Why Does Working Out Make Breasts Smaller?

A woman is bench pressing in a gym.
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One of the many reasons the average person works out is to see a change in physique. One unexpected change that many women experience is a reduction in breast size. Exercise helps you reach a caloric deficit. Over time, using more calories than you eat forces your body to use its fat stores, including those in the breasts.


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Breast Anatomy

Your breasts are made up of lobes, lobules, milk ducts, lymph nodes, blood vessels and connective tissue, although a large percentage of your breasts are fat. The breasts are one reason why a woman has a larger percentage of body fat than a man.


Aerobic Exercise

When you participate in an aerobic exercise routine, you use fat for fuel. During prolonged bouts of mild- to moderate-intensity exercise, the primary source of energy is fat. There is no way to spot train to tell your body where to take fat from. Your body will use fat from any storage area, including the fat inside your breasts. The rate and area of fat use depends on your gender, genetics and age.


Chronic Adaptations

The more you work out, the more fat is used during exercise. Using fat as fuel takes extra work from your body. It must be liberated and transported from its storage site into your working muscles. A chronic adaptation of regular exercise is the ability to use fat as fuel during higher intensity exercise. This mechanism spares glucose and glycogen stores either for later, more intense exercise or for the cells that can only run on glucose, like the brain. So, the fitter you become, the more you may be using your breast fat as fuel.


Consider Resistance Training

If you are concerned about the look and shape of your breasts, consider adding some chest resistance exercises to your routine. Try the chest fly and press. You can perform these exercises either lying on the ground or supported on your back with a stability ball. For the chest fly, start with a dumbbell in each hand. With your arms extended out to each side and your palms facing up, slowly lift each arm up above your chest until your hands meet, then slowly lower them back down to start and repeat. For the chest press, start with a weight in each hand and your arms positioned so that your elbows are bent and your hands are even with your sternum. Slowly push the weights up above your chest by straightening your elbow, hold for a second, then lower them back down. Work up to performing one to three sets of 10 to 15 repetitions.