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Corresponding Author, E-mail: ccrofts@aut.ac.nz. 1Human Potential Centre & 2Biostatistics and Epidemiology, Auckland University of
Technology (AUT), PO Box 92006, Auckland 1142, New Zealand. Copyright: © 2015 The Authors. This is an open-access article
distributed under the terms of the Creative Commons Attribution License.
Catherine A.P Crofts*1, Caryn Zinn1, Mark C Wheldon2, Grant M Schofield1
Diabesity 2015; 1 (4): 34-43 doi: 10.15562/diabesity.2015.19
www.diabesity.ejournals.ca
REVIEW
ABSTRACT
Globally, there is an increasing prevalence of non-communicable diseases. The morbidity and mortality from
these conditions confer a greater economic societal burden. Epidemiological research associates insulin
resistance in the etiology of these diseases, but there is limited evidence for the mechanism of damage.
Emerging research suggests that hyperinsulinemia, a symptom of insulin resistance, may cause these
pathological changes, and therefore be an independent contributor to these diseases. This review shows that
hyperinsulinemia, or excessive insulin secretion, should be considered independently to insulin resistance, defined
as glucose uptake rate, even though the two conditions are intertwined and will co-exist under normal
conditions. Hyperinsulinemia directly and indirectly contributes to a vast array of metabolic diseases including all
inflammatory conditions, all vascular diseases, gestational and type 2 diabetes, non-alcoholic fatty liver disease,
obesity and certain cancers and dementias. The mechanisms include increased production of: insulin growth
factor-1; reactive oxidative species and advanced glycation end-products; and triglyceride and fatty acids.
Hyperinsulinemia also directly and indirectly affects many other hormones and cytokine mechanisms including
leptin, adiponectin and estrogen. There is limited research standardizing the hyperinsulinemia diagnostic process.
Methodological concerns and lack of standardized reference ranges preclude the use of fasting insulin. Most
research has also focused on insulin resistance and it is unknown whether these methods translate to
hyperinsulinemia.
Keywords:
Hyperinsulinemia, hyperglycemia, type 2 diabetes, insulin resistance, secretagogue, syndrome x
Introduction
Impaired insulin homeostasis encompasses
both hyperinsulinemia and hypoinsulinemia. Although
the latter is well recognised as type 1 diabetes, there is
little literature on the former, despite being first
hypothesised in the early 1920s.1 Currently, a close
approximation to hyperinsulinemia research is that
conducted on insulin resistance. Insulin resistance is
well-established as underpinning many significant
chronic health conditions including type 2 diabetes,
metabolic syndrome, cardiovascular disease, some
cancers and Alzheimer’s disease.2-5 This insulin
resistance is invariably accompanied by increased
demands for insulin so that the body can maintain
euglycemia. Excess insulin, termed hyperinsulinemia,
may be endogenous from bodily compensation, or
exogenous via modern medicine. In this paper we
contend that hyperinsulinemia, in concert with insulin
resistance, should be considered as an important
independent health risk. We exclude isolated
hyperinsulinemia, such as that caused by an
insulinoma.
It is well recognised that earliest detection of
any disease state allows for the best possible
outcomes. It is agreed that hyperinsulinemia precedes
hyperglycemia, by up to 24 years.3, 4, 6 There is a strong
argument that hyperglycemia indicates pancreatic β-
cell attrition; essentially end-stage organ damage.3, 7 We
contend that the under-recognition of
hyperinsulinemia is an important clinical issue because
there are no standard diagnostic reference values, is
most accurately diagnosed with dynamic glucose and
insulin testing, and has few (pharmaceutical)
management options. This review will discuss
pathophysiology and diagnosis of hyperinsulinemia.
Hyperinsulinemia was first theorised in 19241,
but it was not until the 1960’s that direct insulin
measurements became possible.8 Since then, there has
been a wealth of research in the field of insulin
resistance with a small amount of research into
hyperinsulinemia. Therefore, we highlight the disease
states that are both directly and/or indirectly
associated with hyperinsulinemia. We also discuss the
availability and limitations of current diagnostic
Hyperinsulinemia: A unifying theory of chronic disease?
A unifying theory of hyperinsulinemia… Catherine Crofts et al.
Diabesity 2015; 1(4): 34-43. doi: 10.15562/diabesity.2015.19 www.diabesity.ejournals.ca
35
methods for hyperinsulinemia and why further
investigations are needed.
