title: How covert are covertly manipulated diets?
abstract: OBJECTIVE
  To quantitatively assess subjects' ability to detect hedonic (palatability), sensory and nutritional differences between covertly manipulated high-fat (HF) and low-fat (LF) diets. SUBJECTS AND DIETS: This study examined the response of 16 subjects (eight men, eight women) to 20 LF and 20 HF versions of manipulated foods. Average percentage protein:fat:carbohydrate (by energy) and energy density (ED) of the two diets were 13:25:62, 371 kJ/100 g and 13:50:37, 672 kJ/100 g, respectively.
  
  
  METHODS
  Subjects were asked to simultaneously assess the HF and LF versions of each food in terms of (i) subjective pleasantness of each food, (ii) perceived differences in appearance, smell, taste and texture of the foods, and (iii) for each sample to assess whether it was high or low in energy, protein, carbohydrate, fat, fibre, sugar and salt.
  
  
  METHODS
  Perceived pleasantness of HF and LF versions of the foods was compared by analysis of variance. Comparisons used chi-squared tests of independence to assess departure from the null hypothesis of no perceived difference in remaining parameters (ii-iii).
  
  
  RESULTS
  On average, subjects exhibited no significant preference for LF or HF versions of the foods (no difference 15 foods, three HF foods more pleasant, two LF foods more pleasant (P<0.03)). On average there were no general differences in comparison of sensory attributes that were consistently ascribable to the LF or HF foods, although there were numerous significant differences for individual foods. Subjects appeared unable to distinguish the HF foods as being HF (66% of estimates) and guessed correctly 33% of the time. They were better able to categorize the LF foods correctly (53% correct). On aggregate 43% of HF and LF foods were correctly identified. Subjects appeared able to detect sensory differences between foods but not to relate them to energy or nutrient content of these foods.
  
  
  CONCLUSIONS
  These data suggest that subjects are on average not able to perceive large differences in the fat content of diets manipulated in this manner. In general the assumption that the manipulation of such foods is covert appears to hold, but subjects were far better at correctly identifying certain food types (dairy-based) over others.
authors.affiliation: Rowett Research Institute, Greenburn Road, Bucksburn, Aberdeen, UK. j.stubbs@rri.ac.uk
authors.forename: R J
authors.initials: RJ
authors.lastname: Stubbs
authors.forename: S
authors.initials: S
authors.lastname: Mullen
authors.forename: A M
authors.initials: AM
authors.lastname: Johnstone
authors.forename: M
authors.initials: M
authors.lastname: Rist
authors.forename: A
authors.initials: A
authors.lastname: Kracht
authors.forename: C
authors.initials: C
authors.lastname: Reid
mesh_terms: D000328:Adult; D004041:Dietary Fats / Q000008:administration & dosage*; D004435:Eating; D002149:Energy Intake; D005260:Female; D005502:Food; D005518:Food Preferences; D006801:Humans; D008297:Male; D011795:Surveys and Questionnaires; D013649:Taste
chemical_list: D004041:Dietary Fats