When women lose weight, they usually want their hips and belly to shrink -- not their breasts. You can't control from where you lose fatty tissue when you lose weight, though. The amount your chest reduces depends on your genetics and the amount of weight you lose. There's no supplement or special strategy to prevent breast shrinkage during weight loss.
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Breast Tissue Basics
Breasts are composed of fat, as well as ductile and lobule tissue essential for breast-feeding. Larger breasts usually get their size from a greater amount of fat tissue. When your breasts shrink as you lose weight, it's because you've lost some of the volume in this fat tissue. Breast tissue fat works like other fat in the body. When you gain weight, the fat cells plump up. Likewise, when you lose weight, they deflate and you slim down.
Targeting Fat Loss
You can't target a specific body part for fat loss, and you can't protect one from it either. When you create a calorie deficit to lose weight, your body mobilizes fat cells from all over your body to convert their stored triglycerides into glycerol and fatty acids to use for energy. You can't pick and choose which fat cells are metabolized; your body loses weight in a pattern set by your genetics.
The amount of fat volume you lose from your breasts depends on their composition. If you have denser breasts -- meaning a lesser amount of fat tissue in relation to duct and lobule tissue -- you may notice less loss in your breasts as you lose weight. Medical imaging offers the only way to truly know the composition of your breasts.
Changes in Cup Size With Weight Loss
When you do notice your breasts shrink somewhat during weight loss, it's unlikely to radically change your bra size unless you lose a substantial amount of weight. It takes a 20 percent loss in your body weight to drop one cup size. That means a 200-pound woman would have to lose 40 pounds before noticing this change. Women who lose massive amounts of weight, such as 100 to 200 pounds, will probably see a decrease in breast size.
Be wary of what losing and gaining weight in a yo-yo pattern may do to your breasts. The skin can stretch out as they increase and decrease in size -- even if those changes are mild. The collagen and elastin that provide elasticity and spring to the breasts' skin prematurely wear out -- leaving you saggy.
Shrinking to a New Breast Size
Unfortunately, you can't do much to stop breasts from shrinking noticeably when you lose weight, if that's your genetic pattern. The health benefits of losing weight, though, should be a solid trade for the aesthetic change. If you drop even 5 to 10 percent of your body size, you experience positive changes in health markers such as blood pressure and blood sugar levels.
Exercises that promise to "boost" your breasts aren't effective. When you do chest presses, flyes and other breast-targeting moves, you're simply training the pectoral muscles that lie underneath your breasts. Training these muscles don't change the size of the tissue of your breasts. These exercises are important parts of a strength-training program, but don't expect them to preserve breast tissue size as you lose weight.