With days to go before Christmas, my thoughts are (as ever) turning to the culinary delights that await me. For many of us, myself included, Christmas is the holiday of eating – a huge turkey dinner, mince pies, Christmas pudding, ice cream, chocolate, cheese… All consumed throughout the day resulting in several hours dozing on the sofa in front of the Downton Abbey Christmas Special. Bring it on.
But why do we eat so much at Christmas? Why don’t I think “no, I don’t need that 17th roast potato” and push my plate away? For many of us, overeating at Christmas doesn’t stop on December 31st, it continues well into January. With obesity and health related issues one of the biggest killers in society, scientists are studying chemicals produced by our bodies to better understand the problem.
Humans have an in-built regulatory system designed to prevent us from overeating. When we consume large amounts of fat, a hormone called leptin is produced which sends a message to our brain, signalling that we are full. This should prevent us from continuing to eat. If fat reserves are low, signals are sent encouraging us to feed. In addition to this, feedback pathways run from our gut to the brain, signalling satiety and limit the amount of food we consume in a meal. Whilst many of the incidences of obesity are down to lifestyle, it has been suggested that some cases could stem from a failure in these systems .
One such failure is the problem of leptin resistance. This can be caused by genetic mutation, limited tissue access or leptin self-regulation. There is evidence to suggest obesity is caused by leptin resistance and that obesity of this kind can damage tissues such as the liver and pancreas .
Another hormone involved in digestion is insulin. Insulin helps our cells absorb glucose from the blood to use as energy. When we digest food, insulin is released by our pancreas into the blood – the amount varies depending on the amount and type of food consumed, with carbohydrates and sugars being the most effective. A study by Loughborough University showed that adults who consumed 50% more calories in a week, 60-65% of those calories being fat, showed decreased sensitivity to insulin by 25% when their eating behaviours returned to normal. Lower insulin sensitivity means more insulin must be produced to encourage cells to take up blood sugars. High amounts of insulin in the blood can result in diabetes and weight gain, amongst other metabolic disorders .
Leptin and insulin is not the only substances linked to overeating. Pleasurable behaviours, like eating and drug use, can release a neurotransmitter called dopamine which makes us feel good. This response is often mentioned when discussing substance addiction. A 2001 study in The Lancet found a small number of dopamine receptors in the brains of obese people, similar to the number found in individuals addicted to drugs and alcohol. Rats with low numbers of dopamine receptors were found to immediately start overeating when given access to a high fat diet. This could suggest people who have lower levels of dopamine receptors could be predisposed to developing overeating behaviour .
However, humans can’t exactly go cold turkey and give up food – so how do we stop our overeating behaviour? On December 25th many of us will be basking in the glory of our epic meals, feeling sleepy and vowing to never eat again, but come January I’m sure many of us will be buying salad ingredients and joining the gym faster than you can say “no joining fees”! Changing our eating behaviour is tough – high fat, high sugar foods are all around us. Rats fed on a high fat, high sugar diet of sausages and sweets for 40 days refused to eat healthy food for 14 days after their diets ended . However, unlike rats, we are constantly informed of the calorie content of our food and whether it is good for us which in theory encourages us to make more informed, healthy choices when it comes to food.
Let the battle of hormones and neurotransmitters vs. willpower commence! But first, let me just finish this mince pie….
- Editorial (2000) Fat and free will http://www.nature.com/neuro/journal/v3/n11/full/nn1100_1057.html Nature Neuroscience
- Martin S et al (2008) Leptin Resistance: A Possible Interface of Inflammation and Metabolism in Obesity-Related Cardiovascular Disease http://www.sciencedirect.com/science/article/pii/S0735109708024352 Journal of the American College of Cardiology
- Natural History Museum (2013) New research into overeating to be revealed at Science Uncovered http://www.nhm.ac.uk/about-us/news/2013/september/new-research-into-overeating-to-be-revealed-at-science-uncovered123943.html
- Harmon K (2010) Addicted to Fat: Overeating May Alter the Brain as Much as Hard Drugs http://www.scientificamerican.com/article/addicted-to-fat-eating/ Scientific American