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Your symptom: You lost a noticeable amount of weight without trying to.

Best-case scenario: Truly unintentional weight loss is almost always bad, says Gross, but it could be due to a serious (but rarely fatal on its own) condition like Crohn's disease, which makes it harder for your body to absorb nutrients from food. Or if you've also been peeing more than usual, it could be Type 1 diabetes, which typically affects children and young adults but is possible at any age. You cells can't get the sugar they need to produce energy, so your body burns fat and muscle instead, leading to an inadvertent slim down.

Why it could be more serious: An unexplained weight loss of 10 pounds or more can be the first sign of cancer, and it's most commonly linked to cancers of the pancreas, stomach, esophagus and lungs. Cancer cells drain energy from your body to fuel their growth, which may explain the smaller numbers on the scale. If your normal weight is down by at least 10 pounds within the last 6 to 12 months and you really didn't make any changes to your activity levels or eating habits, bring it up with your doctor. That's particularly important if you've got risk factors like a history of smoking.