BURN BABY BURN

On an unashamedly superficial and consumerist whim, I decided to take a look at the current product du jour in the metabolism boosting market. Coats are being shed, skin is being bared, and quite frankly, revving up our sluggish winter metabolisms doesn’t sound like such a bad idea.

So what’s on offer for those hoping the way to a bikini-bod is via the health store rather than the gym? The latest miracle-in-a-bottle in this bewilderingly crowded market is Aspire, a calorie burning soft drink “proven to burn over 200 calories“.

Aspire was launched at the end of 2010 to an eager national media, excited by its claim that... wait for it... a scientific study proved it actually worked! Gasp. Faint. But what exactly do they mean when they say the drink ‘works’?   


Press coverage at the time just regurgitated the company’s snappy soundbites and their website was devoid of anything remotely useful (http://www.aspiredrink.co.uk/), so to find out a bit more about this study, I asked Aspire’s customer support team.  Just some basic questions about the trial design and outcomes, and why Aspire was different from other products on the market containing pretty much the same group of stimulants (taurine, green tea, guarana, caffeine, ginger and l-carnithine).  Apparently the website is being updated, but in the meantime, I would have to make do with this rather vague summary - a mixed group of men and women of all ages burnt on average 209 calories in a resting state over a 180 minute period after consumption of Aspire. 

So I went straight to those who conducted the study – the Carnegie Research Institute at Leeds Metropolitan University - who happily sent me a brief report of the results.  20 participants (tiny for a clinical study – generally this size is only used during the phase I stage of medical trials, when all the investigators are trying to determine is safety rather than efficacy) consumed the standard 250ml can of Aspire.  The effect on their resting energy expenditure (REE) was measured over the subsequent 180 minute period (REE is similar to basal metabolic rate, but is less stringent and considered to be more applicable to ’real-world’ scenarios away from the lab and the respiratory chamber).

The increase in REE by Aspire compared with the placebo looks impressive – 14.7%. And crucially this is statistically significant (P = 0.04) – those of you who remember your GCSE maths may recall that the P value refers to how likely it is that the result could have been obtained by pure chance (the null hypothesis), so a low P value indicates the REE increase is due to Aspire. Ta da!

But stats, as we all know, can be rather misleading. Is a 14.7% increase in calories burned over 3 hours significant in the context of human metabolism? What is obvious from the report but absent from anything provided by Aspire, is that those taking the placebo burnt off 184 calories over the same period of time. Which is to be entirely expected, as we burn calories every minute of every day just keeping ourselves alive.

On average, participants consuming the Aspire drink burnt an additional 25 calories over 3 hours.  Which sounds a lot less impressive than 200 and is roughly equivalent to a brisk 5 minute walk.  Given that Aspire retails at £1.69 a pop, I’m not convinced that’s great value for money.  Also, would the effects last day in, day out?  Might your body become used to the stimulants and cease the elevated REE response?  Even assuming these trial results could be maintained, to lose 1lb of fat, you need to expend 3,500 calories more than you consume, so that’s 140 days or 5 months of daily Aspire consumption. Purely for research purposes, I tried the drink myself, and I’m pretty sure I’d find the synthetic, acrid cranberry flavour a little repugnant after that long.

With the latest offering not quite living up to the hype, is there any hope for future products? Do we just need to improve the potency and type of stimulants? Or is the basic premise – that metabolism is malleable and can be manipulated at will – fundamentally incorrect?

Metabolism 101

The human metabolic system is a hugely complex system of substrates and enzymes and hormones and genes all interconnected and regulated by even more signalling molecules and neurotransmitters. But in short, the energy equation is actually very straightforward.  The energy you expend each day is determined by your personal basal metabolic rate (the rate at which you burn calories while doing nothing), the energy consumed during physical activity and a phenomenon known as diet-induced thermogenesis (from the Greek ‘to produce heat’).  When we digest food – a process involving the absorption, oxidation and storage of nutrients – we actually use up about 10% of the calories contained in the food itself¹. As different nutrients undergo different metabolic processes, the exact percentage of calories burned is not universal for all food types.  Alcohol and protein have the highest thermogenic effect, followed by carbohydrates and lastly, fat. 

