The consensus among unbiased nutrition experts is that meat and other protein foods remain in the digestive system longer than fruits and vegetables, and that we should not confuse the digestibility of food with the time that food remains in the digestive system. For example, beef is "highly" digestible when compared to "less digestible" fruits and vegetables. Yet, beef remains in the digestive system significantly longer than fruits or vegetables. This has to do with how digestive enzymes break down the molecules of various foods.
But, its not just about how digestive enzymes work. Just as importantly, it is about the digestive health of the individual. Many Americans today have low levels of digestive enzymes and, hence, an impaired digestive system. Causes of insufficient enzyme production include aging, overuse of antibiotics and certain other medications, and abuse of alcohol and drugs. But, perhaps the most common cause is the high-fat, low-fiber Standard American Diet (SAD) which consists of mostly cooked and processed foods, including junk and fast food, and is sorely lacking in whole, unrefined foods.
A person with low digestive enzyme levels can expect to have problems digesting various kinds of foods - not just meat. That said, meat and other animal proteins are devoid of dietary fiber and are generally high in saturated fat, which happens to be difficult to digest. Here, plant-based protein sources may have some advantage. Many plant-based protein sources do not have saturated fat and, those that do, generally also contain fiber. For example, many types of beans commonly contain saturated fat. But, generally, beans are also high in fiber, which is thought to aid in the digestive and elimination process.
Regardless of what side of the meat consumption issue you are on, it seems that a person's ability to efficiently digest protein (from whatever source) is highly individual and depends in great part on his or her dietary history and general digestive health.
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