A body is composed of water, muscle, bone, vital
organs and fat. Adipose tissue (body
fat) is a normal constituent of the human body that serves the important
function of storing energy as fat for metabolic demands.
There are different types of fat cells that make
up fat tissue. What most people are
referring to when they speak about the biology of “fat” are white fat cells,
especially those located just under the skin (i.e. subcutaneous fat).
Large white
blobs (termed “lipid droplets”) in the white fat cells are what house our
triglycerides (TAGs). The lipid droplets grow and shrink as TAGs are used by
the body metabolism.
Chemically speaking, TAGs come in different
varieties, but are always made up of a glycerol molecule plus three fat molecules.
You may have heard of Omega-3 Fatty Acids, which
have the third carbon from the right with a double bond. To clarify “saturated”
versus “unsaturated” fat: This terminology has to do with the “saturation” of
hydrogen atoms around the carbon atoms. Unsaturated
fat has double bonds between one or more of its carbon atoms (i.e. the double
line things) and saturated fat are those that don’t.
Know that
your body needs both to function properly.
What causes TAGs to accumulate in lipid droplets
within white fat cells? High blood
glucose levels, mainly
In order for our bodies to start pulling TAGs
out of lipid droplets, the appropriate signals need to be sent to receptors on
the surface of fat cells. When glucose
resources are low, this activates a wide variety of hormones including (to
varying degrees and roles) epinephrine, norepinephrine, corticotropin (ATCH),
thyroid stimulating hormone (TSH), cortisol, ghrelin, testosterone, human
growth hormone (HGH), adiponectin, cholecystokinin (CCK), glucagon, and leptin.
To lose weight all you need to do is keep
glucose levels down, make sure that these hormones are triggered effectively,
and eat fewer calories than you burn.
This
simple concept would be ideal but a body never functions relying on a single
process. The blood glucose levels alone do not control
metabolism. Everyone has a different
“metabolism” (i.e. the rates and efficiencies of the metabolic pathways and
biochemistry described above), and these factors always change as we get older.
In addition to how we actively manage
glucose, genetic and environmental factors will influence body mass index
despite “classic” diet and exercise, and most Americans are destined for an
increasing BMI until the age of 60 or so.
Said plainly, by just limiting calories, one will probably still carry
excess body weight.
In Lyn Genet’s work, she explains how
inflammation affects metabolism, influencing weight gain and loss. Her book, The Plan is an excellent resource
for details. Paraphrased from her blog;
‘elevated cortisol produces glucose, which leads to increased blood sugar
levels. This will start to increase yeast growth as they have specialized
glucose sensors and their growth in population will alter gut flora affecting
your body’s defense system. A high yeast
population means never ending sugar and carb cravings (and high sugar and carb
intake is definitely a factor behind the high rates of type 2 diabetes in the
US). Altered gut flora leads to a
weakened immune response as the balance of our intestinal bacteria is thrown
off’. Here you have just a single
example of the multiple factors working to manipulate glucose metabolism.
Fat metabolism is defined as the biochemical
process by which fats are broken down, incorporated, and used by the cells of
the body. Fats provide more food energy (9 kcal/g) than carbohydrates (4.1
kcal/g). Fat catabolism begins with the hydrolysis of fats (triglycerides) into
glycerol and fatty acids.
Losing one pound of body fat is equivalent to
3,500 calories. To lose two pounds per week, you must eliminate 1,000 calories
per day (all approximations).
The concentration of glucose in your blood is
the critical switch that places your body into a “fat-storing” or “fat-burning”
state.
As you eat carbohydrates, you metabolize them,
and eventually store them as fats. With
energy expenditure, your body will draw on the fat stores, and you burn
fat. If your intake of carbohydrate is
lower than your fat burning energy expenditure, then overall you will lose
fat.
In general though cardiovascular exercise rarely
has a significant impact on the overall quantity of TAGs in your fat cells
because while
you expend energy and burn through
glycogen and fat when you workout for long enough , your
body adjusts for this via increased hunger and consumption of carbohydrates,
fats, proteins, etc. Building muscle
does raise your basal metabolic rate (so
it’s a good idea to weight train). But again, your body adjusts to this new
rate by compelling you to eat more. In
fact, in order to gain muscle you need to be in energy-positive situation and
let insulin do its work of bringing in energy and amino acids in for “muscle
building.” It is essential to eat smart
for exercise to assist with fat burning.
Selecting foods to eat with lower glycemic indices may help a bit with balancing the caloric
need/energy expenditure. The Glycemic
Index (GI) is a relative ranking of carbohydrate in foods according to how they
affect blood glucose levels. Carbohydrates
with a low GI value (55 or less) are more slowly digested, absorbed and metabolized
and cause a lower and slower rise in blood glucose and, therefore insulin
levels. There exists an exhaustive set
of resources presenting foods and their
glycemic ratings to assist in making
smart food choices.
Normal healthy values usually quoted for total
body fat are 15% (12-19%) of body mass for young men and 27% (25-30%) for
women, both increasing by about 5% from late teens to sixties. Essential fat in the tissues and organs
(including bone marrow, nervous system and muscle) averages 3% body mass for
men and 12% for women (extra related to reproductive function); it is not a
labile energy reserve, but a vital component for normal structure and function.
Storage fat represents the energy reserve that accumulates as adipose tissue
beneath the skin and in visceral depots, averaging 12% body mass for men and
15% body mass for women. Methods most
commonly used for estimating percentage fat are: measurement of skinfold
thickness at prescribed sites, body density measurement, usually hydrodensitometry
(the Quetelet index) and bio electrical impedance analysis. None of these methods provides a precise
indicator of body composition, but bioelectrical impedance is the simplest,
least expensive, and most nearly accurate.
Understanding body fat empowers one to manage their
metabolism, weight and health more realistically and responsibly. There is so much more at stake than the
prefect bikini body. Lower body fat has
been associated with lower risks of diabetes, cardiovascular and heart disease. Improved metabolic rates enhance energy, improve mobility and strength
reducing the effects of osteoarthritis and osteoporosis. Overall, we just feel better with lower body fat
percentages and idealized body fat ratios mainly because we are using more
efficient energy stores.