Methodology
For this narrative review, literature was reviewed on
hyperinsulinemia and insulin resistance, targeting full-
text English language studies. There was no date
criterion. Articles were selected on the basis of having
a minimum of both a plausible biological mechanism
and established clinical association. Initially, the
academic database search included EBSCO, Medline
and Google Scholar, using variants of the terms
‘hyperinsulinemia’, ‘insulin resistance’, ‘metabolic
syndrome’, and ‘syndrome x’, individually and
conjunction with ‘non-communicable disease’,
‘mechanism’, ‘atherosclerosis’, and ‘cardiovascular
disease’. As subsequent metabolic diseases and/or
mechanisms were eluded to in the initial search, search
terms were widened so that no disease state was
excluded. Subsequent metabolic diseases included, but
were not limited to, conditions such as ‘non-alcoholic
fatty liver disease’, ‘cancer’, ‘dementia’. The final
selection of references was based on the authors’
judgment of relevance, completeness, and
compatibility with clinical, epidemiological,
pathological and biochemical criteria.
Hyperinsulinemia
Definition
There is no precise definition of
hyperinsulinemia. It is often described as ‘more insulin
than normal to achieve euglycemia’; essentially the
same as insulin resistance. Where a reference range is
available, it is normally based on fasting levels and
include 5-13 µU/mL9, ≤ 30 µU/mL10, and 18-173
pmol/L (3-28 µU/mL)11 However, there are very few
studies where a ‘normal level of insulin’ is defined as
many studies define hyperinsulinemia based on
quantiles.12-14 Few studies have been more specific.
Both a fasting serum insulin of ≥12.2 µU/mL in the
presence of euglycemia15 and a range of 8-11 µU/mL
‘between meals’ and up to 60 µU/mL ‘after meals’16
have been proposed. There are also practical,
methodological issues with determining insulin
resistance under the World Health Organization
(WHO) conditions that will be discussed later in this
review.
Etiology
The etiology of hyperinsulinemia is not yet
fully elucidated. Although there are several theories,
further research will likely show a multimodal
pathology. What can be deduced from physiological
principles is:
1. Healthy cells are subjected to acute hyperglycemia.
2. Although many cells can absorb glucose without
using insulin (GLUT1 transportation) hyperglycemia
causes insulin to be released from pancreatic cells to
facilitate absorption, especially in muscle and adipose
cells (GLUT 4 transportation).17 3. Insulin binds to
cellular insulin receptors and facilitates translocation of
GLUT4 to the cellular surface. During this process the
insulin and its receptor are absorbed into the cell to be
replaced from the internal pool of insulin receptors.18
4. This acute insulin resistance is of no consequence as
long as the cell has viable GLUT4 on the cellular
surface. However, GLUT4 have a relatively short half-
life.19 5. If hyperglycemia persists, the pancreas
maintains insulin secretion. This may deplete the
insulin receptors faster than they can be replaced. 6.
During this period where the cells are replacing their
insulin receptors, moderately elevated blood glucose
levels, (such as that immediately found after a normal
meal) may need slightly higher than normal insulin
levels to restore normoglycemia. This moderate
hyperinsulinemia may delay the return to normal
insulin receptor function (acute insulin resistance).7
This state of insulin resistance due to down-regulated
insulin receptors is reversible should the person not be
subjected to further episodes of hyperglycemia. It does
not matter whether this is via high, but acute, blood
glucose elevations, or moderately elevated glucose
levels over a prolonged period. 8. Prolonged impaired
insulin signaling impedes GLUT4 translocation to the
cellular surface thus causing impaired glucose uptake
and prolonging hyperglycemia, causing a positive
feedback cycle. This will both aggravate and prolong
the insulin resistance, potentially turning it from a
transitory state to a persistent or chronic state.
The complexity of the insulin receptor
regulation, combined with the availability of GLUT4
and factors that influence insulin secretion mean that
it is impossible to generalize whether insulin resistance
precedes or follows hyperinsulinemia. It is more
plausible that different individuals have different
triggers in the cycle. These triggers may include
genetic factors, excessive carbohydrate, corticosteroids
(endogenous or exogenous), free fatty acids, leptin, or
certain medications; each of these are discussed below.
