Insulin: too much of a good thing is bad
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Insulin: too much of a good thing is bad Hubert Kolb1,2, Kerstin Kempf2*, Martin Röhling2 and Stephan Martin1,2
Abstract Background: Insulin shares a limited physiological concentration range with other endocrine hormones. Not only too low, but also too high systemic insulin levels are detrimental for body functions. Main body: The physiological function and clinical relevance of insulin are usually seen in association with its role in maintaining glucose homeostasis. However, insulin is an anabolic hormone which stimulates a large number of cellular responses. Not only too low, but also excess insulin concentrations are detrimental to the physiological balance. Although the glucoregulatory activity of insulin is mitigated during hyperinsulinemia by dampening the efficiency of insulin signaling (“insulin resistance”), this is not the case for most other hormonal actions of insulin, including the promotion of protein synthesis, de novo lipogenesis, and cell proliferation; the inhibition of lipolysis, of autophagy-dependent cellular turnover, and of nuclear factor E2-related factor-2 (Nrf2)-dependent antioxidative; and other defense mechanisms. Hence, there is no general insulin resistance but selective impairment of insulin signaling which causes less glucose uptake from the blood and reduced activation of endothelial NO synthase (eNOS). Because of the largely unrestricted insulin signaling, hyperinsulinemia increases the risk of obesity, type 2 diabetes, and cardiovascular disease and decreases health span and life expectancy. In epidemiological studies, high-dose insulin therapy is associated with an increased risk of cardiovascular disease. Randomized controlled trials of insulin treatment did not observe any effect on disease risk, but these trials only studied low insulin doses up to 40 IU/day. Proof for a causal link between elevated insulin levels and cardiovascular disease risk comes from Mendelian randomization studies comparing individuals with genetically controlled low or high insulin production. Conclusions: The detrimental actions of prolonged high insulin concentrations, seen also in cell culture, argue in favor of a lifestyle that limits circadian insulin levels. The health risks associated with hyperinsulinemia may have implications for treatment regimens used in type 2 diabetes. Keywords: Hyperinsulinemia, Insulin resistance, Nrf2, Autophagy, eNOS, Obesity, Type 2 diabetes mellitus, Inflammation, Oxidative stress, Cardiovascular morbidity and mortality
Background Most endocrine hormones exhibit a window of optimal physiological concentrations, with compromised function of the organism at levels below or above that range. For instance, subnormal levels of thyroid hormone define the clinical condition of hypothyroidism, above normal levels represent hyperthyroidism which usually requires therapy. Addison’s disease is characterized by * Correspondence: [email protected] 2 West-German Centre of Diabetes and Health, Duesseldorf Catholic Hospital Group, Hohensandweg 37, 40591 D
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