title: Caffeinated coffee, decaffeinated coffee, and caffeine in relation to plasma C-peptide levels, a marker of insulin secretion, in U.S. women.
abstract: OBJECTIVE
  Coffee consumption is associated with reduced risk of type 2 diabetes, but the mechanism is not clearly understood. Elevated C-peptide, as a marker of insulin secretion, has been linked to insulin-resistant type 2 diabetes. In this study, we examined consumption of caffeinated and decaffeinated coffee and total caffeine in relation to concentrations of plasma C-peptide.
  
  
  METHODS
  Plasma C-peptide concentrations were measured in a cross-sectional setting among 2,112 healthy women from the Nurses' Health Study I who provided blood samples in 1989-1990. Consumption of caffeinated and decaffeinated coffee and total caffeine was assessed using a semiquantitative food-frequency questionnaire in 1990.
  
  
  RESULTS
  Intakes of caffeinated and decaffeinated coffee and caffeine in 1990 were each inversely associated with C-peptide concentration in age-adjusted, BMI-adjusted, and multivariable-adjusted analyses. In multivariable analysis, concentrations of C-peptide were 16% less in women who drank >4 cups/day of caffeinated or decaffeinated coffee compared with nondrinkers (P < 0.005 for each). Women in the highest quintile compared with the lowest quintile of caffeine intake had 10% lower C-peptide levels (P = 0.02). We did not find any association between tea and C-peptide. The inverse association between caffeinated coffee and C-peptide was considerably stronger in obese (27% reduction) and overweight women (20% reduction) than in normal weight women (11% reduction) (P = 0.005).
  
  
  CONCLUSIONS
  Our findings suggest a potential reduction of insulin secretion by coffee in women. This reduction may be related to other components in coffee rather than caffeine.
authors.affiliation: Department of Nutrition, Harvard School of Public Health, 655 Huntington Ave., Boston, MA 02115, USA. tianying@hsph.harvard.edu
authors.forename: Tianying
authors.initials: T
authors.lastname: Wu
authors.forename: Walter C
authors.initials: WC
authors.lastname: Willett
authors.forename: Susan E
authors.initials: SE
authors.lastname: Hankinson
authors.forename: Edward
authors.initials: E
authors.lastname: Giovannucci
mesh_terms: D001628:Beverages; D015992:Body Mass Index; D002096:C-Peptide / Q000097:blood*; D002110:Caffeine / Q000494:pharmacology*; D003069:Coffee; D005247:Feeding Behavior; D005260:Female; D006801:Humans; D007328:Insulin / Q000097:blood / Q000378:metabolism*; D000078790:Insulin Secretion; D008875:Middle Aged; D013662:Tea; D014481:United States
chemical_list: D002096:C-Peptide; D003069:Coffee; D007328:Insulin; D013662:Tea; D002110:Caffeine