Many books, papers, and articles state that one pound of human body
fat has the energy equivalent of 3,500 calories. Unfortunately, I have
never seen a primary reference cited for that statement. Note that 9
calories per gram of fat and 453.6 grams per pound multiply out to
4046 calories per pound of fat. So presumably the difference has to be
accounted for by water content or other cell types or tissue content
in adipose tissue.  I have found a primary reference for the statement
that ?9 calories per gram of fat? at: Merril, Annable L., Watt,
Bernice K: Energy Value of Foods . . . Basis and Derivation, Human
Nutritional Research Branch, Agricultural Research Service, United
States Department of Agriculture, Agriculture Handbook No. 74,
Slightly revised February, 1973. See especially table 20 on page 49.
At: http://bookstore.gpo.gov/support/index.htmlhttp:/www.nal.uhttp:/www.nal.usda.gov/fnic/foodcomp/Data/Classics/ah74.pdfsda.gov/fnic/foodcomp/Data/Classics/ah74.pdf

A primary reference is one that is written by the original researchers
who determined the figure, or a well respected reference source (e.g.
well respected encyclopedia, dictionary, handbook, or desk reference)
that provides the information.

Such a great question! You got me thinking as well. I am a Registered
Dietitian and this is something we always learned. I hit some
resources that I have lying around that I thought might be helpful.

American Dietetic Association Complete Food and Nutrition Guide 2nd
edition, Roberta Larson Duyff,MS,RD,FADA,CFCS; page 29.

Krause's Food, Nutrition and Diet Therapy, L.Kathleen Mahan, Marian Arlin;
pg. 323

quote: "The diet should furnish 500 kcal/day less than maintenance
requirements. This daily deficit should provide a loss of .45 kg
(1lb)/wk (National Institutes of Health, 1985)."

Hope this helps you out!


www.reallivingnutrition.com

While this is not a primary source, it does provide an explanation of
the discrepancy.

http://www.hhp.ufl.edu/keepingfit/ARTICLE/fatcalories.HTM

Basically 1 pound of body fat is 10% water.  The other 5% represents
calories consumed that pass through the body without being absorbed.

Hopefully the information above helps you track down a primary source
for the information that you seek.

The kcals are determined from bomb calorimetry.  Thus, the 9
kcals/gram of fat is the amount of energy created when a gram of fat
is combuted/oxidized.  To the best of my knowledge, I have not seen a
primary reference for the 3,500 kcals/lb of fat.  As one noted on the
ufl.edu link that 1 lb of fat is not all fat (15%  other -- water,
etc.) -- thus, the discrepency.  The key aspect to understand is what
is the ultimate question.  For example, how many calories are in a
pound of stored human fat? (3,500) is different than how much energy
is needed to "burn" a pound of fat? (4,086).  The other piece to the
puzzle is how much of each fuel (carbs, fat, protein) are being
oxidized -- it is theoretically possible to oxidize only fat.  So, it
will require more energy expenditure to burn that 1 lb. of fat.  That
said, if someone does exercise and oxidize all fuels, the amount of
fat stored over time will be dependent on energy intake even if 50% of
energy was expended by carbs and fat (assuming protein oxidation is
negligible).  This is critical as fat oxidation from stored fat will
occur at a greater rate if energy intake is decreased.

The kcals are determined from bomb calorimetry.  Thus, the 9
kcals/gram of fat is the amount of energy created when a gram of fat
is combuted/oxidized.  To the best of my knowledge, I have not seen a
primary reference for the 3,500 kcals/lb of fat.  As one noted on the
ufl.edu link that 1 lb of fat is not all fat (15%  other -- water,
etc.) -- thus, the discrepency.  The key aspect to understand is what
is the ultimate question.  For example, how many calories are in a
pound of stored human fat? (3,500) is different than how much energy
is needed to "burn" a pound of fat? (4,086).  The other piece to the
puzzle is how much of each fuel (carbs, fat, protein) are being
oxidized -- it is theoretically impossible to oxidize only fat.  So,
it will require more energy expenditure to burn that 1 lb. of fat. 
That said, if someone does exercise and oxidize all fuels, the amount
of fat stored over time will be dependent on energy intake even if 50%
of energy was expended by carbs and fat (assuming protein oxidation is
negligible).  This is critical as fat oxidation from stored fat will
occur at a greater rate if energy intake is decreased.