California blonde. Brown recently beat Mayer at the USA Boxing Nationals. Where is her commercial?

"My body gets tired," Brown says, moving to the stationary bike, where she rides 11km. "My mind gets tired."

Sometimes she thinks about quitting. She will defend her title at next year's World match, then move on. She got her bachelor's degree in criminal justice from Columbus State University. She could study to become a US marshal. It is time to move on with her life.

It will be hard. She remembers that gold medal fight in China, the one that got the whole stadium screaming. Sandra Kruk of Poland was one of the strongest women she had ever faced. Kruk was just as strong as Brown, and just as skilled. After a week of brutal battles, Brown's left shoulder was shot, so she couldn't rely on her jab. The two stood toe-to-toe in centre ring, exchanging combinations.

It took every Headbangers trick she knew. Every shoulder roll, every sidestep: she took it all out and put it on the table. "I knew I couldn't come home without the gold," she says. "What's the point of just getting to the semis? I want to win."

In the end, her will was stronger. Lots of people thought she deserved the title of fighter of the tournament. Ireland's Katie Taylor won that, and Taylor is a great fighter, but Brown was the one who had the whole stadium screaming.

People are always lecturing Brown, saying boxing is un-Christian, that it's violent. She tries to explain that it's just a sport. When the bell rings, you have bad intentions. Once it's over, you hug.

Obviously, God must want Tiara to box. If not, why did He make her so good?

Brown was raised Christian, but in the past few years, her faith has grown into its power. She feels as though God spoke to her, helped her mourn the murder of her brother, helped her pick up the pieces of her broken heart. She says she does not judge other people's religions. She says she does not sign her text messages "Matthew 22:37" because she is trying to convert or preach – rather, she signs them that way so she can see it: "Thou shalt love the Lord thy God with all thy heart, and with all thy soul, and with all thy mind."

Everyone else has gone home by now, and the gym is hushed. "This is my favourite place to be," Brown says. "This is my favourite place in the whole world." She turns on some gospel music. She skips five more rounds of rope, and then, because she forgot to do it this morning, shadowboxes for half an hour.

It's been a month since she's been home to Fort Myers, Florida. She misses her mom and her cousin Ronnie.

Brown could tell you some stories. Given her experiences, there is a chance she could be on the streets selling drugs, like some of her family members. Or dead, like her brother, who was shot eight times. Brown could tell you some stories, but she won't. Forgiveness is part of her religion.

While she is still wet from the shower, she flexes for the locker room mirror. The towel is around her waist, revealing her lean, ropy fighter's torso.

God must want her to box. Why else would he have given her this body?

She was 56.7kg this morning. Now she is 55.8. This means that in a few days she can eat four sweet potato pie cupcakes.

She dresses slowly in her Headbangers regalia. Around her neck goes the wooden amulet a teammate made her in shop class. It is painted with her nickname, "The Dark Menace".

She will go for a long walk now, to loosen her legs. Around 5 o'clock she will eat baked chicken and salad. She will go to sleep, wake up, and do it all over again.

Earlier this month she flew to Los Angeles without buying a return ticket, but she knew everything would work out. Back home, her family passed around a hat. Some pizza places and an auto body shop wrote her cheques: $200 here, $100 there.

"I pray that I stay humble," she says. "I think it's easy for me because I'm always broke." On her way out of Headbangers, she drinks from the fountain.

She knows it sounds silly, but she saw this movie once about Santa and his elves, and there was this part where they showed the elves in the mailroom, sorting the letters. The elf in the thank-you department was all alone.

Tiara thinks maybe God is like that. Maybe He's up there waiting for people to stop asking for things and just say thank you for what they have. Thank you for this breeze, thank you for this boxing gym, thank you for this water I drink every day.

This article appeared in Guardian Weekly, which incorporates material from the Washington Post