Fructose: Fructose is metabolized in liver into ATP
and/or triglycerides in a process that is competitive
with, and preferential to, glucose. If excessive fructose
is consumed, glucose will not be metabolized causing
hyperglycemia and subsequent hyperinsulinemia.20, 21
Excessive fructose also results in hyperuricemia which
A unifying theory of hyperinsulinemia… Catherine Crofts et al.
Diabesity 2015; 1(4): 34-43. doi: 10.15562/diabesity.2015.19 www.diabesity.ejournals.ca
36
is associated with reduced endothelial nitric oxide
causing vasoconstriction, endothelial dysfunction and
insulin resistance.21
Hyperglycemia: Hyperglycemia alone can aggravate
insulin resistance.22 Along with excessive carbohydrate
ingestion, other mechanisms for this mechanism
include hepatic insulin resistance. Increased plasma
insulin slows hepatic gluconeogenesis but this process
can be impaired by hepatic insulin resistance leading to
peripheral hyperglycemia and further insulin
secretion.23
Corticosteroids: It is known that corticosteroids,
especially endogenous cortisol, cause a down
regulation of GLUT-4 receptors, thus preventing
glucose uptake and provoking hyperinsulinemia in the
presence of hyperglycemia. Long-term courses of
exogenous corticosteroids, such as prednisone, are
known to cause ‘drug-induced’ type 2 diabetes, which
may resolve after the medication is discontinued. Not
every patient on long-term corticosteroids will develop
drug-induced diabetes. Therefore, it is plausible that
the patient’s degree of insulin resistance at baseline
influences disease development/progression. Given
that stress causes a temporary rise in cortisol levels, it
is also plausible that prolonged stress may be another
cause of hyperinsulinemia.24
Leptin: Appetite control is mediated from the
hypothalamus in response to a balance between leptin
and insulin controlling neuropeptide Y expression.25
This balance is believed important to manage caloric
intake over longer periods of time when meals can
vary in size, frequency and composition. Leptin
secretion is slow to change as it is influenced by total
body fat mass and total caloric intake, while insulin
secretion is highly responsive to food ingestion and
will change quickly with every meal. Leptin is also
highly influenced by insulin as it is released from fat
stores by mechanisms that appear to involve glucose
flux.25 Experimental evidence shows that reducing
insulin secretion reduces leptin resistance, suggesting a
relationship between hyperinsulinemia and
hyperleptinemia.26 It is not yet clear whether
hyperleptinemia is causative of hyperinsulinemia
beyond the association of obesity and an increase in
free fatty acids.
Medication-induced: There are a number of medications
known or suspected to cause hyperinsulinemia and/or
contribute to insulin resistance. Exogenous
corticosteroids (prednisone) and exogenous insulin
and insulin secretagogues (sulphonylureas) have had
their mechanisms discussed. Other medications
include the antipsychotics (e.g. clozapine), and
statins.27 The mechanisms for these medications
causing hyperinsulinemia are currently unknown.
Due to the nature of insulin receptor
regulation, it is also plausible that insulin sensitivity of
the cells can be restored. This would require the
absence of both hyperinsulinemia and hyperglycemia.
Case studies indicate that a carbohydrate restricted diet
may facilitate this effect.10
Overall, it should be recognized that
hyperinsulinemia is independent to insulin resistance:
Hyperinsulinemia is excessive insulin secretion, while
insulin resistance is impaired glucose uptake. This
review investigates the both the mechanistic and
epidemiological evidence that links hyperinsulinemia
to metabolic disease. Although there is good quality
research mechanistically linking hyperinsulinemia to
subsequent pathologies, there is a paucity of good
epidemiological evidence. Given the intertwined
nature between insulin resistance and hyperinsulinemia
as depicted above, it can be assumed that the majority
of people with insulin resistance are also
hyperinsulinemic. Therefore, if no epidemiological
data was available, this review used epidemiological
research based on insulin resistance as a proxy for
hyperinsulinemia.
Direct effects of hyperinsulinemia
As shown in Table 1, hyperinsulinemia can be
mechanistically and epidemiologically linked to
metabolic syndrome, gestational and type 2 diabetes
and therefore, cardiovascular and other diseases with
an increased prevalence in those with metabolic
syndrome.2-4, 28 It is also an independent risk factor for
a number of other diverse conditions including diet-
induced obesity, osteoarthritis, certain cancers,
especially breast and colon/rectum, and Alzheimer’s
disease and other dementias.5, 6, 29-32
Other conditions that may be associated with
hyperinsulinemia, via either epidemiological evidence
or potential mechanism of action, include gout,
tinnitus, schizophrenia and autism.33-36 Further
research is needed to confirm these associations.
