You'll see magazine articles galore promising you an exercise solution to perk up sagging breasts or increase your size. Your breasts, however, are made up mostly of fat tissue, and you can't tone fat with exercise. You can only lose it — to reduce size — or strengthen the muscles around your chest to create the illusion of size and lift.
Because your breasts are made up of fatty tissue, you can't "tone" them as you'd tone a muscle. They will grow or diminish according to the way your genetics store (or burn) fat in your body, however.
Fat Loss and Your Breasts
A woman's breasts are made up of fat tissue, with some duct and lobule tissue mixed in for breastfeeding purposes. Some women are genetically predisposed to have more generous fat deposits in their chests, while others are less well-endowed. Your weight also affects the size of your breasts. So if you exercise with lots of cardio to lose fat, inevitably you'll lose some volume from your chest.
You can't be sure how much volume you'll lose — some women find that the moment they start to lose weight, their breasts diminish in size. Other women lose breast weight last and can achieve slim hips, thighs and stomachs without losing a cup size. This is just the way the DNA dice roll. You can create the environment for fat loss, but from where in your body you lose that fat is up to your personal genetic code.
Female bodybuilders and other competitive athletes may appear to have small breasts because they're at such low levels of body fat. Their breast size isn't due to the type of exercise they do, it's because much of the fat in their breasts has shrunk.
Muscle Gain in the Chest
When you break down muscle with resistance training, it grows back thicker, stronger and often more voluminous. Fat doesn't respond the same way. So you can't simply do chest exercises, such as pushups and chest flyes, in hopes of changing your cup size.
Your pectoralis major muscle does lie underneath the breast tissue, however. When you build this muscle, you might experience a slight increase in the circumference of your chest and a mild lift to the breasts themselves.
Exercise for Breast Health
Exercise can definitely offer a guaranteed positive to a woman's breast health. A meta analysis published in a 2009 issue of Methods in Molecular Biology confirmed that exercise and physical activity have a beneficial role in breast cancer prevention and rehabilitation.
Exercise helps a woman maintain good cardiovascular, respiratory and immune health. It also affords women suffering from breast cancer psychological and social benefits, according to a 2012 study published in Women & Health. Women who enrolled in a year-long regular exercise program at various phases of their diagnosis reported feeling greater control over their bodies, greater acceptance of their diagnosis and improved mental states.
A Complicated Relationship
It might be more important to consider the effects the breasts have on exercise, instead of exercise's effects on the breast. In a 2015 issue of the Journal of Physical Activity & Health, a study by British researchers identified breast insecurities to be the fourth-greatest barrier to physical activity among 250 women surveyed. These women expressed feeling self-conscious about excessive breast movement and discomfort from poor-fitting bras during exercise. Motivation, time and health were the first three major barriers to women becoming more active.