Christmas means different things to different people. For children, of course, it means presents and the magic of the festive season, for the spiritual, it’s a time for prayer and reflection and for the exhausted, it’s an opportunity to hibernate and watch all the box sets that have accumulated during the year.
But most of us have one thing in common. The festive season gives us an excuse to stuff our faces with as much food as we possibly can. Some of this food is ok. Turkey and sprouts, for example, are fine, upstanding healthy foods. But a great deal of our yuletide intake consists of sugary and salty snacks that at best make us put on weight and at worst may lead to one or more of the various unpleasant conditions that come under the umbrella of the metabolic syndrome. And because it’s Christmas, we allow ourselves to eat both good and bad food in humungous quantities.
Food intake is controlled by the balance between appetite and satiety. Two peptide hormones, leptin and ghrelin, play an important role in the modulation of these alternating states. When we put food in our stomachs, leptin is released by adipocytes, ultimately making us feel full. We stop eating and leptin levels begin to decrease. Ghrelin is then released from cells in the gastrointestinal tract, resulting in hunger. Leptin and ghrelin interact with neurons in overlapping parts of the central nervous system, including the hypothalamus and the mesolimbic reward pathway, causing release of neurotransmitters that make us modify our eating behaviour. In an ideal world, this hormonal and neuronal balancing act should allow us to control our food intake without even thinking about it.
In reality, however, satiety is a complex affair, involving many physiological and psychological factors. The types of foods we eat can have an impact. High fat, energy dense foods appear to compromise satiety and appetite control, often causing us to eat too much. On the other hand, higher protein intakes enhance satiety. In one study, for example, consumption of a high-protein snack with low energy density (yoghurt) three hours after lunch was more effective than an energy-dense, high fat snack (chocolate) at reducing afternoon hunger, delaying subsequent eating initiation and decreasing the number of calories consumed at dinner. Interestingly, coffee consumption was shown to decrease ghrelin levels and reduce energy intake and body fat, whereas ingestion of the umami flavour compounds, monosodium glutamate and inosine monophosphate, in a soup had a biphasic effect, enhancing appetite when the soup was first tasted, but also improving satiety after ingestion.
But of course many people eat not only in response to metabolic need, but also to hedonic hunger, which is the result of motivational, cognitive and emotional influences. This type of hunger leads to increased consumption of foods high in fat and sugar. It makes people feel better or happier and generation of these feelings reinforces the desire to each such foods more frequently. Hedonic eating habits can sometimes resemble addiction-like behaviour, comparable to that shown by alcoholics and drug users. Interactions have been observed between hedonic eating patterns and appetite regulatory hormones. For instance, women suffering from both stress and cravings for sweet foods were shown to have higher basal leptin levels than those who didn’t experience the cravings.
But none of this fully explains why most of us would find it easier to wolf down a high calorie load in the form of a large packet of a salty, savoury snack or a big bar of chocolate than to munch their way through half a raw cabbage, despite its much lower energy content. Nor does it provide a decent justification for the gluttony that takes place at Christmas.
After a traditional Christmas dinner of turkey and all the trimmings, most people sit back in their chairs sighing, paper hats askew and crumpled on their heads. They can hardly bring themselves to look at the ravaged carcass in the centre of the table and the thought of eating a single extra sprout is enough to send them into a stupor. But look what happens ten minutes later. The main course is cleared away and everyone springs to life as the Christmas pudding arrives in all its flaming glory, possibly accompanied by a chocolate yule log and maybe a sherry trifle. Plus a generous jug of cream or brandy sauce. Most of the diners state cautiously that they could just about manage a small portion of something and then go on to eat quite a lot of it. After this, the festive crew all declare that they’ll never eat again and fall asleep in front of the telly. But then the chocolates come out and at some point in the evening, the prospect of tasty supper begins to look attractive - perhaps a couple of turkey sandwiches, a slice of cheese, some pickles and a nice bit of pork pie. Even though the revellers are by now well and truly stuffed, they continue to eat as long as they are presented with a variety of different foods.
This Christmas eating phenomenon is an example of sensory specific satiety (SSS), defined as a hedonic phenomenon in which the desire for and the satisfaction generated by a particular food decreases as a person continues to eat it, but the appetite for other foods remains relatively unaffected. SSS is thought to be mechanism to ensure that the body receives a varied diet. It’s responsible for the fact that people eat larger amounts of food at buffets than they do when presented with courses consisting of single foods and for the observation that adding a sauce to a bowl of pasta half way through a meal can make you eat more of it.
SSS is influenced by many factors. The elderly experience relatively low levels of SSS and adolescents are more susceptible to it than the average adult. Perhaps surprisingly, obese people experience it to the same extent as their normal weight counterparts – it’s the variety of foods on offer that makes them put on weight. SSS kicks in more quickly when high protein foods are being eaten, but unfortunately, the higher the energy content of a food, the less likely it is that SSS will become activated.
Some foods, particularly savoury snacks, are engineered to override SSS. I can think of one product, which shall remain nameless, that is actually marketed with a snappy slogan to the effect that having started eating it (or them) the average consumer won’t be able to stop until they’ve polished off the lot. I can attest that for me at least, this is the truth. The secret lies in the composition of these so called “junk foods”. They contain a mix of ingredients, including a large amount of fat and a range of chemicals that combine to taste great but don’t produce one single strong flavour that would cause SSS to kick in and tell our poor brains to stop eating. Many of these items also have melt-in-the-mouth properties fooling us into thinking we have consumed fewer calories than is actually the case.
I suppose the only way to avoid obesity is to avoid these clever and devious foods, eat a monotonous diet and let ghrelin and leptin do their job. But for me, this is too much to ask. Particularly at Christmas…