Pathophysiological mechanisms
Hyperinsulinemia affects the body via five
main mechanisms: Increased reactive oxidative species
and advanced glycation end-products; increased
insulin-like growth factor-1 (IGF-1); hyperglycemia;
increased fatty acid/triglyceride production; and by
affecting different hormones and cytokines.
Reactive oxidative species
Reactive oxygen species is a collective term that
includes both oxygen radicals and non-radical
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Diabesity 2015; 1(4): 34-43. doi: 10.15562/diabesity.2015.19 www.diabesity.ejournals.ca
37
Table 1. Biological systems and disease states affected by hyperinsulinemia, and associated mechanisms of action
Biological
System
Disease
Mechanism
Direct or
indirect
mechanism
References
Mechanism
of action
Epidemiology
Cancer*
Cancer
(Breast, ovarian, colon,
bladder, pancreas &
liver)
Increased insulin-like growth factor IGF-1 enhances cellular
growth and proliferation.
Direct
(5, 48)
(29)
Enhanced glucose uptake and utilization enhances cellular
growth and proliferation.
Both
(29)
(29)
Increased production of reactive oxidative species causes
derangement of DNA and enzymes involved with repair
mechanisms (enhanced by hyperglycemia).
Indirect
(2, 37, 38)
(2, 37, 38)
Increased sex-hormone production and decreased sex
hormone binding globulin causes increased cellular growth
and proliferation (enhanced by obesity).
Direct
(29)
(29)
Circulatory
Atherosclerosis
Arterial wall damage caused by inflammation, increased
proliferation and migration of arterial smooth muscle cells.
Stimulation of the mitogen-activated protein kinase pathway.
Both
(28, 40)
(28, 47, 63,
64)
Cardiomyopathy
Microvascular disease, including changes to capillary
permeability, microaneurysm formation, vasoconstriction and
microthrombi.
Both
(65, 66)
(65, 66)
Increased myocardial fibrosis by increased reactive
oxidative species, deranged collagen production.
Diabetic neuropathy causes changes to catecholamines, which
further impairs myocardial function.
Endothelial dysfunction
Vasoconstriction and pro-atherosclerotic effects from
decreased nitric oxide bioavailability and action and
increased thromboxane.
Both
(2, 41, 67)
(64)
Enhanced by increased reactive oxidative species and
advanced glycation end-products.
Thrombosis
Hyperinsulinemia causes increased fibrinolysis while
hyperglycemia causes increased blood coagulability
.
Indirect
(42)
(64)
Gastrointestinal
Diabetes: Gestational
Pre-existing insulin resistance and increased demand for
insulin.
Direct
(68)
(68)
Diabetes:
Type 2
Prolonged insulin resistance eventuating in beta-cell failure.
Down-regulation of glucose transporter-4.
Direct
(3, 69, 70)
(4)
Hyper- triglyceridemia
Increased triglyceride production.
Direct
(43, 71)
(72)
Non-alcoholic fatty
liver disease
Fatty acid production exceeds distribution capacity.
Aggravated by inflammation and oxidative stress.
Direct
(71)
(72)
Endocrine
Chronic inflammation
Stimulation of mitogen-activated protein kinase pathway;
glycemic variability; hyperglycemia and/or obesity
influences increased cytokine production.
Indirect
(40, 48)
(73)
Obesity
Decreased lipolysis.
Direct
(74)
(75)
Lack of appetite suppression.
Direct
(25, 26)
(76)
Nervous
Alzheimer's disease
and vascular dementia
Endothelial dysfunction resulting in microvascular disease,
metabolic disturbances and neuronal damage.
Direct
(2, 67, 77)
(30, 78, 79)
Increased blood coagulability and/or fibrinolysis cause
multiple thrombotic events.
Both
(42, 80)
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Diabesity 2015; 1(4): 34-43. doi: 10.15562/diabesity.2015.19 www.diabesity.ejournals.ca
38
Changed regulation of beta-amyloid and tau protein
(Alzheimer's disease).
Direct
(77, 81)
Decreased synaptic plasticity caused by dysregulated PSA-
NCAM interactions (Alzheimer's disease).
