London - Ever wondered why some days all you want to do is snack, yet on other days you don’t feel the slightest bit hungry?
Under normal circumstances we feel hungry when we have burnt up the food we have eaten as energy and our blood sugar and insulin levels begin to drop. Ghrelin, a hormone connected to appetite, then communicates this to the brain, which is how we feel the need to eat.
But all sorts of things can interfere with this process. Earlier this month, researchers from Aberdeen University revealed that brain cells vital to regulating appetite slow down as we age, leading to middle-aged spread.
This, they said, is because it takes us longer to feel full, so we eat more than we should - and our weight creeps up, usually at a rate of about 1lb (450g) a year.
So what else has an impact on how hungry we feel?
Here, with the help of experts, we look at what could be influencing our appetite without us so much as looking at a slice of cake...
You get peckish when you're tired
We all know that we feel a bit more hungry when we are tired, but the effects are more profound for women.
According to a 2011 study by researchers at Columbia University in the US, those who are sleep-deprived eat almost 300 calories a day more than those who get enough sleep.
This is because levels of the hormone ghrelin, which tells the brain we need to eat, increase when we don’t get enough sleep, so we are more hungry than usual.
However, the Columbia researchers also noted that the women who didn’t get enough sleep were hungrier than the men, with their average intake of fat rising by around 30g on sleep-deprived days - four times as much as the average increase for men. “It’s possible that when we are awake we need more energy to sustain that state of wakefulness, which is why the body may want food,” says Professor Russell Foster, director of the Sleep and Circadian Neuroscience Institute at Oxford University.
“It seems that the body especially craves carbohydrates and sugars such as bread and cake, which are energy-giving foods.”
The problem with eating on the go
If you tend to bolt down your toast as you run out of the house in the morning, or have your lunch standing up at the kitchen counter, it could make you hungrier.
Eating in this way tends to make us eat faster, so there isn’t enough time for signals that tell us when we are full to kick in.
“When we eat we have to give enough time for the messages and responses from the stomach to reach the appetite centres of the brain,” explains consultant cardiovascular epidemiologist Dr David Ashton, medical director of the Healthier Weight Centres. “This usually takes about 20 minutes from when we finish eating. But eating quickly overwhelms this mechanism, so the body doesn’t get the chance to feel full and therefore there is an increase in appetite.”
Menopause makes women ravenous
Fluctuations in levels of the hormone progesterone in the lead-up to the menopause can make women more hungry.
“Levels of progesterone vary in the perimenopause - the two to five years leading up to the menopause - and this seems to affect appetite,” says Leila Hannah, a gynaecologist at Queen Mary’s Hospital, Sidcup.
Progesterone relaxes the body’s smooth muscles - these are the ones that work areas such as the bladder and gastrointestinal tract. When levels are higher, it increases the need for energy. This is what causes the craving for food, especially energy-dense carbohydrates.
However, Miss Hannah adds that this tends to level out after the menopause has passed.
Feeling unusually hungry can also be a sign of pregnancy. “Pregnancy is a time when tissues are being laid down for the growth of the embryo,” explains Manchester GP Dr Jude Gunasekera. “This uses up a lot of energy, which is why women who are pregnant may feel so hungry.”
It could be your hay fever pills
It’s common for people to go off their food if they are depressed, as sufferers become so overwhelmed by feelings of bleakness that they lose interest in food.
However, taking antidepressants - and especially those containing the drug mirtazapine, for reasons that are not clear - can stimulate hunger.
“Antidepressants disturb the function of the hypothalamus, the part of the brain that controls and regulates appetite, and this is why appetite can increase when people take them,” explains consultant psychiatrist Professor Jonathan Chick, medical director of the Castle Craig rehabilitation clinic in Peeblesshire, Scotland.
Antihistamines, given for allergies and taken by many for hay fever, also affect the hypothalamus and therefore increase our appetite, though this tends to be cumulative, rather than happening after a single dose.
There is no easy way to combat the problem if your medication is making you hungry - but simply being aware that this may happen can help you keep an eye on your food intake.
