Stool color is usually explained by diet, but certain shades — especially tarry black or unexplained bright red — can signal a condition that needs medical attention.
Yellow bowel movements are very common and are not necessarily bad. In young babies, it can simply be a sign that they are breastfeeding. However, yellow bowel movements that look and smell unpleasant may indicate that your digestive system isn't properly processing fat. This is especially common in people who have had their gallbladder removed or who have been diagnosed with celiac disease, along with other digestive tract conditions. Undigested fats will appear greasy and will smell bad, which is when it is time to see a doctor. Your physician can help you to adjust your diet to prevent this.

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