Direct
(33)
Peripheral neuropathy
Increased production of reactive oxidative species and
advanced glycation end-products enhanced by
hyperglycemia.
Indirect
(2, 41)
(64, 82)
Insulin resistance in the dorsal root ganglion neurons.
Both
(83)
Retinopathy
Hyperglycemia and endothelial dysfunction contribute blood-
retinal barrier breakdown. Aggravated by excess advanced
glycation end-products.
Direct
(41, 64, 84)
(41, 64, 84)
Skeletal
Osteoporosis
Increased reactive oxidative species and/hyperglycemia
cause collagen breakdown, impairs new collagen synthesis
and compromises mensenchymal cells.
Indirect
(31)
(31)
Urinary
Nephropathy
Microvascular disease, including changes to capillary
permeability, microaneurysm formation, vasoconstriction and
microthrombi.
Direct
(67, 85)
(64, 86)
Increased production of reactive oxidative species and
advanced glycation end-products enhanced by
hyperglycemia.
Indirect
(41, 87)
*While cancer is not typically classified as a “biological system”, due to its recognition and impact as a key chronic disease, it was decided that it warrants a
classification on its own, rather than be integrated into individual biological systems. PSA-NCAM-polysialic acid - neural cell adhesion molecule.
oxidising agents such as hydrogen peroxide.37 Reactive
oxidative species are also produced during, and
involved in, many metabolic processes including
enzymatic reactions, gene expression and signal
transduction.37 Generally, the actions of intracellular
reducing agents such as antioxidants prevent reactive
oxidative species-mediated damage. However, a
number of factors can contribute to excessive
production of reactive oxidative species including
excessive calorie consumption and the presence of
various pro-inflammatory mediators, including tumor
necrosis factor-α.37 Once produced, reactive oxidative
species can interact with numerous cellular
components including DNA, lipids, and amino acids.
Damage to DNA is likely to be the underlying
mechanism for reactive oxidative species being
associated with cancer and early aging.38
Polyunsaturated fatty acids are considered very
susceptible to reactive oxidative species damage,
triggering lipid peroxidation, which can affect cell
membrane fluidity and integrity, potentially being the
mechanism for endothelial damage.37 Amino acids
such as cysteine and methionine are very susceptible to
reactive oxidative species damage. Changes to these
amino acids are implicated in the development of
Alzheimer’s disease.39
Hyperinsulinemia is associated with increased
reactive oxidative species, although the exact
mechanism is disputed. Hyperinsulinemia is
mechanistically linked to excessive serum glucose and
free fatty acids. Either substrate can cause increased
reactive oxidative species production.2 Insulin has also
been demonstrated to have some inhibitory effects on
reactive oxidative species production that may be
independent of its effects on glycemia.40 However,
reducing insulin-stimulated nutrient uptake into the
cell is also believed to decrease reactive oxidative
species production.2 Further research is required to
better understand these mechanisms.
Over-nutrition is also thought to be
responsible for the formation of advanced glycation
end-products via non-enzymatic glycation and
glycooxidation processes.41 Defective renal excretion
of advanced glycation end-products, as seen with
diabetic nephropathy, and consumption of exogenous
advanced glycation end-products increases plasma
advanced glycation end-product levels. Advanced
glycation end-products are believed to contribute to
changes in the microvascular systems and also
promote changes to inflammatory, oxidative and other
degenerative processes of various chronic diseases
including neuropathies.41
Growth factors (IGF, vascular endothelial growth
factor)
Insulin, IGF-1 and other substances such as
vascular endothelial growth factor (VEGF) can
stimulate the growth and division of many cells.
Insulin can mediate cellular division but may also
stimulate cancer cell proliferation and metastasis.29
Most importantly, insulin increases the bioavailability
of IGF-1, thus insulin is indirectly implicated in all
IGF-1 mediated processes. These processes include
changes to vascular structures, increases to cellular
division and prevention of apoptosis
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Diabesity 2015; 1(4): 34-43. doi: 10.15562/diabesity.2015.19 www.diabesity.ejournals.ca
39
Hyperglycemia
Hyperglycemia commonly follows
hyperinsulinemia3 but there is little information to
suggest whether fasting glucose, peak glucose, or area-
under-the curve (AUC) have the most adverse health
impact. Cancer cells have a continuously high glucose
uptake, which enhances cellular growth and
proliferation29; hyperglycemia augments this process.
