Behind every public figure's appearance lies a personal wellness routine. This series examines the diets, workouts, treatments, and beauty and fashion habits of notable figures as practical lifestyle information.

[Yoo I's SNS]
[Yoo I's SNS]

Singer and actor Yoo I, 38, has drawn attention after sharing body composition data showing she gained muscle while losing fat. Training for HYROX — an indoor fitness competition that combines running with functional strength exercises — over roughly four months increased her skeletal muscle mass by 1.4 kilograms and brought her body fat percentage down to 19 percent.

Yoo I posted the body composition results on her SNS on Tuesday, writing, "I had no idea preparing for a HYROX competition would be this enormous!" The records show her skeletal muscle mass rose from 22.5 kilograms on July 9 to 23.9 kilograms on Nov. 3. Her body fat percentage fell 6.2 percentage points, from 25.2 percent on Aug. 10 to 19 percent on Nov. 3, while her total body fat dropped 3.3 kilograms, from 13.6 kilograms to 10.3 kilograms over the same period.

Interest in managing the ratio of muscle to fat — rather than focusing solely on the number on the scale — has been growing. Research findings have continued to show that combining aerobic exercise with strength training helps reduce body fat while maintaining or increasing muscle mass.

Running 8 km while pushing sleds and lifting weights — the fitness effects of HYROX

HYROX, the competition Yoo I is training for, is an indoor fitness event in which participants alternate between running and functional strength exercises. Competitors run 1 kilometer, perform one designated exercise, and repeat the cycle eight times. A defining feature is that the full 8-kilometer course engages muscles across the upper body, lower body and core.

According to the official HYROX website, participants complete eight exercises: the SkiErg, which mimics a cross-country skiing motion; heavy sled pushes and pulls; burpee broad jumps; rowing; farmer's carries; sandbag lunges; and wall balls.

Running and rowing raise the heart rate, building cardiovascular endurance and increasing energy expenditure. Pairing those with sled pushes and weighted carries adds resistance to multiple muscle groups, including the thighs, glutes, back and core.

This approach of combining aerobic and resistance exercise is known as concurrent training. It draws on the benefits of both modalities to develop cardiovascular capacity and muscular strength simultaneously, making it particularly useful for those who want to reduce body fat while preserving muscle.

A 2025 analysis confirmed these effects. Researchers examined 36 studies involving a total of 1,564 participants and compared changes in body composition among those who did aerobic exercise only, strength training only, or both.

Among participants who exercised for 10 weeks or more, aerobic exercise was more effective than strength training at reducing body fat, while strength training was relatively better at preserving lean mass. The group that combined both types also showed reductions in body fat. The researchers said total exercise volume and duration were as important as exercise type in driving fat loss.

Same weight, different body — why preserving muscle matters in dieting

One concern during weight loss is the simultaneous loss of muscle alongside fat. When caloric intake is severely restricted, the body may break down not only stored fat but also the protein that makes up muscle tissue for energy.

Muscle handles everyday movement and serves as a key tissue for storing and using glucose. Maintaining adequate muscle mass and strength makes it easier to sustain physical activity and supports blood sugar regulation and overall metabolic health.

As a result, even at the same body weight, differences in the ratio of muscle to fat can affect both physical function and appearance. Even when the number on the scale barely changes, an increase in muscle mass alongside a decrease in body fat represents an improvement in body composition.

An analysis published in 2025 in BMJ Open Sport & Exercise Medicine highlights the importance of strength training during weight loss.

Researchers compared overweight and obese adults who followed dietary restriction alone with those who combined it with strength training. Total weight loss did not differ significantly between the groups, but those who added strength training lost less lean mass and achieved better results in fat reduction and strength gains.

Sustaining and building muscle requires adequate nutrition alongside exercise. Protein supports the recovery and synthesis of muscle tissue stimulated by training, while carbohydrates serve as a key energy source for endurance activities such as running.

Drastically cutting food intake while repeatedly performing high-intensity exercise can lead to accumulated fatigue and delayed recovery. Gradually reducing body fat while eating enough to match training demands is a more sustainable approach to long-term body composition management.

Jumping straight into HYROX risks injury — start with both aerobic and strength training

There is no need to enter a HYROX competition or take on high-intensity exercise from the outset to improve body composition. The benefits of concurrent training can also be achieved by consistently walking or running and gradually adding strength exercises.

The US Centers for Disease Control and Prevention recommends that adults get at least 150 minutes of moderate-intensity aerobic exercise per week along with strength training on two or more days. Brisk walking or cycling can build cardiovascular fitness, while squats, lunges and push-ups engage major muscle groups.

For those with limited exercise experience, starting with about 20 to 30 minutes of walking and light strength work, then gradually increasing intensity as fitness improves, is advisable. Events like HYROX, which combine running with repeated heavy-load movements, demand not only cardiovascular endurance but also joint stability and muscular strength.

Sled pushes, lunges and burpees in particular can place considerable stress on the knees, lower back and ankles. Those who are generally inactive or have joint pain should learn proper form before adjusting exercise intensity and repetitions.

Consistency in measurement conditions also matters when tracking body composition. Commonly used body composition analyzers estimate muscle and fat mass by passing a small electrical current through the body, a method sensitive to hydration levels, meal timing and fluid shifts immediately after exercise. Repeated measurements taken under similar conditions and at similar times of day are therefore better suited to tracking long-term changes.

What stands out in Yoo I's case is that her skeletal muscle mass increased even as her body fat percentage fell. Rather than focusing solely on changes in body weight, combining adequate nutrition with both aerobic and strength training to manage body composition is a more effective path to building a healthy body.


rainbow@heraldcorp.com