According to nutrition science calories, proteins, fats, carbohydrates, essential fatty acids, essential amino acids, vitamins and minerals required by the human body to grow and maintain good health need to be supplied through the food consumed. There are well laid down and universally accepted norms of nutrition vis-a-vis requirements of different nutrients. While carbohydrates in the food consumed provide bulk of the calories required by the body, fats account for about 30% of calorie needs. Proteins, though can supply energy, are used by the body mainly for building of tissues and generation of hundreds of enzymes that catalyze all the metabolic reactions in the body. Vitamins and minerals have their own roles to play in assisting various cellular functions. It is well accepted that a gram of carbohydrate and a gram of protein can supply 4 calories each under ideal conditions whereas a gram of fat yields 9 calories. A human being requires about 2000-2500 calories a day to maintain normal activity, about 50 gms each of proteins and fats besides recommended levels of micro nutrients and dietary fiber.
What happens to the food after it is eaten is well known and even an elementary school student can elaborate on the sequence of changes that take place once food is ingested. Just to recap, the teeth are provided to chew the food and after mastication the fluid material goes through the stomach for protein break down, followed by further break down of carbohydrates and fats to simple molecules in the small intestine, suitable for absorption into the blood. The residue after "digestion" goes into the large intestine for resorption of water and fecal evacuation. Of course the entire digestion process is so complex that only those having necessary specialized knowledge about human anatomy and physiology can explain the same correctly and correctly. It is generally believed that 50% of the food passsing into the stomach is emptied in about 3 hrs time while 100% leaves in about 4-5 hrs. Similarly 50% of the content coming to the small intestine gets out in about 3 hours while it takes 30-40 hrs for the entire post digestion residue to transit through the colon. Having stated this, it has to be admitted that there are significant variations in these values from person to person and from food to food. Those who do not have regular bowel movement or others eating foods with low fiber may have fecal transit time as long as 48-72 hours.
One may ask why this interest now on an old topic like digestion in human body? Provocation comes from two recent scientific reports that the relationship between calorie content of a food and its actual availability to the body after it is consumed may vary enormously and the calorie content declared on front of the pack labels needs to be moderated to factor the individual variations in digestibility from person to person. For example it has been reported that when nuts like Pistachio and Almonds are eaten about 5-20% of the fat present in these nuts appear undigested in the feces indicating that human GI tract is not able to get the fat in some foods completely released for the same to be hydrolyzed and absorbed. There is a parallel one can find in protein utilization also and that is the reason why the terms like Biological Value, Protein Efficiency Ratio, Net Protein Utilization etc have been coined to reflect the extent of protein used from a food by the body. Similarly Glycemic Index values are useful in understanding the efficiency of carbohydrate utilization in human body though this parameter has more relevance to diabetic people with insulin insufficiency.
While person to person variability, depending on many factors, is understandable, what complicates the issue is the nature of food consumed and the physical properties of the same. It is just common sense that food components in aqueous solutions are readily digested and absorbed fat, the process takes longer time and becomes more and more inefficient as the food eaten is under cooked or under processed putting more burden on the system. Food industry of modern days excel in making the product more and more GI tract riendly through the technologies used to process foods, most of them into ready to eat condition. Ultra mills, emulsifiers, homogenizers, grinders, extrusion cookers, pre-cookers, etc which are extensively used by the industry make the food readily digestible in double quick time.