Within the well-fed state, glucose is absorbed and transported to
the liver. Excess energy is converted to fat in the liver and transported, along
with dietary fat, to the adipose tissues.
The pancreas releases high levels of insulin in response to these events. Insulin signals the liver to convert
glucose to glycogen. The adipose tissues synthesize and deposit fats.
NB: Don't allow your glycogen store to be overloaded; engage in exercises/fasting
(e.g. brain works, since your brain always reach out for glucose first as a
source of energy). This avid quest for glucose will ultimately stimulate the
breakdown of glycogen to glucose.
A deficiency of insulin is a cause of diabetes, characterized by excess levels of blood glucose. In this
disease, glucose is not converted into glycogen or fat, so it remains in the
circulation.
As the individual enters the fasting state,
glycogen stored in the body is broken down into glucose to supply energy for
the tissues.
Simultaneously, gluconeogenesis begins as amino acids, lactate,
and pyruvate from metabolism are cycled into the formation of glucose. As fats
are broken down, the fatty acids supply energy to the peripheral tissues, while
the glycerol from breaking down the triacylglycerols is transported to the
liver and converted to glucose. Gluconeogenesis becomes more important than glycogen
breakdown after about 16 hours of fasting.
Gluconeogenesis is maximal after about two days without food, at which
time ketone bodies are made from fat and transported to the brain. This
transition describes the beginning of starvation,
which can last for six to ten weeks before death occurs. During starvation, the
body breaks down amino acids for glucose; however, ketone bodies and fat supply
most energy requirements. At this time, the body is in negative nitrogen balance, because the amount of nitrogen excreted due
to protein breakdown exceeds the nitrogen eaten in food.
The small amount of glucose made is supplied to brain, kidney, and
red blood cells. The latter two tissues have no alternative energy sources; the
brain uses both ketone bodies and glucose. When fat is gone, the only sources
of energy available are amino acids from muscle. The carbon skeletons are
metabolized, and the nitrogen is excreted. This situation cannot continue for
very long. Eventually, the kidneys fail, or the heart muscle is broken down,
and the individual dies.
