An Article on Metabolism

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"How long does it take for the food I ate to add weight to my body?" -- 
Bunter122

Today's question comes from a reader who would prefer to stay 
anonymous, but it is certainly a thought that many people have had. 
Given that we all have different metabolic speeds, what is the average 
amount of time it takes food to be digested and then shunted off to its 
various purposes?

"We have a pretty good indication about the timeline from the sequence 
of ingestion, digestion, and egestion. The whole thing plays out over a 
span of roughly 12 hours on average," says Dr. David Katz, a HuffPost 
Healthy Living contributor and director of the Yale University 
Prevention Research Center. "By the time the residual waste of food is 
passing out of us, the useable parts have all been put to use -- for 
energy expenditure, as heat generation, or placed into storage, as 
either glycogen [from carbohydrate], or fat."

After you eat, the food passes through your stomach and small intestine 
in about six to eight hours, according to the Mayo Clinic. Then, it 
enters the large intestine where it is further digested. Putting it as 
delicately as possible, Mayo Clinic gastroenterologist Dr. Michael F. 
Picco writes: "Elimination of undigested food residue through the large 
intestine usually begins after a total of 24 hours. Complete 
elimination from the body may take several days."

For a complete understanding of what the body does to digest food, the 
National Institutes of Health has a great explainer. For our purposes, 
it is essential to understand that as food is digested, enzymes and 
hormones are secreted by the stomach and small intestines (as well as 
fat cells) and work to break down the carbohydrates, protein and fat 
we've eaten into usable material for our cells.


A growing number of endocrinologists -- those who study our hormone 
system -- are beginning to see the hormone response as far more 
complicated than previously thought, as well as far more 
individualized: there is a wider variance in how people respond to 
food, they suspect.

"While the question posed seems extraordinarily simple, I don't think 
that there can be a simple answer to it," says Dr. Kaveh Ashrafi, an 
associate professor of physiology at the University of California, San 
Francisco School of Medicine who studies fat metabolism. "The 
prevailing view essentially considers feeding behavior/exercise as the 
sole determinants of body weight. For this to be true, one has to 
assume that the body itself is simply an inert vessel; if you put more 
food in it, it must get fat, if you move it, it must lose weight."

Ashrafi offers this analogy in communication with HuffPost Healthy 
Living:

If one lives in a shack with broken windows, no insulation, and a 
caved-in roof, changes in the temperature outside correlate fairly well 
with changes in the temperature inside of the shack. However, if one 
lives in a modern home, with wonderful walls, windows, roof, and a 
sophisticated heating/cooling system, the relationship between the 
outside and inside temperate becomes much more difficult to guess 
easily. If the thermostat is set at 70°C, it can be -20 or +120 outside 
yet inside will be around 70. Bodyweight regulation is much more like 
the temperature regulation of the modern house than that of the shack 
-- the prevailing view, in my opinion, treats it as if it were a shack.
So while it is easy to say how long it takes for your body to digest 
food, it is very difficult to answer the question, how quickly does it 
add weight to my body? That's a very individual phenomenon, based not 
only on the make-up of someone's diet, but on their unique hormonal and 
physiologic response.