Most of us know that post-Christmas lunch feeling, when even one last wafer-thin offering makes us groan and our stomachs say "no more". Not being Monty Python's Mr Creosote, we head for the nearest armchair, or bed, to recover. It may be some hours before we move again, let alone eat.
New Zealand scientists are trying to capture that feeling in an effort to stop us overeating routinely.
Our gut really does send signals to the brain telling us when to stop eating. Scientists at Plant & Food Research, the University of Auckland and Massey University are chewing through nearly $20 million in taxpayer funding over six years to identify foods that can prevent us overeating by manipulating these natural signalling processes.
Scientists call it satiety - the feeling of being full.
"We're trying to fool the body into thinking it's Christmas Day with smaller amounts of food," says Dr John Ingram, food innovation scientist at Plant & Food's Mt Albert research centre.
For years nutritionists have encouraged us to eat more low glycemic index (GI) foods including fruits, vegetables, pulses and wholegrains, which slow down the release of glucose (energy) into the body and can keep us from feeling hungry for longer. The Foods for Appetite Control project is looking beyond these simple nutritional effects on appetite by investigating how compounds in fruits, vegetables and wholegrains interact with sensory mechanisms in our gut to curb our appetite.
The gut utilises a range of sensory cells including cells containing taste receptors (the same receptors found on the tongue) to sense what type of nutrients are present in food and where in the gut they are. Some types of foods and specific natural plant compounds seem to be better at activating these sensory cells, causing them to release hormones which alter the digestion process and signal to appetite control centres in the brain, basically to keep us feeling full.
"They act as feedback mechanisms," Ingram says. "We are trying to identify compounds which stimulate these mechanisms to send feedback to the brain to keep us feeling fuller for longer."
Many of these compounds are found in plants including fruit, vegetables and grains. The project hopes to identify New Zealand-sourced fruits, vegetables and wholegrains to give our food producers a niche in the $513 billion global weight management foods industry.
It's also acknowledgement that the "eat less, exercise more" mantra of health promotion isn't working. While Government-funded campaigns have tried for decades to tighten belts, New Zealand's obesity rates have soared.
We may make New Year resolutions to stick to a regimen of sprouts and chaff but, for some reason, we cheat. According to the Ministry of Health, more than a third of NZ adults are overweight and a quarter are obese. More disturbing is the way the PlayStation generation is heading. The most recent (2003) Children's Nutrition Survey found 31 per cent of youngsters were overweight or obese, and the prevalence of child obesity had doubled in six years.
Obesity is estimated to cost $830 million a year in direct and indirect costs arising from diabetes, strokes, heart disease, cancers and lost working hours.
"If we feel satisfied eating fewer calories and can suppress that feeling of hunger between meals, it can have a real impact on weight control," says Ingram.
Dieting and exercise prove ineffective for most people long-term, he says. "It's difficult to change lifestyles and many people end up just yo-yo dieting. The reason most people cheat on diets is that they keep feeling hungry."
The World Health Organisation estimates the planet has 2.3 billion overweight adults and 700 million of them are obese. If our scientists can identify New Zealand foods and extracts which help to control appetite, the potential spin-offs for New Zealand food and beverage manufacturers are enormous.
The team of researchers from Plant & Food Research, Auckland University's Human Nutrition Unit and Bioengineering Institute and Massey's Institute of Food Nutrition & Human Health are backed by industry investors Zespri, Sanitarium, Hansells, NZ Extracts, Comvita and Bell Tea and Coffee. By developing whole foods, extracts and ingredients in processed foods, the potential for an additional $98 million in export revenue by 2018 has been identified. A trial will soon begin confronting volunteers with a diet of kiwifruit.
The research is literally stomach-churning, studying the way different food components affect digestion and absorption of food and how sensory mechanisms at different levels of the small intestine (duodenum, jejunum and ileum) and colon (large intestine) relay this information to appetite control centres in our brains.
The idea is to identify food components which target sensory mechanisms at different levels of the gut to enhance short, medium and long-term appetite control. The goal is to keep us feeling full for up to four hours after eating. The scientists may even be able to keep us from snacking between meals.
For short-term control bitter compounds -found in citrus fruits, red wine, green tea and brassicas (cabbages, cauliflowers and broccoli) - are thought to be effective in triggering the release of the satiety hormone cholecystokinin (CCK) from sensory cells lining the duodenum (the upper third of the small intestine). CCK plays an important role in stimulating digestive processes and has been shown to suppress appetite.
The ileum (lower part of the small intestine) may play a key role in modulating appetite over the medium-term (one to four hours). If carbohydrates or fats that are normally absorbed in the duodenum and jejunum reach the ileum they can trigger what's known as the "ileal brake", where additional satiety hormones are released telling the brain to not eat any more as more time is needed for digestion. One aim of the research is to ensure more carbs reach the ileum by identifying natural plant compounds that can delay carbohydrate digestion and absorption. Phytochemicals in apples, kiwifruit, berries, red wine, cinnamon, coffee and leafy green vegetables do this.
The third focus is the colon (large intestine), where bacteria play a key role in harvesting energy from indigestible dietary fibre, fermenting it to metabolites we use as energy sources. Increased levels of these metabolites trigger the release of a further set of satiety hormones.
Ingram concedes there's still work to be done to determine how effective this combination of approaches will be. The first step is to identify the phytochemicals which work most effectively so researchers can predict what fruits, vegetables and whole-grains will work best.
The team is also interested in developing functional ingredients from natural plant sources which grow well in New Zealand. "Processed foods have the convenience consumers desire and make up a large component of people's diets so it's important that we look at ways of incorporating these appetite control ingredients into them.
"We are targeting multiple mechanisms - most of the rest of the world is targeting only one. We are focusing on phytochemicals and trying to bring some novelty to the area around uniquely New Zealand ingredients."
But it's way too early to suggest we should all stuff ourselves on kiwifruit, says Ingram. The only tip he's comfortable with at this early stage is familiar enough - to increase our intake of fruit, vegetables and wholegrains.