This question is a combination of physics, chemistry, biology, and
fitness.  Taken in three related parts, it can be expressed as 1) If I
eat a pound of butter, I gain more than a pound of fat, so where does
the extra mass come from? 2) People talk about fat "changing" to
muscle, but muscle weighs more than fat, so where does that extra mass
come from? and 3) When I lose weight, is the mass burned for fuel, and
if not, what happens to it?  I would prefer a brief explanation with
links to some fitness or biology-oriented websites that explain these
processes (I already have plenty of sites about chemical reactions and
conservation of matter, this is specifically regarding biological
processes, and the person I'm trying to explain this to is not
satisfied with general chemistry and physics examples).

Hi tprjones -- 

I assume that what you are looking for is a "plain English"
explanation to answer your questions so you can explain them to your
friend.  I'll do my best!

I'll address your questions in order ---


1) If I eat a pound of butter, I gain more than a pound of fat, so
where does the extra mass come from?

This is a common misconception among dieters, but not true. Plain and
simple, you can't possibly gain more weight than the weight of the
food you eat. If you eat a pound of food (ANY food, whether butter,
carrots or a juicy hamburger), you will temporarily gain a pound, but
no more.  Then, as your body burns up calories, that initial weight
gain decreases.

The number of calories in the food versus the amount of energy you
expend will determine how much of the original pound of food you
retain.

For example --  a pound of fat equals about 3500 calories. A pound of
carbohydrates equals about 2000 calories. If you eat a pound of fat
you will retain the full pound. If you eat a pound of bread or apples
(mostly complex carbohydrates, the good stuff) you will only gain
4/7ths of a pound.

In any case, you can never gain more than the original pound you have
eaten. So, there is no "extra mass" gained.


2) People talk about fat "changing" to muscle, but muscle weighs more
than fat, so where does that extra mass come from?

Again, this is a myth. Fat does not turn into muscle. Muscle does not
turn into fat. Biologically they are two different kinds of cells. The
kind of cell you build, fat or muscle, depends on your exercise regime
and the food you eat.

Yes, muscle weighs more than fat. That much is true, so if the
calories you consume are converted to fat, you will weigh LESS than if
the calories you consume are converted to muscle.  This is why when
people begin a weight loss program that includes exercise; they often
gain weight, even though they look thinner. (Muscle takes up less
space than fat too).

So again, there is no "extra mass" gained. 

By way of further explanation ---

We are biologically designed to use food in the most effective manner.
 What we need we use, what we don’t need we store.

When we go from being chip-eating couch potatoes to diet and exercise
mavens, we change our body's priorities from storage to usage.

When we exercise, fat is burned first. Fat is our biological storage
unit for calories. It's where we "save food" for a rainy day.  When
sudden exercise kicks in, our body says "Whoa, I need more energy,
better start drawing on the reserves."  And we begin to burn fat.

Also, as a separate process, when we exercise we build muscle because
our body "notices" that we are now using our muscles so begins to send
energy there. The muscles are built directly from the calories we
consume. Again, we won't gain more than a pound of muscle for each
pound of food we eat. In fact, it will be much less since it takes
more energy (calories) to build muscle.

When we exercised, the calories we ate "went to" the muscles instead
of the fat because we are designed to send calories to the places in
our bodies where we need them most. Muscles being exercised a lot need
calories.

If NO muscles are being used, and no other biological functions (such
as pregnancy for example) require extra calories (beyond maintaining
the body), then we just store them as fat.

A brief aside on that topic, if we burn up all of our fat and then
continue to burn more calories than we eat, our body begins to shut
down non-essential functions and also begins to burn muscle mass. As
we continue to deprive our body of energy, even essential systems shut
down. This is the danger that anorexics face. Eventually the body just
can't sustain itself.


3) When I lose weight, is the mass burned for fuel, and if not, what
happens to it?

If, by "mass" you mean the mass of the food, Yes, because we are
talking about a calorie, a unit of heat or energy. When you lose
weight it is because you have expended more energy than you consumed.
This means that you have not only burned off all the food you just ate
(by building muscle and running other bodily functions), you also drew
from your reserves (fat). So yes, you "fuelled" your body with the
mass you consumed.

If you are talking about the mass of the fat we burn, the answer is
still Yes. Our bodies "burn" calories through metabolic processes, by
which enzymes break the carbohydrates (starches) into glucose and
other sugars, the fats into glycerol and fatty acids and the proteins
into amino acids.

These molecules are then transported through the bloodstream to the
cells, where they are either absorbed for immediate use or sent on to
the final stage of metabolism in which they are reacted with oxygen to
release their stored energy.

It helps here to understand the concept of the calorie ---- 

Often the misconceptions about weight gain (and loss) are a result of
people not understanding what we mean by a "calorie". They often think
a calorie is a little "thing" that has to be gotten rid of.   A
calorie is a unit of energy (or heat).  For food it defines how much
energy is needed to burn of a unit measure of a particular food.  For
exercise it means how much energy your body is burning.  So all this
means is that the more calories a food *has* the more energy it takes
to burn it up.


Here are some useful links from which I drew some of the previous
information ---------


"…they think that, if you eat a pound of chocolate, you can gain
more than a pound. I can't understand where the extra weight would
come from."

http://www.homestead.com/prosites-waynewatcher/Support_FatFact.html

A pound is a pound and no more.
http://www.myjanee.com/livelight/eat/weigh.htm

Fat to Muscle
http://shop.store.yahoo.com/homegym/fattomusrevb.html

How is it possible to lose inches but not pounds?
http://www.gymamerica.com/gti/magazine/magazine_qa/0,3291,1_cid_536,00.html

What is a Calorie?
http://www.howstuffworks.com/calorie1.htm

How food works
http://www.howstuffworks.com/food.htm

The Caloric Concept of weight control
http://nbaf.com/nbaf/feb7pgf.htm

How your Metabolism Really Works
http://www.metabolism.com/lifestyles/2001-05-22/

Metabolism - A primer
http://www.mayo.edu/comm/mcs/news/news_1608.html


I've put all of this in the simplest biological terms I could. The
links should provide you with more in-depth discussion. If anything
I've said is not clear, please feel free to ask for a clarification.

Thanks for the great question -

--K~

Search terms

"eat a pound" weight gain
"fat to muscle"
"what is a calorie"
"how we lose weight"
"how metabolism works"

Thank you for a marvelous explanation, k_s!  I think that should do nicely!

"If I eat a pound of butter, I gain more than a pound of fat, so where
does the extra mass come from?"

According to most sources, a pound of butter supplies 3200 calories.
In order to gain a pound of body fat, you must consume 3500 calories.

Cool! Interesting question, and thanks for the answer :)

I believe your answer is not correct.
See the USDA Nutrient Database for Standard Reference
http://www.nalusda.gov/fnic/cgi-bin/nut_search.pl

Any of the oils contains roughly 123.7 Calories / .5 ounces or 14
grams
16 ounces in 1 lb
123 * 2 * 16 = 3936 calories
3500 calories in 1 lb of fat and for digestion / thermic effect of
food roughly 3% of calories are used (fat is stored more effeciently
then carbohydrates and protein)
so we have a net gain of 3817.92 Calories which is more than enough
for a pound of fat mass.

The same can be said of lard and other very fatty foods.  It seems
that the weight is not the issue but rather the energy contained in
the food.... the laws of thermodynamics are what comes into play.