Hyperglycemia allows IGF-1 to stimulate vascular
smooth muscle proliferation, which is a hall-mark of
both cancer and atherosclerosis. Blood coagulability is
also increased by hyperglycemia irrespective of insulin
levels.42
Increased fatty acid and triglyceride production
Hyperinsulinemia influences both free fatty
acid and triglyceride production.43 While the processes
that occur during hepatic de novo lipogenesis are not
disputed, there is debate as to whether
hyperinsulinemia precedes, or are a consequence of
fatty liver.44 Nevertheless, elevated triglyceride levels
are recognized to be a key component of metabolic
syndrome (Table 1) while fatty liver may be considered
a hepatic manifestation of metabolic syndrome and
may progress to cirrhosis or hepatocellular cancer.44
Elevated triglyceride levels may also further impair
leptin resistance.20, 45
Hormone/cytokine production (sex hormones,
inflammation, obesity)
Hyperinsulinemia is involved with adiposity
via increased appetite and triglyceride production,
thereby increasing adiposity.46, 47 Adipose tissue is now
well-established as an endocrine organ and produces
both hormones and cytokines that are used for cellular
communication. Hypertrophic adipose tissues activate
inflammatory and stress pathways and decreases
insulin response. This results in increased cytokine
production including TNF-α, vascular endothelial
growth factor and leptin, while adiponectin expression
is decreased.48 These actions contribute to decreased
glucose and lipid uptake, leading to further reductions
to adiponectin secretion and adipogenesis as well as
contributing to further insulin resistance. Decreased
glucose uptake means there is less glycerol within the
adipocyte to esterify free fatty acids, allowing them to
infiltrate and accumulate in other tissues.
Adiponectin decreases proliferation of cell
types including adipocytes, endothelial cells and cancer
cells.48 The role of leptin is yet to be fully understood,
but it is accepted that hyperinsulinemia and
hyperleptinemia results in central leptin resistance, and
consequent prevention of appetite suppression and
promotion of further obesity.25, 26, 49 Hyperleptinemia
is also linked to increased inflammatory cytokines,
changes in nitric oxide, and further endothelial
injury.49
Hyperinsulinemia is also believed to elevate
plasminogen activator inhibitor type-1 (PAL-1) levels,
with associated increased fibrinolysis and increased
risk of thrombosis. When combined with the
increased coagulation from hyperglycemia, this may
explain why over 80% of people with type 2 diabetes
have a thrombotic death.42
Diagnosis
Diagnosing hyperinsulinemia is challenging
partly because the health effects of insulin resistance
and hyperinsulinemia have been conflated. Further
challenges arise when interpreting the available
literature. As previously discussed, fasting insulin
levels have been assessed as a means of diagnosing
hyperinsulinemia with differing results. But it is not
just the insulin level alone that is problematic. How
and when sampling occurs will also cause variation to
results. Insulin levels are higher in serum compared to
plasma samples meaning that studies reporting serum
insulin cannot be compared directly to plasma
insulin.50, 51 Insulin secretion is pulsatile leading to
significant levels in plasma insulin in a short space of
time. It is recommended that the mean of three
samples taken at five minute intervals be used if a
fasting insulin level is required52, however this rarely
seem to happen in practice. Single fasting insulin
samples can have a coefficient of variation of 25-
50%.53 This variation decreases testing sensitivity and
is perhaps why fasting insulin is not recommended to
be used clinically.54
It is unknown whether insulin resistance
testing can be used to diagnose hyperinsulinemia. The
gold standard for measuring insulin resistance is the
hyperinsulinemic-euglycemic clamp test. The lowest
quartile of glucose uptake rate defines insulin
resistance for that study population. Figures for this
lower quartile have ranged from < 4.7mg/kg ·min to
≤ 6.3M·mU-1·L-1, however differences in insulin
infusion rates, glucose disposal rate calculations, and
background populations under investigation means
that there are limits to the generalizability of these
results.15, 55-58 Furthermore, given the complexity of the
procedure, the hyperinsulinemic-euglycemic clamp
test has little to no clinical application.15
A further complication to using the clamp test
to assess hyperinsulinemia is that the high dose
infusion of insulin will confound any effects of
endogenous insulin secretion. As theorized above, the
damage associated with hyperinsulinemia is due to the
A unifying theory of hyperinsulinemia… Catherine Crofts et al.
