Screening Measures

A prominent feature of activation of the HPA axis by stress is the inter- and intra- individual variability in cortisol response. Research has demonstrated that cortisol responsivity to stress is influenced by numerous moderating and intervening factors (Biondi & Picardi, 1999; Dickerson & Kemeny, 2004; Kudielka, Hellhammer, & Wust, 2009). Awareness of modulatory factors can be used to inform the design of psychobiological research in which cortisol response is a primary outcome variable and potentially improve the signal to noise ratio. The recognition of potential confounding modulators influencing primary outcome variables is important considering the relatively modest effect sizes demonstrated in psychobiological research, and the tendency for small effect sizes in dietary intervention studies. Potential modulators of cortisol responsivity were assessed at screening and used to inform inclusion/exclusion criteria and selection of relevant covariates to include in statistical models used in the analysis of data presented in this thesis. Specific inclusion/exclusion criteria employed in each

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study are detailed in each experimental chapter. Measures employed in the screening of participants are outlined here.

3.3.1

The Perceived Stress Scale (PSS)

The PSS (Cohen, Kamarck, & Mermelstein, 1983) is a 10-item self-report scale used to measure appraisal of perceived stress (Appendix 7). This global measure assesses how frequently respondents have experienced an uncontrollable, unpredictable or overloading situation during the last month, and the perceived effectiveness of individual ability and confidence to cope with this stress (e.g., “In the last month, how often have you felt that you were unable to control the important things in your life?”). Responses are made in reference to a five-point Likert scale: never = 0, almost never = 1,

sometimes = 2, fairly often = 3, and very often = 4. Likert scoring is reversed for

negatively weighted items. The PSS has been translated and validated widely in healthy and clinical populations. Reliability coefficients for the PSS range between rα = .83 – .86 (Cohen et al., 1983).

The PSS was employed as a measure of perceived chronic stress level. Chronic stress can be defined as the prolonged and/or repeated exposure to a stressor or stressors. Animal (Akana et al., 1992; Brodish & Odio, 1989; Pecoraro, Reyes, Gomez, Bhargava, & Dallman, 2004) and human data (Chrousos & Gold, 1992; McEwen & Stellar, 1993) have demonstrated that chronic stress can modulate HPA axis function. There remains some heterogeneity in the literature regarding the effect of chronic stress upon acute cortisol response to challenge with evidence of no effect, hypo-, and hyper-reactivity reported (Kudielka, Hellhammer, & Wust, 2009; Kudielka, von Kanel, Preckel, Zgraggen, Mischler, Fischer et al., 2006; Melamed et al., 2006). The level or stage of chronic stress appears to be a key determinant of the relationship. An initial hyper-reactivity of the HPA axis to acute stress may be demonstrated in the early stages of exposure to chronic stress. Hyporeactivity may emerge later once a stage of exhaustion or burnout is reached (Kudielka, Bellingrath et al., 2006; Kudielka, von Kanel, Preckel, Zgraggen, Mischler, Fischer et al., 2006).

Considering evidence of an association with blunted cortisol responsivity, chronic stress was deemed an important factor to account for in sample selection. To reduce the potential influence of chronic stress upon study outcomes participants reporting high levels of perceived chronic stress (defined as ≥ 30 [PSS score range 0 – 40]) were

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excluded. The PSS score of all study participants was included as a covariate in relevant statistical models.

3.3.2

The Hospital Anxiety and Depression Scale (HADS)

The HADS (Zigmond & Snaith, 1983) is a self-report measure widely used to assess the symptom severity and caseness of anxiety and depression in clinical and general populations (Appendix 8). The 14-item measure is comprised of two, 7-item, subscales: anxiety (HADS-A) and depression (HADS-D). Respondents are required to choose from four-point Likert scale response options specific to each item relating to how they have been feeling over the last week (e.g., “I feel tense of wound up.... most of the time, a lot of the time, from time to time/occasionally, and not at all”). Each item is scored 0 – 3 resulting in scores ranging from 0 – 21 for each subscale. Reliability coefficients for the anxiety subscale range between rα = .68 - .93, and depression rα = .67 - .90 (Bjelland,

Dahl, Haug, & Neckelmann, 2002; Olsson, Mykletun, & Dahl, 2005). A number of different cut-off points have been used in the identification of “caseness” of anxiety or depression. Scores on either scale between 0 – 7 have often been adopted to represent 'no case', 8 – 10 indicate a 'possible case', and 11 – 21 suggestive of a 'probable case’ of anxiety or depression. A review of the literature by Bjelland et al. (2002) revealed an optimal balance between sensitivity and specificity was achieved when a score of ≥ 8 was adopted as a cut-off point for caseness on both scales.

The modulation of cortisol response to acute psychosocial stress by affective mood disorders (e.g., major depression, anxiety disorder, and social phobia) has been widely reported. The difference between clinical and non-clinical populations appears to be particularly prominent when the HPA axis is challenged (Burke et al., 2005; Jessop & Turner-Cobb, 2008; Tsigos & Chrousos, 1994, 2002). Accordingly, the HADS was employed as a screening tool to exclude individuals with suspected affective mood disorders from participation. A cut-off point of ≥ 8 on either HADS subscale was adopted as an exclusion criterion across all studies presented in this thesis as indicative of potential mood disturbance (Bjelland et al., 2002). Respondents scoring above these cut off points were advised to seek further advice from their GP if they had any concerns about their mental health. Contact details of sources of mental health support were also provided in all participant information sheets in accordance with ethical principles.

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3.3.3

Body Mass Index (BMI)

Body mass index is a commonly employed index of human body weight using mass and height to calculate underweight, overweight, and obesity cut off points. Body mass index is calculated by dividing body mass (weight in kg) by the square of height (in m) expressed in units of kg/m2. The World Health Organisation currently classifies a BMI of

< 18.5 kg/m2 as underweight, ≥ 25 kg/m2 as overweight, and ≥ 30 kg/m2 as obese (WHO, 2013).

The abdominal obesity phenotype has been associated with a number of alterations in HPA activity in both sexes. This includes altered ACTH secretion, increased reactivity to laboratory stressors, and hyper-responsiveness of the HPA axis to CRH and AVP (Pasquali et al., 2002; Pasquali et al., 1996). Emerging evidence also highlights a potential for increased impairment of cognitive performance under conditions of acute stress in individuals with abdominal obesity (Lasikiewicz, Hendrickx, Talbot, & Dye, 2013).

Considering the potential moderating effect of obesity on parameters of cortisol response individuals with a measured BMI ≥ 30 kg/m2 were excluded from participation across all studies. The BMI of all eligible participants was included as a covariate in relevant statistical models.

3.3.4

Study Exclusion Criteria

The following exclusion criteria were common across studies (criteria specific to studies are stated in the method sections of respective study chapters):

 BMI ≥ 30 kg/m2

 Medication use (prescribed and ‘over-the-counter’)  Smoking

 Recreational drug use (last month)

 Current psychological affective/mood disorders (HADS subscale score > 8; Zigmond & Snaith, 1983)

 Perceived Stress Score ≥ 30 (Cohen et al., 1983)

 Endocrine, cardiovascular, or other chronic diseases (ascertained by a health screening questionnaire; Appendix 9)

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 Hypertension. Participants with a resting BP exceeding 140/90 mmHg over repeated measurements at screening

 Pregnancy, planning pregnancy or lactating  Night shift work