Huffpost Healthy Living

What Is Metabolism: How Long After I Eat Is Weight Added To My Body?

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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.

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