4.1 Introduction
As discussed in the introductory chapter, eating and food selection in the real world may be viewed as one of a range of health-related behaviours potentially affected by stress. The relationship between stress and other health behaviours has been investigated in several studies; stress being associated with increases in ‘negative health behaviours’ (those which threaten health) such as alcohol drinking and smoking , and decreases in ‘positive health behaviours’ (those which promote health) such as sleep and physical activity (Conway, Vickers, Ward & Rahe, 1981; Steptoe, Wardle, Pollard, Canaan & Davies, 1996).
Dietary selection may incorporate both positive health behaviours (such as high intake of cancer-protective fruit and vegetables) and negative health behaviours (such as high intake of saturated fat, because of its known links with cardiovascular disease and certain cancers) From what has already been demonstrated with regard to other health behaviours and stress, it may be predicted that an effect of stress on food choice might be to increase intake of saturated fat and decrease intake of fruit and vegetables. Existing studies are inconsistent in their support for these predictions. Most have focused on overall energy intake or general changes in eating rather than on patterns of selection. As discussed previously, changes in eating habits during stressful episodes have been reported in several questionnaire studies (Spillman, 1990; Alexander & Walker, 1994; Ogden & Mtandbari, 1997; Oliver & Wardle, 1998) (Chapter 2). Naturalistic studies which have recorded intake in high and low stress periods show varied results. Some
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studies report increased eating under stress (McCann, Wamick & Knopp, 1990; Michaud, Kahn, Musse, Burlet, Nicolas & Mejean, 1990), and a few report hypophagia as the dominant response (Popper, Meiselman, Smits & Hirsch, 1989; Stone & Brownell, 1994). It would seem from the literature that eating more (stress hyperphagia) and eating less (stress hypophagia)are both observed, which may be the reason behind an apparent lack o f effects in some studies. For example, Bellisle et al’s (1990) study of a single meal intake in a group of twelve men pre- and post-surgery, showed no average difference in either energy intake or dietary composition between the two occasions. However the men did in fact show considerable variation, ranging from one man eating 125% more on pre-surgery day, to another who ate 53% less. One study has suggested that the direction of change in amount eaten is a characteristic which is stable over successive episodes of stress (Stone & Brownell, 1994) and which may be determined by a variety o f individual difference parameters.
Few conclusions may be drawn from existing naturalistic studies because of the diversity of stressors being investigated, and the variety of methodologies used, ranging from daily measures of stress and food intake, to retrospective self reports of intake relative to usual intake, in response to a specific stressful event.
The majority of naturalistic studies, have focused on the issue of whether stress increases or decreases eating behaviour in a general sense. As with the laboratory literature, studies addressing the issue of food choice under stress, are rather fewer. It was suggested above that stress may act to increase intake o f dietary fat. There is some evidence to support this hypothesis; these studies invariably also report increases in overall energy intake, presumably due to the higher intake of fat with its high energy
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density. McCann et al’s (1990) study related changes in dietary intake to serum lipids as a result of varying workloads in a small group of office workers. The workers reported a higher percentage of energy intake from fat (and also higher overall energy intake) in two high work load periods compared with a normal work load period, although changes in serum cholesterol did not map onto reported differences in dietary intake. Michaud et al (1990) studied the impact of a major school examination on the eating behaviour of 225 French high-school students. They found that the amount of fat in the diet (and total energy intake) were significantly greater on the day of an examination compared to intake on a control day on which there was no examination. Separate analyses of boys’and girls’ intake revealed that the increased fat intake was apparent only for the boys, whilst overall energy intake was affected only in girls. This suggests that differences in dietary composition, especially in the proportion of fat in the diet, may not be entirely responsible for differences in overall energy intake, as might have been thought. A study by Weidner, Kohlman, Dotzauer & Bums (1996) found similar changes in eating under examination stress.
One weakness of these studies is the lack of non-stressed control groups to dis- confound time and stress level. Consequently it is difficult to exclude the possibility that any changes in diet during stressful periods were due to differences in food availability influencing food choices during low and high stress periods. This is more relevant when formulating a mechanistic explanation of stress-induced food choice; however it is noteworthy that another study of examination stress which did include a control group, found no overall nutritional difference between students taking examinations, and others for whom there were no examinations scheduled at that time (Pollard et al, 1995).
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