I think we all accept that changes in insulin levels over time predispose people to the development of type II diabetes and that this is often accompanied by the central adiposity that distinguishes the metabolic syndrome morphology we have come to look for.
This study, published in PLOS One this year (2010), suggests that besides these clinical indications another slightly less obvious change to body mass affects insulin resistance and increases risk of diabetes.
A cross-sectional analysis of National Health and Nutrition Examination Survey III data was undertaken, utilising subjects of 20 years or older, non-pregnant (N = 14,528). Sarcopenia was identified from bioelectrical impedance measurement of muscle mass and obesity was identified from body mass index.
Outcomes were homeostasis model assessment of insulin resistance (HOMA IR), glycosylated haemoglobin level (HbA1C), and prevalence of pre-diabetes (6.0≤ HbA1C<6.5 and not on medication) and type 2 diabetes. Covariates in multiple regression were age, educational level, ethnicity and sex.
Sarcopenia (from the Greek meaning “poverty of flesh” is the degenerative loss of skeletal muscle mass and strength associated with aging) crosses the population whether the patient is overweight or of normal body mass. As we age muscle mass declines, in part due to the reduced ability to reproduce muscle cells and in the main because people do less resistance exercise.
Sarcopenic obesity, the mutual existence of sarcopenia and obesity, appears in approx 10% of western adults aged in their 60’s. This rises to a staggering 50% in those over 80, with most people demonstrating a 50% muscle mass reduction if they reach the age of 90.
Since muscle is the primary tissue contributing to whole-body insulin-mediated glucose disposal, sarcopenia is an important causal factor in age-induced insulin resistance and type 2 diabetes susceptibility.
The primary causes of sarcopenia include a sedentary lifestyle and malnutrition
The researchers concluded:
Sarcopenia, independent of obesity, is associated with adverse glucose metabolism, and the association is strongest in individuals under 60 years of age, which suggests that low muscle mass may be an early predictor of diabetes susceptibility
Low grade inflammation is also a common finding with sarcopenia and suggests that apart from gross nutritional strategies, that there may well be a role for targeted use of antioxidants and insulin sensitizers along with a graded exercise program.
25OHD may also be important for the maintenance of muscle function, and higher skeletal muscle mass and function, so checking Vit D and supplementing adults with at least 2000iu daily or in combination with the other key fat soluble nutrients makes a sensible intervention strategy
Exercise (both resistance and aerobic) in combination with adequate protein and energy intake is the key component of the prevention and management of sarcopenia. Adequate protein supplementation alone only slows loss of muscle mass. Adequate protein intake (leucine-enriched balanced amino acids and possibly creatine) may enhance muscle strength. Low 25(OH) vitamin D levels require vitamin D replacement.
In view of these findings, dieting to be thin is by itself not enough to reduce the risk of diabetes. It is also important to be fit and, in particular, to have good muscle mass and strength.
 Srikanthan P, Hevener AL, Karlamangla AS, 2010 Sarcopenia Exacerbates Obesity-Associated Insulin Resistance and Dysglycemia: Findings from the National Health and Nutrition Examination Survey III. PLoS ONE 5(5): e10805. doi:10.1371/journal.pone.0010805 View Full Paper
 Davison KK, Ford ES, Cogswell ME, Dietz WH (2002) Percentage of body fat and body mass index are associated with mobility limitations in people aged 70 and older from NHANES III. J Am Geriatr Soc 50: 1802–1809 View Abstract
 Morley JE, Argiles JM, Evans WJ, Bhasin S, Cella D, Deutz NE, Doehner W, Fearon KC, Ferrucci L, Hellerstein MK, Kalantar-Zadeh K, Lochs H, MacDonald N, Mulligan K, Muscaritoli M, Ponikowski P, Posthauer ME, Rossi Fanelli F, Schambelan M, Schols AM, Schuster MW, Anker SD; Society for Sarcopenia, Cachexia, and Wasting Disease. Nutritional recommendations for the management of sarcopenia. J Am Med Dir Assoc. 2010 Jul;11(6):391-6. View Abstract