title: Observational research rigour alone does not justify causal inference.
abstract: BACKGROUND
  Differing opinions exist on whether associations obtained in observational studies can be reliable indicators of a causal effect if the observational study is sufficiently well controlled and executed.
  
  
  METHODS
  To test this, we conducted two animal observational studies that were rigorously controlled and executed beyond what is achieved in studies of humans. In study 1, we randomized 332 genetically identical C57BL/6J mice into three diet groups with differing food energy allotments and recorded individual self-selected daily energy intake and lifespan. In study 2, 60 male mice (CD1) were paired and divided into two groups for a 2-week feeding regimen. We evaluated the association between weight gain and food consumption. Within each pair, one animal was randomly assigned to an S group in which the animals had free access to food. The second paired animal (R group) was provided exactly the same diet that their S partner ate the day before.
  
  
  RESULTS
  In study 1, across all three groups, we found a significant negative effect of energy intake on lifespan. However, we found a positive association between food intake and lifespan among the ad libitum feeding group: 29·99 (95% CI: 8·2-51·7) days per daily kcal. In study 2, we found a significant (P = 0·003) group (randomized vs. self-selected)-by-food consumption interaction effect on weight gain.
  
  
  CONCLUSIONS
  At least in nutrition research, associations derived from observational studies may not be reliable indicators of causal effects, even with the most rigorous study designs achievable.
authors.affiliation: Office of Energetics, School of Public Health, University of Alabama at Birmingham, Birmingham, AL, USA.
authors.forename: Keisuke
authors.initials: K
authors.lastname: Ejima
authors.affiliation: Office of Energetics, School of Public Health, University of Alabama at Birmingham, Birmingham, AL, USA.|Department of Biostatistics, University of Alabama at Birmingham, Birmingham, AL, USA.
authors.forename: Peng
authors.initials: P
authors.lastname: Li
authors.affiliation: Department of Nutrition Sciences, University of Alabama at Birmingham, Birmingham, AL, USA.|Nutrition Obesity Research Center, University of Alabama at Birmingham, Birmingham, AL, USA.|Diabetes Research Center, University of Alabama at Birmingham, Birmingham, AL, USA.
authors.forename: Daniel L
authors.initials: DL
authors.lastname: Smith
authors.affiliation: Department of Nutrition Sciences, University of Alabama at Birmingham, Birmingham, AL, USA.|Nutrition Obesity Research Center, University of Alabama at Birmingham, Birmingham, AL, USA.|Diabetes Research Center, University of Alabama at Birmingham, Birmingham, AL, USA.
authors.forename: Tim R
authors.initials: TR
authors.lastname: Nagy
authors.affiliation: Department of Cell, Developmental & Integrative Biology, University of Alabama at Birmingham, Birmingham, AL, USA.
authors.forename: Inga
authors.initials: I
authors.lastname: Kadish
authors.affiliation: Department of Cell, Developmental & Integrative Biology, University of Alabama at Birmingham, Birmingham, AL, USA.
authors.forename: Thomas
authors.initials: T
authors.lastname: van Groen
authors.affiliation: Office of Energetics, School of Public Health, University of Alabama at Birmingham, Birmingham, AL, USA.
authors.forename: John A
authors.initials: JA
authors.lastname: Dawson
authors.affiliation: Department of Nutrition Sciences, University of Alabama at Birmingham, Birmingham, AL, USA.|Nutrition Obesity Research Center, University of Alabama at Birmingham, Birmingham, AL, USA.
authors.forename: Yongbin
authors.initials: Y
authors.lastname: Yang
authors.affiliation: Department of Biostatistics, University of Alabama at Birmingham, Birmingham, AL, USA.
authors.forename: Amit
authors.initials: A
authors.lastname: Patki
authors.affiliation: Office of Energetics, School of Public Health, University of Alabama at Birmingham, Birmingham, AL, USA.|Department of Biostatistics, University of Alabama at Birmingham, Birmingham, AL, USA.|Department of Nutrition Sciences, University of Alabama at Birmingham, Birmingham, AL, USA.|Nutrition Obesity Research Center, University of Alabama at Birmingham, Birmingham, AL, USA.|Diabetes Research Center, University of Alabama at Birmingham, Birmingham, AL, USA.
authors.forename: David B
authors.initials: DB
authors.lastname: Allison
mesh_terms: D000818:Animals; D015984:Causality; D004435:Eating; D002149:Energy Intake; D005247:Feeding Behavior; D005260:Female; D008136:Longevity; D008297:Male; D051379:Mice; D008810:Mice, Inbred C57BL; D064887:Observational Studies as Topic; D011897:Random Allocation; D012107:Research Design; D015430:Weight Gain
keywords: Causality; nutritional sciences; observational study; randomized controlled trial; research design