Naughton, Shaan (2017) Dietary fats in the Australian diet and their impact on obesity, appetite and metabolism. PhD thesis, Victoria University.

Abstract

Obesity is a worldwide problem, and as such a large body of research has focused on potential treatments to decrease excess body weight and the associated loss of adequate metabolic health. Obesity risk is influenced by more sedentary lifestyles, higher energy intakes and greater consumption of convenience and processed foods. Worldwide evidence indicates that these factors occur as part of nutrition transitions, typified by an increase in energy density and plant derived fats, refined carbohydrates and a reduction in the variety of food consumed. It is believed that this increase in availability of particular plant derived fats can influence health, with the monounsaturated fatty acid (MUFA) oleic acid (OA) appearing to preserve metabolic health, and the polyunsaturated fatty acid (PUFA), linoleic acid (LA), having potential negative impacts, though this has not been fully explored. The first study of this thesis aimed to investigate changes in the Australian diet from historically available population level food disappearance data and how the availability of common dietary fats, specifically OA and LA changed, and what foods have influenced this. This study found that over the time period 1961-2009 total available energy (TAE) from lipids increased 16.67 %, with a slight reduction in TAE from carbohydrates. Cumulative change in TAE from LA was +120.48 %, found to result from total plant oils having a + 627.19 % cumulative change in LA availability. As the effect of these fatty acids (FA) on weight gain and metabolic health is yet to be fully elucidated, a rodent study was performed, with 60 Sprague Dawley rats being fed a high fat ‘Western’ style diet to induce obesity for 9 weeks. The animals were then switched to diets with varying FA compositions, namely a standard ‘Western’ style diet, a high OA ‘Mediterranean’ style diet and a high LA diet for 6 weeks. The impact of these diets on body weight and composition, energy intake, glucose handling and adipose tissue and skeletal muscle metabolism was also investigated. As adiponectin is an insulin sensitising adipokine the ability of this to interact with FA and influence gene expression relating to FA metabolism in skeletal muscle was also investigated through incubation of excised muscles in physiological levels of globular adiponectin. 6 weeks of specific high fat diet feeding in male Sprague Dawley rats resulted in no significant differences between body weights, food intake, blood pressure, percentage body fat, or adipose depot specific change to FA metabolism genes for the 3 high fat fed groups. Despite this, this study does show that there is a potential for the type of dietary fat to modulate food intake and energy efficiency (week 6 high OA vs. high LA, 0.020 ± 0.005 and 0.037 ± 0.003, g gain/mJ, p=0.033), with reductions in both of these occurring for the high OA consuming animals towards the end of the study. Additionally, investigation of changes to gene expression in the muscles from these animals found the ability of adiponectin incubation to increase FAT/CD36, β-HAD and CPT1 mRNA expression in both the EDL and soleus from high OA fed animals (all p≤ 0.05 compared to tissue incubated without adiponectin), a response which was not seen in any of the other diet groups. This may indicate that high OA feeding preserves adiponectin sensitivity in both of these muscle types and may confer metabolic benefits not seen in the other high fat diets tested here. Finally, the ability of different FA to modulate acute appetite and hormonal responses following consumption of isoenergetic breakfasts containing elevated contents of carbohydrate, OA and LA respectively, was investigated in eight overweight and obese individuals (age 45.8 ± 3.6 years, BMI 32.0 ± 1.3 kg/m2 ) in a 3-way crossover single-blinded study. Consumption of all meals resulted in a significant increase in fullness and a reduction in desire to eat, with the control and high OA meals decreasing prospective food intake (p= 0.006 and p= 0.049 respectively), though this did not occur following ingestion of the high LA meal (p= 0.183). Additionally, following the consumption of the LA meal there was a significant increase in ghrelin production (compared to the control meal, p= 0.018). This was coupled with a spike in resistin production at the one hour time point, indicating potential impairment of insulin signalling. Taken together this indicates that high LA meals may promote excess energy intake and impair glucose handling, though further investigation is required. The results of this thesis indicate that LA availability in Australia has increased over the preceding 5 decades as a result of increased plant oil availabilities, as has total fat, possibly contributing to the increasing rates of obesity and obesity associated co-morbidities. Results from the animal and human studies indicate that a higher OA intake as opposed to LA may help to preserve skeletal muscle FA metabolism and potentially limit food intake, indicated by the reduction in food intake and energy efficiency seen in the high OA diet consuming rats in the rodent study and the increase in the appetite and food intake stimulating hormone ghrelin following consumption of the high LA meal in the human study.

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