Abstract
Nuts are rich sources of cis-unsaturated fatty acids, vegetable protein, dietary fibre, phytochemicals, and essential vitamins and minerals. Epidemiological studies have consistently demonstrated an inverse association between frequent nut consumption and the risk of cardiovascular disease (CVD). Randomised controlled trials (RCTs) have shown a significant improvement in blood lipid profiles when following a diet enriched in different types of nuts.
Interestingly, although nuts are high in fat and energy, several studies report that nut consumption is not associated with adverse weight gain. This raises the question as to whether nuts have unique properties that protect against unwanted weight gain. No studies to date have been designed to determine whether regular nut consumption is different to the frequent ingestion of other energy-dense foods in terms of body weight regulation.
Recent evidence suggests that frequent nut consumption may have beneficial effects above and beyond their cholesterol-lowering effect. For instance, nut consumption may also improve novel risk factors for CVD such as oxidative stress, inflammatory markers, and endothelial dysfunction. The current recommended daily serving of nuts (30 g) is largely based on their lipid-lowering properties. The dose of nuts required to influence the aforementioned novel risk factors is unknown.
To achieve optimal health benefits, nuts must be consumed regularly and in sufficient quantity. Sensory-specific satiety (SSS) and acceptance (‘desire’ and ‘liking’) have been shown to influence intake both acutely and over the long-term. For example, an increase in SSS may result in early termination of consumption, which may compromise adherence to dietary guidelines. Furthermore, a decline in acceptance over time is likely to reduce compliance to consume nuts on a regular basis.
The overall aims of this thesis were: (i) to examine the effects of regular nut consumption on body weight and SSS in comparison to other energy-dense snack foods; (ii) to determine the effects of consuming nuts at different doses on novel risk factors for CVD; and (iii) to assess the effects of long-term nut consumption on acceptance. To achieve these overall aims, two randomised, controlled, parallel studies were conducted.
In part one of the first study (Chapter 3), 118 healthy normocholesterolaemic individuals were randomly allocated to receive daily portions of ~1100 kJ/d of hazelnuts (42 g), chocolate (50 g), potato crisps (50 g), or no snack food (control group) for 12 weeks. Body composition, resting metabolic rate (RMR), blood lipid profiles, and diet quality were measured at baseline and at week 12. The results showed no statistically significant differences in changes in body composition, RMR, and blood lipid profiles from baseline to week 12 between the groups, after adjusting for baseline value, sex, baseline age, and baseline body mass index (BMI) (all P ≥ 0.106). However, diet quality improved significantly in the nut group. Compared to all other groups, the percentage of total energy derived from saturated fatty acids (SFA) (all P ≤ 0.045) and carbohydrate (all P ≤ 0.006) was significantly lower whereas vitamin E intake (all P ≤ 0.007), the percentage of energy derived from monounsaturated fatty acids (MUFA) (all P ≤ 0.001) and polyunsaturated fatty acids (PUFA) (all P ≤ 0.011) was significantly higher in the nut group at week 12.
In part two of this study (Chapter 4), SSS and ad libitum intake for the snack foods were measured during a tasting session at baseline and at week 12. In addition, ‘desire to consume’ and ‘overall liking’ for the snack foods were also assessed at daily intervals during the 12-week exposure period. Twelve-week consumption of snack foods resulted in a statistically significant reduction in SSS in all three snack groups (P = 0.015). However no such changes were seen in the control group (P = 0.608). Ad libitum energy intake for the snack food increased over the study across all groups including the control group (P = 0.039). Daily exposure to chocolate and potato crisps for 12 weeks caused a significant decline in ‘liking’ (P = 0.002, P = 0.031 respectively). However, nuts were found to be resistant to such an effect, and ‘liking’ for nuts was stable throughout the exposure period.
Two important findings of this first study were that regular consumption of 42 g of nuts on a daily basis did not adversely affect body weight, and acceptance of the nuts remained high over 12 weeks. Whether the consumption of higher doses of nuts elicits similar results is unknown. Although 30 g of nuts is recommended to reduce blood lipid and lipoprotein concentrations, the dose of nuts required to influence novel risk factors for CVD may be higher. Therefore, a second study was conducted to investigate whether there was a dose-response effect of nut consumption on markers of inflammation and endothelial function. Importantly, body composition and acceptance were also assessed given the higher dose was in excess of current recommendations.
In the second study (Chapter 5), 107 overweight and obese individuals were randomly allocated to one of the three treatment arms: no nuts (control group), 30 g/d of hazelnuts or 60 g/d of hazelnuts for 12 weeks. These individuals were chosen as previous research demonstrates that chronic inflammation and impaired endothelial function is more pronounced among overweight and obese individuals. Body composition, blood lipid profiles, markers of inflammation and endothelial function were assessed at baseline, week 6, and week 12. In addition, ‘desire to consume’ and ‘liking’ for nuts were assessed at weekly intervals during the twelve-week exposure period. The results showed no statistically significant change in body composition from baseline to week 12 in both nut groups. Although 30 g/d and 60 g/d of hazelnuts significantly improved blood lipid profiles and endothelial function compared with baseline, these changes were not statistically significantly different from the control group after adjusting for baseline value, sex, baseline age, and baseline BMI (all P ≥ 0.065). However, hazelnut consumption significantly improved diet quality in a dose-response manner. Compared to the control group, both hazelnut-enriched diets significantly reduced carbohydrate intake (overall P < 0.001) whereas the intakes of MUFA (overall P < 0.001), vitamin E (overall P < 0.001), and potassium (overall P = 0.038) increased significantly. The ‘desire’ and ‘liking’ for nuts remained stable throughout the exposure period in the 30 g/d nut group, whereas both measures declined significantly in the 60 g/d nut group (both P < 0.001).
Unlike previous literature, hazelnut consumption had little influence on blood lipid profiles in both studies reported in this thesis. It is important to note that the participants in the first study were normocholesterolaemic while the second study was comprised of overweight and obese individuals. Recent evidence suggests that the response to a cholesterol-lowering diet is less pronounced in weight-stable individuals with high BMI and among those with low baseline cholesterol concentrations.
Our study disagrees with previous nut studies, which have shown significant improvements in novel risk factors such as endothelial function. An important difference between the current study and previous research is that our study simply added nuts to participants’ habitual diet. All other studies have not only included nuts, but have also altered the background diet or provided additional dietary advice.
An important finding of this thesis is that moderate nut consumption (up to 60 g/d) does not adversely affect body composition. In addition, nut consumption significantly improves diet quality in a way that would be expected to improve CVD risk. On the basis of SSS and acceptance data, the current guideline to consume one serving (30 to 42 g/d) of nuts regularly appears to be an achievable and sustainable behaviour. However, higher intakes (i.e. 60 g/d) compromised participants’ ‘desire’ and ‘liking’ for them. Therefore, this thesis reinforces the recommendation to consume a moderate serving (30 to 42 g/d) of nuts regularly as part of the heart-healthy diet.