"Sucking in the stomach and keeping your abdominal muscles tense as you go about your day might seem innocuous, but... [it] can have physical and mental consequences over time."

"Beyond potentially affecting the pelvic floor muscles, which are involved in posture, urination, bowel movements and sex, sucking in your gut all the time 'could alter the mechanics of your abdomen; it could alter its ability to respond to demands in the environment.... It could change your breath patterns.'... Habitually contracting your oblique abdominal muscles can exert force down on the pelvic floor muscles and potentially cause the pelvic floor to become overwhelmed, which could have consequences such as incontinence and pelvic organ prolapse.... 'When you activate those upper abdominal muscles, you’re creating an increase in intra-abdominal pressure at baseline.... Many people’s fixation with having a flat stomach, or at least the appearance of one, has largely been fueled by a well-intentioned idea about health and fitness — having a strong core — that has been taken to the extreme.... While you should engage those muscles if you’re lifting a barbell, you shouldn’t be tensing them at full strength when you’re standing in line at the grocery store.... Consistently sucking in your stomach could alter the appearance of your abdomen, including a protrusion of the lower abdomen and a flaring of the rib cage. To begin retraining your muscles, experts recommend consciously trying to relax and let your stomach out...."

From "Stop sucking in your stomach all the time — it could be bad for your health" (WaPo).

Who knew this was a problem?! I suspect it's symbolic of a whole set of problems — shallow overachievement leaving you in worse condition than if you would just relax and take it easy.