Which molecule can diffuse from the digestive tract into human bloodstream without first being digested?

Glucose.

Explanation:

Glucose can diffuse first from the digestive tract into human bloodstream without being digested. Glucose gets digested in both the stomach and small intestine. When the food gets digested in the stomach then the carbohydrate present in the food breaks into smaller molecules of sugar which is also known as glucose.

The stomach and small intestine both absorbs the glucose and release it into bloodstream. That's why our body needs to store glucose for energy.

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