What does chocolate ice-cream taste like? A simple enough question, you might think: sweet and creamy, with a slightly bitter cocoa kick. Delve a bit deeper, though, and the exercise becomes impossibly subjective, because what you taste when you eat ice-cream is not the same as the next person's experience. Your tongue and your taste buds are unique, and a sweet taste that seems strong to you might be almost undetectable to someone else.
Perhaps more importantly, individuals also vary greatly in how pleasurable and satiating they find ice-cream, or any number of other foods. Could someone's taste perceptions and preferences be a major influence on their weight?
The emergence of obesity as the world's largest preventable health disorder gives urgency to this question. Although the drivers of obesity are far more complex than simply a sweet tooth, study after study suggests that shifting taste preferences are a big part of the puzzle. The latest findings are forcing us to fundamentally re-examine our understanding of taste perception itself.
“If you're fat, you like sweet and fat better — that's part of what keeps you fat.”
Research into a link between an appreciation for sweetness and body weight goes back at least to the 1950s. And almost from the start, the evidence has been contradictory. A landmark 2006 review1 by Linda Bartoshuk, a sensory scientist at the University of Florida in Gainesville, offered an explanation for why the results have been so mixed. Bartoshuk argued that a taste described as “extremely sweet” by a lean person might not seem so sweet to someone overweight, because their food experiences are different — and ignoring this divide masks the difference in their taste preferences. “We discovered something that should have been obvious — that if you're fat, you like sweet and fat better — that's part of what keeps you fat,” she says.
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Although studies such as Bartoshuk's point to a slight preference for sweet foods among obese people, the link becomes much stronger when a food's fat content is also considered. “There's a relatively clear link between sweet and fat together — obese people tend to like sweet fatty foods,” says Lucy Donaldson, who studies taste perception and ill health at the University of Bristol, UK. “But they also tend to like fatty savoury things,” she adds.
Recent research into links between obesity and savoury preferences has not revealed any clear relationship with salt or umami taste perception, however. Only one link seems to hold firm. “What is strongly associated with obesity is not the consumption of carbo-hydrates or sweets — it is the consumption of fat,” says Yanina Pepino, who studies taste perception and disease at Washington University in St Louis, Missouri. “The obese crave more fat.”
Fat is not one of the established five tastes (sweet, sour, salty, umami and bitter). However, a growing group of researchers suspect that the sense of taste is implicated (see 'Hardwired for taste', page S7). For one thing, the tongue has taste receptors for two of the three macronutrients we need in our food: sweet corresponds to carbohydrates, and umami indicates protein. “It makes logical sense that we have some form of taste response to the other macronutrient: fat,” says Russell Keast, a sensory scientist at Deakin University in Melbourne, Australia. Fat content affects not only a food's taste, but also its appearance, texture and perhaps even smell. Keast and his colleagues have devised tests to assess whether people can detect fatty acid in custard when all sensory cues apart from taste are removed. “The sense of smell and any textural effects are minimized, and everything is under red light just in case there are any visual differences,” he says.
Just as carbohydrates and protein are revealed by the taste of their breakdown products, Keast thinks we can taste the breakdown products of fat — fatty acids. And intriguingly, the obese and overweight seem to be less sensitive at detecting them2. Last year, using their 'sensory-matched custards', Keast and his colleagues showed that people with low sensitivity for fatty acids tended to consume significantly more fat in their diet, and have a higher body mass index (BMI), than people with high sensitivity for flavour3.
Pepino is also researching fatty tastes. “We still cannot claim that fat is a basic taste, but within the limitations that we do know, the relationship between taste perception and obesity is strongest with fat,” she says.
The prime candidate for the fatty acid taste receptor is a protein called CD36. The initial evidence came from rodents: mice genetically engineered to lack CD36, for example, lose their natural preference for fat, whereas their affinity for other tastes remains unaffected4. Pepino is investigating similar patterns in people with natural genetic variants that lead to either over— or under — expression of CD36. “We hypothesized that if you are in the group with the higher expression level then you will be more sensitive to detect fat at very small concentrations. And that's exactly what we found,” she says.
Even so, the idea that a 'fat taste' even exists is not universally accepted. Bartoshuk, for example, questions how fatty acid detection could play a role in sensing dietary fat in the mouth. “Fat doesn't usually break down until it gets to the stomach,” she says. “And if you taste fatty acids in their pure form, they are bitter — really nasty. So how is something nasty, that's barely detectable, going to help you regulate fat at all?”
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Could insensitive taste receptors be to blame when you reach for that extra doughnut?
Keast argues that the explanation lies in the fact that fatty foods naturally contain low levels of free fatty acids alongside the fat itself. “As the fat level rises, so do the levels of naturally free fatty acids,” he says.
What's more, the unpleasant taste of fatty acids doesn't become apparent until their concentration is much higher than our threshold detection level — which is quite low, he adds. As food degrades naturally, the level of free fatty acids increases “until they can be recognized. This is unpleasant and indicates the food is past its best.” It's a similar story for other tastes, he adds — too much salt, for example, turns an enjoyable flavour into something unpleasant.
Cause or effect?
Now the correlation is emerging, the next big question pertains to causation. Does a reduced ability to taste fat tip someone towards eating an excess of fatty food, or does eating a fat-filled diet depress our sensitivity to fatty tastes?
The answer probably lies in the middle, with a balance of genetics and environment. “Whether you like fat depends on many different things, starting with what you're exposed to in utero and as a neonate, which sets quite a lot of your preferences,” says Donaldson (see 'Partners in flavour', page S4). But some studies have shown that these preferences are still malleable after childhood. Change your diet, and your preferences can be partly reset — for better or worse.
The early work on dietary malleability was carried out with salt. Consuming too much salt has long been associated with high blood pressure and hence an elevated risk of cardiovascular disease. Reducing salt intake over a period of time diminishes someone's desire for salty food. Donaldson and colleagues have recently shown with sugar that things can also swing the other way5. A group of young adults were asked to consume two sweet sports drinks a day for a month in addition to their regular diet. “We wanted to know if you could change sweet preference in a relatively short period of time — and we were quite surprised when we did,” Donaldson says. “The people who didn't prefer sweet things at the beginning preferred sweet things at the end.”
Keast is starting to see similar shifts with fat taste perception6. “If we go on a low-fat diet, our sensitivity to fats increases,” he says. In one experiment, “both lean and obese people responded to a very-low-fat four-week diet, becoming orally more sensitive to fat”. Keast is now beginning longer-term trials to assess whether this increased sensitivity can be translated into a decreased preference for — and hence consumption of — fatty food.
Obesity is a multifaceted disease with no simple, single cause. But for Pepino, one take-home message is already becoming clear. “Promoting a healthy diet and a healthy lifestyle should always be the first line of treatment for obesity,” she says. “For people who want to reduce their fat intake, they just have to persevere and it will become easier.”