It achieves the impossible – its makers say – by making sure the quantities you desire are not very great. Sensa comes as granules that are added to every meal and snack you eat. Put simply, the Sensa "sprinkles" are designed to enhance the sensory experience of eating, stimulating taste and smell to an extent that fools the brain into thinking you've eaten more than you have. Users have reported the novel experience of happily leaving food untouched on their plates.
Depending on which expert you talk to, taste is between 75 and 90 per cent about smell, and Sensa is not the only new product on the market in the States that claims to exploit the apparent connection between strong smells and smaller appetites. SlimScents are pens filled with fruity or minty smells, sniffed before meals. Aroma Patch is vanilla scented and worn permanently, like a nicotine patch. All boast scientific validity.
A limited number of studies have been done. Dr Alan Hirsch, the scientist behind Sensa, conducted his own research in 2005 on what would later become Sensa granules. The study followed over 1,400 subjects over a six-month period, and recorded an average weight loss of 30.5lb, and a five-point drop in Body Mass Index.
Kimberly Tobman, a spokeswoman for Sensa, says those results have since been duplicated in a smaller study carried out by an independent laboratory.
And last year Dr Bryan Raudenbush, an associate professor of psychology at the Wheeling Jesuit University in West Virginia, conducted a small study which found that subjects who regularly sniffed a peppermint aroma consumed, on average, 1,800 calories fewer over the course of a week than normal.
Raudenbush is not convinced by the miraculous claims of Sensa and others, and suggests we take them "with a grain of salt and cautiousness". But he does think something is going on.
"From what we have found in other studies, peppermint scent can distract you from painful stimulation," he says. In one of them, participants held their hands in cold water for prolonged periods. "Participants who were administered peppermint scent held their hand in the water for a longer period of time and rated the pain as less severe."
He believes that something similar may be at work in the appetite experiments: strong smells are distracting participants from physical discomfort, whether that means pain or hunger.
Professor Tim Jacob, an expert in smell and taste at Cardiff University's School of Biosciences, is more sceptical of the connection between strong scents and weight loss, not least because we tend to get habituated to smells very quickly. But he thinks the idea that scents can distract us from pain or allow us to endure more of it is valid.
"The olfactory (sense of smell) system and pain share some brain networks and it's thought that the positive consequences of experiencing pleasant or familiar odours offsets pain to a measurable extent," he says.
In fact, there's increasing excitement in the scientific community about the power of our sense of smell, and what consequences this may have for psychological and physiological health. Though much of the research is in its infancy, various studies have shown that scents like peppermint, vanilla and coffee may have therapeutic effects.
In a recent study published in the Journal of Agricultural and Food Chemistry, for example, researchers at the University of Tokyo found that inhaling Linalool, a natural chemical found in flowers and spices, significantly reduced stress levels in rats. And a study at Tubingen University in Germany showed that vanilla fragrance reduced the startle reflex, making us calmer.
Scientists involved in this research are keen to distance themselves from what many see as the quack principles of aromatherapy – the complimentary therapy that recommends administering pleasant smells for anything from cancer to the common cold – which Professor Jacob calls "nonsense".
But Jacob and others in the field of olfactory research believe the connection between smell and memory – and the associative power of odour – represents a hugely promising avenue of investigation.
"Using conditioned association we could use smell therapeutically, to treat sleep problems, anxiety, blood pressure, etc; and even clinically, possibly for immune system pathologies, intractable medical conditions, for example lower back pain; and use it for drug rehabilitation," says Jacob. "Smell, once conditioned, can re-evoke a psychophysiological state. It relies upon the association of smell and memory."
That might sound ambitious, but it's exactly what Jacob and colleagues are researching in his Cardiff lab at the moment. To put it simply, if you associate a certain odour with a particular state of mind – being relaxed, for example, or positive – you may be able to recreate that state of mind, at least to some extent, by simply introducing the associated smell. In fact, many of the powers ascribed to the scent of vanilla could be down to the link between memory and smell. Vanilla is a universally positive childhood aroma – holiday ice creams, perhaps, or the smell of baking on cosy Sunday afternoons. Recreating positive childhood emotions may explain vanilla's oft-vaunted powers of relaxation or stress relief.
And, as Professor Jacob suggests, it may be possible to programme smell associations for particular therapeutic tasks. In the most famous study of this kind, healthy male volunteers were injected with insulin every day for four days and their blood sugar fell. At the same time, they were exposed to a smell. On the fifth day they were just given the smell, and their blood sugar still fell.
Such findings hold out the promise of some pretty mind- boggling medical advances, from diabetics with inhalers instead of injections, to insomniacs cured by a smell they associate with sleepiness. We're not quite there yet, but as Jacob says, "watch this space".
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