besity is a complex metabolic disorder that is characterised by insulin resistance. This insulin resistance prevents the correct utilisation of food for energy. Ingested energy is instead partitioned to adipose tissue for storage where it accumulates as abdominal body fat. As insulin sensitivity deteriorates due to poor diet containing refined carbohydrates, the ability of the skeletal muscles to effectively oxidise either glucose or fatty acids is severely curtailed. The result of the diversion of food energy from oxidation to storage is measurable as a blunted thermic effect of food (TEF). Normally food ingestion causes a large increase in body temperature due to thermogenesis in the postabsorptive state. However, in obesity that response is measurably reduced. This reduction in the TEF is associated with a reduction in arterial adrenaline (here) suggesting that blunted thermogenesis might be caused by a metabolic abnormality originating in the sympathetic nervous system.
Forced energy restriction in obese individuals causes skeletal muscle loss and this lowers the resting metabolic rate (RMR). The combination of the reduction in RMR and the blunted TEF increases significantly the risk of weight regain. This is demonstrated by the considerably lower energy needs of previously obese subjects following weight loss. Increasing the energy expenditure of such previously obese individuals is therefore of paramount importance to ensure permanent weight loss. Caffeine and other methylxanthines present in tea and coffee are known to increase thermogenesis in humans and may be an effective strategy at causing long-term successful weight loss. Methylxanthines stimulate the sympathetic nervous system, which may be dysfunctional in obese individuals. Some studies have used large doses of caffeine to cause thermogenesis, but research suggests that the concentrations found in normal intakes of tea and coffee are also effective at improving energy expenditure in humans.
For example, researchers1 have investigated the effects of single doses of 100 mg oral caffeine on the RMR of 9 lean and 9 previously obese individuals. The lean subjects were selected because they had no difficulty maintaining normal weight. The previously obese subjects had been overweight with mild obesity and had lost bodyweight through a forced energy restrictive diet. Administration of a 300 kcal liquid meal caused an increase in thermogenesis in lean subjects that was higher (+25 %) than in the previously obese subjects (+15 %). Both groups maintained increased thermogenesis for about an hour after which the rate declined in both groups, but faster in the previously obese subjects. Consumption of caffeine with the same meal increased the thermogenic response by 12 % in lean subjects but by 25 to 30 % in previously obese subjects, thus ameliorating some of their blunted thermogenesis.
When the researchers administered repeated caffeine intakes every 2 hours for 12 hours, they caused increases in RMR of around 8 % in the previously obese and 12 % in the lean subjects. The authors calculated that this was equivalent to an increase in daily energy expenditure of 79 and 150 kcal, respectively. Interestingly, the researchers calculated that the food intake of the previously obese subjects in this study was 25 % lower than the lean counterparts, suggesting that weight loss had seriously impaired their RMR, most likely through skeletal muscle loss. The previously obese subjects maintained their body weight on 1600 kcal/d, which was 500 kcal/d less than the lean controls. The RMR of the previously obese subjects was 8 % lower than the lean subjects when expressed as a percentage of their body weight, although in absolute terms it was not significantly different.
These result suggest that caffeine may be effective at preventing weight regain in previously obese individual because it can increase RMR and the TEF. Methylxanthines can stimulate the sympathetic nervous system, which in turn increases the oxidation of fatty acids, and this may be one way that they are effective at increasing energy expenditure. Green tea has shown weight loss effects over and above that provide by the caffeine content (here), and so might be a better alternative to other methylxanthine containing drinks. Synergism between nutrients is vastly under researched in the nutritional science, but anecdotal evidence suggests that combining beneficial nutrients may be an effective strategy. Calcium is effective at causing weight loss and preventing muscle wasting (here) and is beneficial at increasing RMR. It would therefore make sense to combine methylxanthine consumption with a high calcium diet, if weight loss was a priority.
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