Despite losing all of that weight, her ratio of fat to muscle was around 25 percent, 5 percent from where she started and dangerously close to what some researchers say is an unhealthy situation.
"I was just shocked. I thought that it was a lot lower, and I thought that I was healthy," Sumpter said.
There are others like Sumpter. As many as 30 million Americans who are considered average weight may actually have what scientists call normal weight obesity, according to a recent study by the Mayo Clinic.
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The study, which followed 6,171 Americans over nine years, found 20 percent to 30 percent of people considered normal weight still have an alarmingly high percentage of body fat.
The Mayo Clinic says that generally, women should have a body fat percentage below 30 percent, while men should have a fat to muscle ratio of less than 20 percent to 25 percent.
Sumpter, a 34-year-old mother of one, is 5 feet 8 inches tall and weighs about 140 pounds, which is in the normal weight range for her height.
However, if 42 of those pounds -- or 30 percent of her weight -- are made up of fat, Sumpter would actually be considered normal weight obese.
That diagnosis means a higher risk of obesity-related diseases, such diabetes, high cholesterol and heart disease, which is the No. 1 killer of women.
"Women with normal weight obesity, meaning those who have high fat and a normal weight have a two times increased risk for death or dying from heart problems or a stroke," said Dr. Francisco Lopez Jimenez, who led the Mayo Clinic study.
Sumpter changed her workout routine, adding more weight-bearing exercises to build lean muscle mass instead of only doing calorie-burning cardio.
"Resistance training is the key," said Sumpter, who is now certified as a personal trainer and works at Equinox in New York. "There are three key components to a healthy lifestyle, which is healthy eating habits, resistance training to build muscle and of course cardio to burn calories and for a healthy heart, but it's a combination of all three."
Today she weighs 20 pounds more than her lowest weight, but her body fat percentage is down to 14 percent.
Inspired by Sumpter's story, ABC News' Juju Chang asked researchers at the Human Body Composition Laboratory at St. Luke's-Roosevelt Hospital in New York City to measure her body fat.
Chang's body mass index measured between normal and overweight but was far from obese.
According to the World Health Organization, 23 to 35 percentage of body fat for a woman her age is acceptable. But she measured 37 percent, putting her at risk.
"So on some level this study points to people like me who need to be aware of their body fat," Chang said. "I didn't need a full body scan to know that I'm out of shape and I need to exercise."
Doctors said they do not recommend people getting their body fat tested because of this study. Instead they ask that people look at their waistline.
Because belly fat is considered more dangerous than fat in the butt or thighs, doctors say if you have fat around the waist or love handles, that is an indication that you could be over your normal, healthy weight.