Thermic Effects of Some Scottish Plant Foods

Obesity prevalence is increasing around the world and the United Kingdom is no exception. Latest statistics show that approximately 25% of the UK population is obese and is predicted to rise to 70% by 2020 if present trends continue. Obesity is particularly widespread in Scotland where approximately 62% of the population are of a BMI over 25kg/m2. Curtailing obesity rates is a public health priority as excess weight is also the greatest predictor of other co-morbidities such as cardiovascular disease, cancer and diabetes, and hence collectively exerts a significant toll on the health and economic burden of the country.

Weight gain occurs primarily due to an imbalance in energy balance status where energy intake outweighs expenditure. Energy expenditure (EE) in adults consists of three components: basal metabolism (50-70% of EE), physical activity (30-40% of EE) and diet induced thermogenesis (DIT). The latter is described as the thermic effect of food and is the increase in EE above Basal Metabolic Rate after a meal, and usually ranges between 3-10% of total EE (depending on the composition of the meal). Whilst cornerstone approaches to weight control have focused on strategies to reduce energy intake and increase physical activity, the still escalating obesity levels show that this approach is ineffective in the long term possibly due to poor long-term compliance to dietary regimes and adequate physical activity. Thus it is being increasingly recognised that the most sustainable long-term strategy for reducing obesity is making small diet and lifestyle changes that would positively influence energy regulatory mechanisms targeted at preventing a positive energy balance. It is also now recognised that genetic predispositions to a slow mechanism characterised by a slow metabolic rate and capacity for fat oxidation also determines an individual's propensity to weight gain. Furthermore, studies have shown that weight loss and reduced food intake result in an attenuation of EE possibly due to the loss of lean mass and improved metabolic efficiency. The regular consumption of high DIT foods which could increase EE and fat oxidation whilst circumventing these energy sparing mechanisms may have significant potential in long-term weight control. Indeed studies demonstrating pronounced obesity in mice whose DIT mechanism were inactivated indicates the important role thermogenesis plays in weight control

Food components such as chilli (capsaicin), ginger, mustard, pepper, caffeine, green tea (flavanols) and medium-chain triglycerides have been demonstrated as increasing DIT. The thermogenic effects of these compounds have been shown to be mainly via the sympathetic nervous system. Pharmacological agents such as ephedrine and salicylate as well as food constituents such as methylxanthines (theophylline and caffeine) and caffeine have been shown to possess sympathomimetic properties and induce fat oxidation and thermogenesis in effector cells by stimulating the release of noradrenalin. Flavanoids such as green tea catechins have also been shown to stimulate this pathway. Sympathetically released noradrenalin has been shown to activate Beta3adrenoceptors and thereby induce thermogenesis and fat oxidation in peripheral tissues as well as activate uncoupling protein (UCP1) that mediates thermogenesis in brown adipose tissue (BAT). Plant phenolics have also been shown to demonstrate thermogenic effects, particularly Oleuropein, green tea catechins, Kaempferol, flavanol glycosides, stilbenes and quercetin.

The objective of the present pilot study is to assess the thermogenic effects of some plant foods native to Scotland that contain the above phytochemicals. Specifically, the study will investigate the thermogenic effects of berries (blackberries [Rubus sp.], blueberries [Vaccinium corymbosum] and red raspberries [Rubus idaeus L]), wild garlic (Allium ursinum) and sea buckthorn (Hippophae rhamnoides). Blackberries, blueberries and raspberries have been shown to be rich in flavonoids and phenolics including catechins and raspberry ketones which have been suggested to be thermogenic. These berries are also rich in anthocyanins such as cyanidin-3-glucoside which has been shown to potentially promote thermogenesis by upregulating UCP1. Furthermore, blueberries have been shown to be a good source of stilbenes such as resveratrol and pterostilbene which have demonstrated thermic effects elsewhere. Similarly, sea buckthorn has been reported to contain catechins, epicatechins and flavonols such as isorhamnetin, quercetin and kaempferol. Sea buckthorn is also rich in carotenoids, especially those with provitamin A activity that have been shown to upregulate the expression of UCP1 and thereby influence thermogenesis. Wild garlic contains Sulphur containing compounds such as alliin and Allicin derivatives (diallyldisulphide) that in studies using garlic have shown thermogenic activity by enhancing noradrenalin secretion and stimulating brown adipose tissue. Indeed the total thiosulfinate content in wild garlic has been shown to be comparable to garlic.

The presence of thermogenic compounds in these plant products logically suggests that they may have a thermic effect in humans and this forms the rationale for the present study. No studies have investigated the thermic effects of these foods in humans and their therapeutic potential in weight control from a thermogenesis perspective. The proposed pilot study will provide preliminary data on the thermic effects of these Scottish foods and provide a basis for future research.