Eating Behaviour and Appetite
Sarah - The eating habits we develop early in life can be very hard to change, so it's no wonder that people can struggle to maintain a diet and find it very hard to shift weight that they've already gained. Marion Hetherington is Professor of Biopsychology at the University of Leeds, where she works on understanding appetite and eating behaviour. So Marion, what are the factors that we think influence how we behave towards food?
Marion - Well, there's some evidence to suggest that influences on food intake actually occur very early on in life. In fact, in-utero experiences seem to be really interesting and really important. There's some very elegant data from Julie Mennella who's at the Monell Chemical Senses Centre in Philadelphia and she has shown that the foetus is exposed to flavours from the maternal diet. So right from the very beginning of life, there's an experience of flavour, an experience of odours, and that these can have an important effect on later food intake. Also, there's the environment that the maternal diet brings. There's the environment supplied by toxins in the environment. For example, if the mother is smoking, this will influence the child's growth, but also, it will therefore entrain appetite.
And then, of course, whenever the baby is born the decision to breastfeed or bottle feed with formula, this will have an impact on whether the child is able to accept lots of different flavours. For example, breastfed babies tend to be very good at trying new flavours and new tastes, and new foods, and formula fed babies tend to be a little bit more resistant to trying new foods. So the environment in utero is important. The environment that you provide postnatally is important and of course, on top of all of that is the genetic inheritance that you get from each of your parents. We know that food intake can be driven by genotype.
Sarah - Our experiences early in life, do we learn our eating behaviours from the sort of foods our parents provide and also the way that they behave around food as well?
Marion - The infant is born highly adapted to accommodate a milk-based diet, so they're very good at accommodating breast milk. After 6 months on being on breast milk, they have to then start acquiring a liking for new foods and they have to do that through learning. However, they're equipped to like sweetness right from birth. There was a beautiful study by Yakov Steiner in the 1970s; he exposed newborn babies before they'd been given any breast milk or any colostrum, any type of feeding, to distilled water or to citric acid and water, quinine sulphate in water and to sucrose in solution, and he found that sucrose in solution produced a positive, affective response. So, newborn babies are positively affected by sucrose whereas with bitter flavours, the response of the infants was to gape and to let the fluid drop off the mouth. With bitter tastes, we have to acquire them through experience and through learning. But sugar, you don't have to learn to like because you like it right from the very beginning.
So when parents are offering different foods to their children, they have to bear in mind perhaps that sweetness is something that they don't have to expose them to a lot because they already like it and it's a very powerful food stimulus. Whereas with something bitter like vegetables, they have to be quite persistent and quite positive. The optimal number of times that you need to offer novel food like a vegetable is 8 to 10 times. But on average in the UK, mothers tend to offer vegetables about 2 or 3 times before they decide, "Well, my baby doesn't like this". But actually, if the optimal amount of time is 8 to 10 times, they've perhaps have missed that opportunity to encourage the child to accept these novel foods.
Sarah - Is there a window of opportunity to get children interested in eating vegetables?
Marion - There's some really nice work by Gillian Harris at the University of Birmingham and she has suggested that there are sensitive periods in early infancy. This suggests, for example, there are really good times to introduce texture, there are good times to introduce flavour, and very early on in life - infants around about six months are very accepting of lots of different fruits and vegetables and flavours. Textures are particularly good to start to bring in just after about 6 months. There is data from the longitudinal study based in the Bristol in Avon area - ALSPAC data - suggesting that if you don't introduce lumpy foods by 9 months of age then children can be quite fussy later on in life. So, these crucial periods, these sensitive periods, suggest that parents do have a window of opportunity to offer different flavours and different foods, and if those chances are missed then it's actually quite difficult to get children to try new foods, and also, to like new foods. We've done research with school age children and it's really difficult because once they get to school, they're already 5, they have 5 years of experience. They already are quite resistant to trying particularly foods that they consider to be novel and different, and we get children telling us that foods are slimy like spinach and that kind of thing, so it's really tough. It's not impossible but it's harder when they're older and much better if you try to introduce these foods early on.
Sarah - If someone wasn't perhaps exposed to these things as a child and they are now not very keen on eating healthy food, is there any way to kind of re-learn the behaviour of liking this kind of thing?
Marion - I think the key to that is being positive and being persistent. There are some really nice data from the Food Dudes programme which was started in Bangor by Fergus Lowe and his colleagues. Basically, they had children who really disliked vegetables, and they started to introduce vegetables using stickers and using rewards, and using praise. These kinds of efforts showed that actually, children can learn to like foods like vegetables, like raw red pepper, and celery, and all of these different things, but this is with social reward. There's also data from Jane Wardle's lab in UCL showing exactly the same thing; that if you give social praise to children and if you give tangible rewards like stickers, the children appreciate the reward and they will eat the food more. It's actually quite a durable effect. So several weeks later if you go back and test the children on their liking for these new vegetables, they will like them again.
It does seem like it's never too late to learn, but it's much harder when they're a bit older and they have to use all these behavioural strategies such as tangible rewards and social praise and modelling. And certainly, if parents are modelling vegetable intake and healthy eating from the very early stages of life including the maternal diet in utero, for example, then that will set up children best for healthy eating later in life. But then, all of that gets turned on its head when you talk about teenagers because they eat very differently again. But, they resume, it seems, to their previous eating habits later in life. So if they've been set up with good healthy eating habits early on in life, it's easier to resume those later in life.