Singer Shin Ji's shock confession: what happens to your body when you lack muscle

Singer Shin Ji's disclosure that her weight once fell to 42 kilograms, along with her experience of being told she had a serious muscle deficiency, has resurfaced in public discussion, drawing fresh attention to the health effects of being excessively thin.

Shin Ji recently posted photos of her current appearance on Instagram. When fans expressed concern that she looked thinner than before, she said she was healthy, adding that she had "just had a rough time with a cold."

Earlier, in a YouTube video in July, Shin Ji shared the results of a body composition test showing she was deficient in muscle mass. A traditional Korean medicine doctor advised her to gain about 12 kilograms of muscle and maintain her weight in the low 50-kilogram range.

Even people who weigh relatively little can experience problems with physical function and metabolic health if they lack sufficient muscle. Beyond enabling movement, muscle plays a role in regulating blood sugar, supporting bone health and aiding physical recovery. Maintaining not just muscle mass but also actual muscle strength and muscle quality is particularly important.

Low muscle mass — even at a low body weight — can harm blood sugar control and bone health.

Muscle is one of the body's primary energy-consuming tissues. It enables movement and posture, and also absorbs and stores glucose from the bloodstream.

When food is consumed, blood sugar rises and the pancreas releases insulin, which helps glucose move from the blood into various tissues including muscle and fat.

Skeletal muscle in particular is a key tissue for processing glucose after meals. Because glucose uptake is also promoted when muscles contract, regular strength training helps improve insulin sensitivity and maintain stable blood sugar levels.

A study published in 2025 in the international journal Physiological Reviews emphasized that skeletal muscle is a primary organ for storing glucose after meals and using it as energy during physical activity. The researchers said exercise-stimulated glucose uptake in muscle plays an important role in improving insulin resistance.

When muscle mass is severely lacking or physical activity declines, these metabolic functions can deteriorate. Even at a low body weight, people who have a high body fat ratio relative to muscle and insufficient physical activity should pay attention to their metabolic health.

Muscle loss also affects bone health.

When muscles contract, they transmit physical stimulation to bones through tendons. This stimulation is involved in bone formation and maintenance, and consistent strength training can help slow the decline in bone density.

Conversely, prolonged severe dietary restriction and nutritional deficiency can deplete the energy and nutrients needed to maintain both muscle and bone.

The US National Institute of Arthritis and Musculoskeletal and Skin Diseases says strength training and weight-bearing exercise help strengthen bones and prevent bone loss.

Weakened muscles also reduce the ability to maintain balance, increasing the risk of falls and fractures. In middle-aged and older adults in particular, the simultaneous decline of muscle strength and bone density can place a significant burden on physical function.

Low grip strength raises respiratory disease risk by 31% — muscle 'quality' matters too.

Muscle condition has also been found to be closely linked to respiratory health.

A 2025 study published in the Journal of Cachexia, Sarcopenia and Muscle analyzed the relationship between muscle-related indicators and the risk of developing respiratory disease among 17,510 Chinese adults.

Researchers from the University of Oxford in the United Kingdom and Peking University in China, among other institutions, participated in the study. Participants were adults between 38 and 88 years old, and the researchers tracked their health for about 4.8 years.

The group with low grip strength had a 31 percent higher risk of developing respiratory disease than the group with high grip strength.

The group with low arm muscle quality also showed a 25 percent higher risk of respiratory disease. Muscle quality was assessed as the ratio of grip strength to arm muscle mass.

By contrast, no clear association was found between overall muscle mass indicators and the incidence of respiratory disease.

The researchers concluded that muscle strength and muscle quality may be more useful indicators than muscle mass alone for assessing respiratory health.

Muscle also plays an important role in breathing. The diaphragm and the intercostal muscles between the ribs regulate chest cavity movement, allowing air to flow in and out of the lungs. Various respiratory muscles and abdominal muscles are also engaged when coughing.

A general decline in muscle strength throughout the body can affect respiratory function and the ability to cough effectively.

However, the study was an observational study analyzing the association between muscle strength and the incidence of respiratory disease. It did not demonstrate that improving muscle strength alone can prevent respiratory disease.

When managing muscle condition, it is important to look not only at muscle mass as measured by body composition tests but also at actual physical function.

Grip strength, the ability to repeatedly rise from a chair, and movement when climbing stairs can all be used to assess muscle strength. If a person tires more easily than before or finds everyday movements more difficult, it may be worth checking for a decline in muscle strength.

Building muscle requires strength training at least twice a week — and total caloric intake matters as much as protein.

Building muscle mass requires combining strength training with adequate nutrition.

WHO recommends that adults get at least 150 minutes of moderate-intensity aerobic exercise per week, along with muscle-strengthening activities at least twice a week.

Strength training can include squats, lunges, push-ups and dumbbell exercises. Using major muscle groups evenly and gradually increasing exercise intensity according to one's fitness level is the most effective approach.

Beginners are advised to start with movements they can repeat eight to 12 times with a light weight. As they grow comfortable with the movements, they can gradually increase the number of repetitions and the weight to provide new stimulation to the muscles.

Muscle grows as it adapts to the stimulation generated during exercise. Sufficient rest and nutrition after exercise are necessary for muscle protein synthesis to proceed smoothly.

Protein in particular is the key nutrient for building and repairing muscle.

The International Society of Sports Nutrition suggests that 1.4 to 2.0 grams of protein per kilogram of body weight per day is sufficient for muscle building and maintenance in healthy adults who exercise regularly.

For a person weighing 50 kilograms, that range works out to roughly 70 to 100 grams per day. However, actual requirements vary depending on activity level, health status and dietary composition.

Protein can be obtained from a variety of foods, including eggs, fish, meat, tofu, legumes and dairy products. Spreading intake across multiple meals rather than concentrating it in one sitting makes it easier to structure a balanced diet.

For people who are significantly underweight or who regularly eat too little, getting enough total calories — not just protein — is also important.

When energy intake is insufficient, the body draws on stored energy to make up the shortfall, and prolonged nutritional deficiency can increase the breakdown of muscle protein.

Carbohydrates supply the energy needed for exercise, while fat is involved in cell function and hormone production. Maintaining a balanced diet alongside consistent strength training is the most effective approach to building muscle.

Even when trying to lose weight, exercise and diet should be managed in a way that minimizes muscle loss.

If sudden weight loss occurs or stamina continues to decline, it is advisable to check nutritional status and whether any underlying conditions are present. Managing not just the number on the scale but also muscle strength, physical function and dietary habits is key to maintaining long-term health.