MadSci Network: General Biology

Re: how long does it take for the calories in your body to transform into fat?

Date: Tue Mar 29 17:58:19 2005
Posted By: Peter Bosani, Independent
Area of science: General Biology
ID: 1108418103.Gb

Hello, Lindsey.

Just to clarify, calories do not transform food into fat, nor are they 
stored.  When food is oxidized, (burned), it provides the body with 
energy.  The amount of energy a particular food source provides is 
measured in calories.  Therefore, calories are simply a unit of measure.

The body's first priority is to produce energy.  After one's energy 
requirements have been met, other nutrients are used to build and repair 
tissue, and carry out functions that essentially keeps us alive and 
healthy.  Any calories obtained from food in excess of what we really 
need, is converted into fat and stored.  Eating fat, does not make one 
fat.  The conversion to fat can just as well come from sugars, protein, or 
alcohol, as long as more is being consumed than is actually required.  For 
every 9 kilocalories of food eaten in excess of the body's requirements, 1 
gram of fat is stored.

The transformation of food into fat involves the breakdown of food into 
simpler components, a task handled by enzymes.  Depending on the length of 
the fat molecules, they may have to be rebuilt prior to leaving the small 
intestine.  Nutrients are not really "in" the body, until they get 
absorbed into the circulatory system.  Once there, they usually head to 
the liver for furthur processing, where they leave wrapped in special 
protein coats, destined for some of the 12 to 18 trillion fat cells in an 
average adult body.

It is impractical to give you a precise timeline for the conversion of 
food into fat, because there are too many variables.  Are you referring to 
the formation of fat while still in the small intestine, or only when the 
fat gets stored into fat cells?  The latter occurs at a later time.
Rates of digestion are very variable, ranging from 6 to 14 hours, in the 
stomach and small intestine alone!
A faster metabolism will convert excess calories into fat more readily 
than a slower one.  Muscular people likewise have higher metabolic rates.
The type of meal or food one is eating is important, because in the race 
to be digested, carbohydrates advance faster than proteins, while fats are 
the slowest.
Age is another factor, as older people have slower biochemical processes 
than younger people.
Hormones count too, - higher insulin levels cause fats to be formed more 
quickly than lower insulin levels.
Neither is the presence of fat in the bloodstream gauged to your latest 
meal.  Rather, there is a constant dynamic exchange between the fat stored 
in tissues and the bloodstream.  An equilibrium is maintained throughout 
the day.

If we must come up with an estimation of a timeline for fat formation from 
excess calories, we would need to disregard the previously mentioned 
variables, and focus solely on average digestion times, prior to its 
absorption into the bloodstream.  With this in mind, fat can be formed at 
approximately 5 to 8 hours, on average, from the start of a meal.

However, in my opinion, our focus should not be diverted by how fast fat 
is being formed, but rather, on how quickly we're getting fat.

As the comedian Buddy Hackett once said, "I was reading one of those 
weight-and-height charts the other day, and I discovered something:  I'm 
not too fat, I'm just too short."

Hope that helps.
Peter Bosani.

For furthur reading on food and digestion, log on to and 
key in my answer to - How long does the entire digestive procees take?   

References:  www.howstuffworks
             Wholesome Diet - TimeLife Books
             Human Body - Life Science Library     

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