The race against obesity continues apace. Last week, for instance, the bid to save New York's sugar addicts, by banning large bottles of soft drinks, fell flat. Meanwhile a small study published in the journal Neuroscience Letters discovered receptors for stress hormones in taste buds. This could help explain why people end up preferring sweeter foods in times of trouble.
"Stress and sweet are obviously linked. We've known that for a very long time," says Rocky Parker, lead author on the study at the Monell Chemical Senses Centre in Philadelphia. For instance, people with Addison's disease, a rare disorder that causes a deficiency of stress hormones, experience dramatically heightened sensitivity to sweet and salt tastes. But if you give someone with Addison's a shot of these stress hormones, their taste immediately returns to within normal realms of sensitivity.
The stress study
Parker's hunch was that when stress hormones are released in a healthy body, this might make it less sensitive to sweet taste, which could lead to, say, its regular dose of sugar in coffee no longer cutting it. The results of Parker's experiment (on mice) illustrated how this might occur. He found that oral taste cells (housed within taste buds) that detect sweet, savoury and bitter contain an abundance of receptors for the stress hormones, glucocorticoids (GC).
If three tastes are affected, why is sweet singled out and made the headline? First, the GC receptors were more prevalent in cells that pick up just sweet and umami, and second, says Parker, "umami cells make up a smaller percentage of the population of sweet/umami cells. The majority of them respond to sweet."
Of course, there are different types of stress. Being chased by a lion will dump a whole load of GC in your liver to convert fat stores into instant glucose energy. This is great both in terms of survival and slimming. However, the modern malaise of grinding, daily ongoing stress, often referred to as chronic stress, has the opposite effect. You don't exercise enough and your body goes into energy storage mode, while simultaneously, says Parker, after months of stress (our bodies can only autoregulate against these effects for so long), the GC "leads to long-term changes in gene expression, decreasing the amount of sweet-taste receptor you produce in your taste buds". That, says Parker, is the rub.
He readily admits that this is just one tiny aspect of how stress might be making us fat. Parker cites research by Mary Dallman, a stress and physiology expert at the University of California in San Francisco, showing that "sweet foods actually ameliorate the effects of stress, which I think is just so cool". It has to be nutritive sweetness, though. Artificial sweeteners have no effect. The body wants real sugar in the bloodstream, says Parker. "Your central nervous system needs that to basically not freak out."
Comfort food habits
In a review paper a few years ago Dallman described how CG also increase insulin secretion, which leads to craving palatable (fatty, salty and sugary) foods, while stress hormones also activate the parts of the brain used for learning and memory. And so if sweeties are making us feel less stressed, better even, then our brains can easily wire themselves into a dangerous cycle of comfort-eating. "Once stress-induced feeding becomes habitual," she writes, "the problem-solver, executive part of the prefrontal cortex may no longer be actively engaged in the outcome; 'comfort food' intake may become a reflex." While occasionally eating something pleasurable to relieve stress will not cause obesity, she reasons, "habitual relief of life's discomforts using this means inevitably leads to obesity".
To make matters worse, she says, "emotional 'comfort feeding', when used repeatedly, results in primarily abdominal obesity", which is the kind most commonly associated with heart disease and type 2 diabetes. This is because abdominal adipose (fat storing) tissue is more sensitive to the combined signals of insulin and glucocorticoids than in other parts of the body.
It's beginning to sound like a conspiracy. How can we ever possibly escape this cycle without banning sugar refinement globally? Well, there is some consolation in Dallman's statistics on how many people gain weight as a result of stress. She has it that 20% of the population's eating habits don't change, 40% or less consume fewer calories and lose weight, leaving the remaining 40% or more as the "gainers". That's still quite a lot of us getting fat. However, forewarned is forearmed, right? As Parker says, "if you notice that you're getting less pleasure than usual from sweet foods, it could be the effects of stress".
Dallman's advice is to "deliberately increase training of our cognitive, executive prefrontal brains to overcome emotional, habitual responses, using techniques like mindfulness and meditation". These practices, I hear, are also rather good stress-relief strategies. Or you can just say to yourself, as I am wont to on occasion, "I'm too busy, and besides, there are zillions of contradictory studies out every day", and have a piece of cake. So what's it to be: mindfulness or cake?
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