Final answer:
Osmotic diarrhea typically improves with fasting as non-absorbable substances are no longer consumed, while secretory diarrhea does not improve during fasting because it's caused by excessive secretion into the intestines which persists regardless of food intake.
Step-by-step explanation:
Osmotic diarrhea usually improves with fasting because the cause of this type of diarrhea is often due to the presence of non-absorbable substances in the intestines, which draw water into the gut. When fasting, the intake of these substances decreases, leading to an improvement in symptoms. On the other hand, secretory diarrhea usually does not improve during fasting as it is caused by the excessive secretion of electrolytes and water into the intestine, often due to an underlying disease or the secretion of enterotoxins.