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continuous action of insulin in the tissues. The
amount of insulin normally present in the tissues
cannot be measured during the clamp process. It is
unknown whether glucose uptake rates correlate with
insulin secretion.
A number of tests have been developed that
are validated against the hyperinsulinemic-euglycemic
clamp that has more clinical applicability. Those based
on fasting insulin include homeostatic model
assessment (HOMA or HOMA2), McAuley Index,
and the quantitative insulin sensitivity check index
(QUICKI).15, 56, 59 Although HOMA has since been
refined to the HOMA2 model, both are modelled on
the combination of fasting insulin to fasting glucose.
The original HOMA has a 89% sensitivity and 67%
specificity compared to hyperinsulinemia-euglycemic
clamp.57 The McAuley index is calculated from fasting
insulin and fasting triglyceride levels with 61%
sensitivity and 85% specificity.15
Another insulin resistance test, the oral
glucose sensitivity index (OGIS), is modelled on the
results derived from an oral glucose tolerance test.56
OGIS uses both blood insulin and glucose levels at
baseline, 120 min and 180 min. A spreadsheet is
recommended for the calculations (available from
http://webmet.pd.cnr.it/ogis/download.php). The
OGIS is validated against the hyperinsulinemic-
euglycemic clamp assessments for insulin resistance,
but as previously stated, the generalizability of clamps
is limited.
Both the OGIS and tests based on fasting
insulin levels have more clinical applicability for
assessing insulin resistance compared to the
hyperinsulinemic-euglycemic clamp test. However,
insulin resistance testing has never translated to
improvements in disease risk calculations. The WHO
definition for insulin resistance means that one in four
people would be diagnosed with insulin resistance; a
figure that may be unrelated to their actual health
risks.60 Analysis from the Women’s Health Initiative
Biomarkers study showed that although HOMA-IR
had a positive association with cardiovascular risk, this
was became non-significant after adjusting for other
risk factors such as HDL cholesterol.61 There is an
argument that HOMA-IR should be used in
combination with HOMA-%B for assessing insulin
resistance.52
Emerging research now suggests that insulin
response patterns following an oral glucose load may
determine hyperinsulinemic status. Kraft7,10
demonstrated the variability of insulin response to a
100 g glucose load over 3-5 hours, especially with
respect to timing and magnitude of the insulin peak
and rate of response decline. Five main insulin
response patterns are clearly identifiable, with pattern I
being considered normal insulin tolerance. From this
research Kraft concluded that the most accurate
means of assessing hyperinsulinemia was a 3-hour oral
glucose tolerance test with insulin levels assessed at
baseline, 30, 60, 120, and, at minimum, 180 minutes
but 240 and 300 minute insulin levels could also be
considered. This study was cross-sectional and there
are no long-term outcomes.
Hayashi and colleagues62 have shown that the
insulinemic pattern produced from sampling every 30
minutes during a 2-hour OGTT can predict the
development of type 2 diabetes. An insulin peak
delayed beyond 60 minutes being associated with
poorer health is common to both Kraft and Hayashi
patterns. Further research is required to understand
how to apply these patterns to clinical practice.
Collectively these studies show that there is a
paucity of research for diagnosing hyperinsulinemia.
Most studies focus on insulin resistance testing, but it
remains unknown whether insulin resistance correlates
with insulin secretion.
Concluding remarks
This review clearly demonstrates that not only
is hyperinsulinemia involved with the etiology of all of
the symptoms of metabolic syndrome, it is also
implicated in many other conditions; some of which
have previously been considered to be idiopathic, such
as tinnitus. This raises many questions with both
clinical and research implications. Firstly, what is the
prevalence of hyperinsulinemia? Given its association
with metabolic syndrome and fatty liver disease, this
warrants investigation. Could early detection and
careful management of hyperinsulinemia decrease the
need for medical interventions later in life? Would
managing hyperinsulinemia improve to both quantity
and quality of life? Yet there are currently too many
questions regarding diagnosis. A reliable and
repeatable result when sampling insulin is still a
challenging task. There is no agreed upon reference
range, and there are only associations between
quantiles and ongoing disease risk. Insulin response
patterning may answer some of these questions, but
patterning requires more resources than a fasting level.
Given the global concerns about the ‘epidemic’ of
metabolic diseases, this research needs to be urgently
addressed.
Conflict of interest
None Declared.
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