Stimulants such as those found in Aspire aim to temporarily raise the metabolic rate by enhancing thermogenesis. Caffeine, green tea, chilli pepper at their ilk are not food sources themselves so don’t enter the metabolic pathways of digestion and thermogenesis – instead they work by stimulating or inhibiting various signalling pathways that control cellular metabolism. 

At the heart of many stimulants' mechanisms of action is the sympathetic nervous system (SNS). The SNS is responsible for our ‘fight or flight’ response and as such, has a wide-ranging effect on many tissues and systems throughout the body. In metabolic terms, kick-starting elements of the SNS is a surefire way to boosting both thermogenesis and fat oxidation². Noradrenaline (NA), a neurotransmitter (a molecule that conveys signals between neurons) is released by the adrenal glands in response to SNS activation.  It increases the body's usage of ATP (the molecular unit of energy) and decreases efficiency of energy conversion, with the result that more energy is wasted as heat – thermogenesis.  As such, NA is one of the prime mediators for stimulant-induced increases in metabolism.

S. Eman

For example, green tea is rich in catechins, a group of compounds for which antioxidant properties are widely attributed (often by modulating the expression of genes encoding proteins to combat stress). One catechin in particular, EGCG, has been shown to inhibit COMT, an enzyme that degrades NA. So more EGCG means more NA.  And as well as enhancing thermogenesis, NA binds to adrenoreceptors on the surface of adipocytes (fat cells) and stimulates lipolysis (the breakdown of fat). The free fatty acids are then released into the bloodstream, increasing their metabolism in muscle and liver cells.

Caffeine may increase NA levels, but also appears to have another mode of action - inhibiting phosphodiesterase, an enzyme that breaks down the cellular signalling molecule, cyclic AMP.  cAMP activates hormone-sensitive lipase which again facilitates lipolysis in adipocytes.  

A boost too far?

Caffeine and green tea are considered to be relatively safe for most of us – the worst side effects being raised blood pressure and mild dizziness.  Unfortunately not all dietary stimulants are quite so innocuous. Ephedrine originates from the Chinese shrub Ephedra sinica and also enhances the neuronal release of NA (and adrenaline). However a few rather scary studies have led to the compound being banned by the FDA for its potential to induce psychiatric symptoms and heart palpitations.

Interestingly, a chemically-similar compound, synephrine, is widely used in weight-loss supplements in place of ephedrine under the label ‘bitter orange peel’.  The US National Center for Complementary and Alternative Medicine explicitly advises against combining synephrine with caffeine due to the increased potential for heart attack or stroke³. But synephrine is entirely legal and is found in many products on sale in the UK (Maximuscle’s Sculptress and Thermabol capsules being just two examples), alongside, you guessed it, caffeine.  I should stress that the data on synephrine isn’t conclusive – it hasn’t been investigated to the extent of ephedrine – but as is symptomatic of the dietary supplement market in general, the clinical studies are few and far between. 

The sympathetic nervous system's involvement in thermogenesis and fat metabolism seems a good place to start when attempting to manipulate our metabolism.  Therefore products containing stimulants such as green tea or caffeine do at least appear to have some scientific rationale.  However I’ve trawled through numerous study papers and their overall conclusion is that calorie-burning stimulants have at best a very small and transient beneficial effect.  

Eventually, we may get there.  With the global weight loss industry expected to reach $568bn by 2014, there is good reason for big pharma and the supplement industry to chase this particular pot of gold⁴. But for this summer at least, the drinks and pills should probably be left on the shelf.

Here I have focussed on stimulants that explicitly aim to alter metabolism.  Others mechanisms to aid weight loss include impeding fat absorption (e.g. orlistat: Alli) or altering body composition (e.g. conjugated linoleic acid).

1.     Westerterp K. Diet Induced Thermogenesis Nutrition & Metabolism 2004, 1:5

2.     Diepvens K, Westerterp K and Westerterp-Plantenga M. Obesity and thermogenesis related to the consumption of caffeine, ephedrine, capsaicin, and green tea Am J Physiol Regul Integr Comp Physiol 292:R77-R85, 2007.

3.     http://nccam.nih.gov/health/bitterorange/

4.     Global Weight Loss and Gain Market (2009 – 2014), published by MarketsandMarkets