Yes, your rib cage can become smaller when you lose weight. This occurs due to losing subcutaneous fat, the layer of fat directly below the skin. Weight gain results from consuming more calories than you use in activity.
The body stores excess calories in fat deposits and this affects your circumference at the rib cage. Although there are variations in body fat distribution, most people lose upper body fat before lower body fat.
Your rib cage can appear to become smaller when you lose the subcutaneous layer of fat beneath your skin.
Bones Don't Lie
Most areas of your body, including hips, torso and even feet and fingers, can become smaller when you lose weight. Bones and body frame size don't change with weight loss, so the seemingly wide rib cage itself won't change in size. But as you lose overall body fat through weight loss, your measurements decrease.
Many people store body fat on their backs and throughout the torso. As this layer of fat over your rib cage and back decreases, your body's size at the rib cage becomes smaller. Women may reduce their bra band size and both sexes will fit into smaller clothing as weight loss progresses.
Body Fat Distribution
Body fat includes subcutaneous fat and abdominal fat. In some people, particularly men and women with the "apple" shape, body fat distribution includes a significant amount of abdominal fat. Abdominal fat often increases with age, due at least in part to age-related muscle loss, according to Harvard Health. Inactivity can accelerate this process.
Abdominal fat increases the size of your torso and rib cage. Deep abdominal fat known as visceral fat covers your internal organs and increases your risk of heart disease and diabetes.
Exercise is the most effective way to burn off all forms of body fat, including both forms of abdominal fat. Combining exercise with a healthy, moderate calorie-restricted diet can help you reach a smaller rib cage size.
Ratio of Muscle to Fat
Exercise helps to make your torso smaller at your rib cage by improving your metabolism and building lean muscle, which takes up less space than body fat pound-for-pound. Exercise, especially weight-bearing exercise and strength training, also increases bone strength — helping to prevent the risk of osteoporosis and fractures according to the Mayo Clinic.
One note: What looks like a large rib cage in a man who lifts weights may not develop into a smaller rib cage size during weight loss, because men who put on a lot of back muscle can actually increase the width of their torso.
In this case, the body develops a healthier ratio of muscle to fat and will have a smaller waist size. For physical fitness and weight loss, perform at least 30 minutes of moderate-intensity cardio five days a week and at least two full-body strength training sessions each week.
Shrink It Down
Women who lift weights build lean muscle too, but it takes a lot more concentrated effort for a woman to add large muscle than a man, thanks to the different hormonal balance between the genders. Because muscle is more compact than body fat, shedding body fat and building muscle may make what appears to be a large rib cage in a woman seem to "shrink," even if her body weight doesn't change much.
To aid weight loss for either gender, concentrate on creating a calorie deficit where you burn more calories than you consume, according to the National Heart, Lung and Blood Institute. The NHLBI recommends that you reduce your daily calorie intake by 500 to 750 calories for a safe rate of weight loss; that would generally yield a loss of 1 to 1.5 pounds per week. Don't reduce daily calories below 1,200 to 1,500 for women and 1,500 to 1,800 for men.
Read more: Recommended Caloric Intake for Weight Loss