78 Introduction to Human Nutrition
(GIP) that are secreted from enteroendocrine cells
within the mucosa of the small bowel. As the glucose
concentration in blood rises above 5 mM after a meal,
these peptide hormones amplify the response of the
β-cells of the endocrine pancreas, resulting in the dis-
charge of the hormone insulin from secretory gran-
ules which fuse with the cell membrane. Insulin has
several effects on metabolism, including facilitating
the transport, by GLUT4, of glucose into adipocytes
and muscle cells.
In healthy people, blood glucose concentration
(glycemia) is homeostatically controlled within a
fairly narrow range. It seldom falls below about 5 mM,
even after a prolonged fast, and returns to this value
within a couple of hours of a meal. In the absence of
uptake from the gut (the postabsorptive state), about
8 g glucose per hour is provided for those tissues with
an obligatory demand for glucose – namely, the brain,
red blood cells, mammary gland, and testis – by
breakdown of stores of glycogen in the liver and
muscle and by gluconeogenesis. The brain of an adult
has a glucose requirement of about 120 g/day. The
amount readily available in glycogen approximates
190 g. In long periods of fasting and starvation glucose
must be formed from noncarbohydrate sources by a
process known as gluconeogenesis. Gluconeogenesis
occurs in the liver (responsible for about 90% of
gluconeogenesis) and kidney and is the synthesis of
glucose from a range of substrates including pyruvate,
lactate, glycerol, and amino acids. Amino acids are
derived by catabolism of the body’s proteins. All
amino acids, with the exceptions of lysine and leucine,
are glucogenic. Triacylglycerols (from adipose tissue)
are catabolized to release glycerol. These gluconeo-
genic processes are triggered by a fall in blood glucose
concentration below about 5 mM and are signaled to
the tissues by the secretion of glucagon and the glu-
cocorticoid hormones.
Diabetes and its consequences
Diabetes may be diagnosed as an exaggerated response
in blood glucose concentration following ingestion of
a fi xed amount of glucose (glucose tolerance test).
The most common forms of diabetes are type 1 dia-
betes (T1DM) and type 2 diabetes (T2DM). T1DM
results from the autoimmune destruction of the β-
cells of the endocrine pancreas (possibly following
viral exposure), the consequence of which is insulin
insuffi ciency. Control of blood glucose concentra-
tions in T1DM requires the exogenous supply of
insulin by injection. Implanted insulin minipumps or
pancreatic β-cells may offer alternative forms of treat-
ment in the future. Symptoms of type 1 diabetes
include the presence of glucose in urine, passage of
large volumes of urine, body weight loss, and, in
extreme cases, ketosis (excess production of acetone,
acetoacetate, and β-hydroxybutyrate). Although there
is good evidence of genetic predisposition to T2DM,
expression of the disease is due mainly to lifestyle
(excess energy intakes and low physical activity),
resulting in obesity, especially when the extra fat is
accumulated on the trunk. The early stages of T2DM
are characterized by insulin insensitivity/resistance,
i.e., failure of the tissues to produce a normal response
to insulin release that can be seen as relatively wide
swings in blood glucose concentrations following a
carbohydrate-containing meal. Raised blood glucose
concentration sustained for several years is believed
to be fundamental to the spectrum of complications,
including macrovascular (atherosclerosis) and micro-
vascular diseases and problems with the kidneys
(nephropathy), nerves (neuropathy), and eyes (reti-
nopathy and cataract) experienced by diabetics.Dietary management of blood
glucose concentration
Glycemic index
As an aid to the dietary management of blood glucose
concentrations in diabetics, Jenkins and colleagues
(1981) introduced the concept of the glycemic index
(GI), which provides a means of comparing quanti-
tatively the blood glucose responses (determined
directly by in vivo experiments) following ingestion
of equivalent amounts of digestible carbohydrate
from different foods. When a range of carbohydrate-
containing foods was ranked according to their GI
values, there was a strong linear relationship with the
rapidly available glucose (RAG) from similar foods
determined in vitro as the sum of free glucose, glucose
from sucrose, and glucose released from starches over
a 20 minute period of hydrolysis with a battery of
enzymes under strictly controlled conditions (Englyst
method; Englyst et al. 1999). This offers the possibil-
ity of assaying foods in vitro for their RAG content,
which will be quicker and cheaper than the current
approach based on GI measurements in vivo.
Studies with glucose and starches enriched with
the stable isotope carbon-13 have demonstrated that