|MadSci Network: General Biology|
This brings up some interesting issues, but not a definitive answer. You might answer that question several ways. If you define digestion as the chemical breakdown of food by enzymes to prepare it for absorption then only a few hours are required. However we usually talk about digestion AND absorption as a continuous process. Your example of 7 years could only refer to both digestion and absorption. If you think about how long the substance of a hot dog resides in the intestinal tract before being expelled, we know from studies that food passes through the body at different rates for different people, taking from around ˝ day to several days. But this is not digestion. If digestion is the process of breaking down food and absorbing nutrients from the food into the body to be carried to cells through the blood and lymph systems then much more time is involved. (This might be referred to as digestion, absorption and metabolism.) An enterprising young investigator surprised most of us with some very clever experiments in the 1950’s. He demonstrated that amino acids, fats and other components of foods are incorporated in most body tissues on a daily basis, that the tissues in our muscles and skin and organs exchange some of the structural molecules on a continuous basis. So, it is conceivable that a molecule of protein or fat from a hotdog might be incorporated into muscle or fat tissue and stay there for a long time. I agree that stating that these molecules would persist for 7 years is unlikely, but I would not suggest it is impossible. Another way you might give a time to “digestion” is to consider the rise and fall of blood sugar following a meal. The blood sugar rises from the fasting state and remains elevated for 4 to 6 hours after food is consumed. With this as a guide you might say it takes 6 hours to digest a hotdog. But if you want to count the time that all molecules from your food passes through your body and are gone from the body, then your estimate would be as good as mine. This makes the statement “You are what you eat” have REAL meaning. Phyllis Stumbo University of Iowa
Try the links in the MadSci Library for more information on General Biology.