Blame the news
Politicians may talk about tightening our belts during lean times - but hearing about economic hardship on the news actually makes us want to eat more, according to scientists at the University of Miami.
In a 2013 study, they found that people who were primed to expect ‘tough times ahead’ ate more food when they were told it was high in calories than people who listened to neutral messages.
When the bad news group was given the same food but told it was low-calorie, they ate 25 percent less.
“Taste [for a particular food] was not what caused the reactions, it was a longing for calories,” said lead researcher Professor Juliano Laran.
It’s thought that hearing about troubled times causes a survival instinct to kick in, making us want to “seize the day”, leading to cravings for high-calorie snacks such as chocolate. The researchers went so far as to suggest that people should “tune out [the] news for a while” if they are looking to cut back.
'Naughty' foods fuel cravings
Tempted by that extra biscuit? Then just enjoy it - as researchers have found that when you feel guilty about a pleasurable experience, the guilt itself becomes pleasurable, so encouraging you to do it more.
In a 2012 study involving 1 000 people, researchers from Northwestern University, in Chicago, gave volunteers sweet treats, and primed them to experience feelings of guilt before eating them by talking about occasions when they had felt guilty in the past.
This group was shown to enjoy the treats more and eat more of them than the volunteers who were not urged to feel guilt.
“If we consider something a naughty pleasure, it can psychologically drive the appetite and the guilt may make you want to eat more,” says Cary Cooper, professor of psychology and health at the University of Lancaster.
The appetite centre of the brain also lies very close to the area governing mood - the limbic system - which is why changes in the way we feel may affect appetite.
Mobiles muddle your metabolism
The bright blue light emitted by devices such as smartphones and tablets may give an unwelcome boost to your appetite.
Research this year found that exposure to the light increases hunger levels for several hours - even if you have just eaten.
Scientists, again from Northwestern University, asked volunteers to be exposed to dim light for three hours in the evening, then on the next day to blue light for three hours. They found that those exposed to the blue light had a marked increase in appetite 15 minutes after initial exposure to the light. Their appetite continued to be increased almost two hours after eating their evening meal.
It also altered their metabolism, as blood tests showed that the blue-light subjects had higher insulin and glucose levels.
One possible explanation is that bright blue light at night confuses our body clock, which has a role in controlling when we feel the need to eat. Natural light is made up of different colours, including blue light, and it is this type that sends the strongest signal to the brain to let it know whether it is day or night.
AND THE THINGS THAT CAN KILL YOUR APPETITE
Picking a posh restaurant
Adding softer lighting and mellow music in restaurants can reduce the amount that diners eat, according to US research. People ate nearly 20 percent fewer calories and were more satisfied with the quality of the food when they ate in a ‘fine dining’ environment, complete with soft lighting.
This was the opposite to what the researchers at Cornell University Food and Brand Lab were expecting. They thought that the relaxing environment would entice people to eat more.
The reason why they didn’t could be because the relaxing atmosphere enabled them to savour their food and eat slowly, which would make them more responsive to signals from their brain that told them when they were full.
In another study this year, Cornell researchers found that people who paid more money found their meal more satisfying than diners who had the same food but paid half as much.
The colour of your crockery
Dieters should eat from red plates, as the colour can actually make you feel less hungry, according to research by scientists at the University of Parma, in Italy.
For the study earlier this year, researchers gave 240 participants snacks of popcorn and chocolate chips served on either red, white or blue plates. Those eating from the red plates ate less overall.
Researchers suggest that this is because we generally associate the colour red with stopping and caution (for example, traffic lights). This subconsciously encourages us to eat less if our food is on red dishes.
Having an egg for breakfast
Eggs are a particularly dense form of protein, says independent food scientist and nutritionist Dr Sam Christie, “which is why you feel fuller for longer after eating them”.
According to a report in the Journal of the American College of Nutrition, eating two scrambled eggs for breakfast resulted in people eating fewer calories for the rest of the day - in fact, for the next 36 hours.
Sprinkling cinnamon on food can also make you feel fuller for longer.
“It seems to have an effect on both insulin uptake and blood glucose levels, which is why you don’t get a spike and then drop in how full you feel,” says Dr Christie.