Dietary trickery

Why do some foods give us the munchies? We just can’t stop at one bite ... or two … or three … think chocolate chip cookies, potato crisps, jelly beans ... No wonder we call them “more-ish”. Compare this to “old-fashioned” foods (think porridge and beans) that positively stick to the ribs. How hard is it to get more than one bowl of steaming oats down our gullets?

Many experts lay the blame for the current epidemic of obesity right at the food industry’s door. Do they have a secret ingredient that makes us eat more, so we can hold up our arms in resignation and say it’s “dietary trickery”? There may be some merit to this idea. Researchers have conjured up a way to rate foods according to how full they make us feel.

The reasons we start and stop eating are complex. Meal size is partly determined by how quickly you “feel full” during eating, and to what extent. This is called "satiation". Similarly, the time between meals is determined by how long you feel full for. This is called "satiety". Psychological factors are of course paramount concerning both, with learned habits, social cues and emotional states having the capacity to override the “basics” underlying eating. However, if one feels full quickly during a meal and for a long time subsequently, psychological factors lessen in significance.

How easy, and enticing, is it to gorge on fruit after a bad day at work? Any takers for eating ten apples in 10 minutes? Conversely, anyone for a warm, moist croissant? Both ten apples and a croissant provide similar amounts of energy, but differ in all other aspects, namely macronutrients, such as fat, protein and carbohydrate, and water, all of which contribute to how full you feel.

“Fullness” is a new buzz word in the world of diet today, and was quantified over a decade ago by Jennie Brand-Miller, Sue Holt and others at Sydney University’s Nutrition Department. Brand-Miller’s team invented a new “scientific” measure of fullness, the “satiety index” (SI). The SI refers to the short-term satiating capacity of a food, i.e. how full one “feels” after ingestion. It was ascertained via a subjective (personal rating) satiety questionnaire, filled out every 15 minutes by healthy human subjects, after a 1000 kilojoule (kJ) portion of a test food was served for breakfast, for a period of two hours.

The higher a subject’s rating of fullness over the two hours, the higher the SI of the food, as compared to a reference food, white bread (which was assigned a SI value of 100). Subjective feelings of satiety were validated by a direct correlation with prospective ad libitum, eat as much as desired, food intake at a buffet lunch after the two-hour test period (i.e. when a subject reported high satiety after breakfast, he or she ate less at lunch). Thirty-eight common foods were tested in this way, each in 12 subjects, with some interesting results.

Croissants had the lowest SI (47) and potatoes the highest (323). In other words, croissants were only half as filling as the same energy load of white bread, while potatoes were over three times more filling. The most satiating foods weighed more and had the highest protein, fibre and water content, less fat content and lower palatability (“tastefulness”). Modern foods, ideally illustrated by the bakery and snacks/confectionery groups, scored low on the SI. Donuts and Mars Bars made subjects feel hungrier and inclined to eat more at lunch. “Wholesome” foods, high in protein and carbohydrate, on the other hand scored highly, with foods like fish and pasta making subjects feel fuller for longer.

The level of distention of the stomach and small intestine is likely to be the main factor eliciting high levels of satiety in the study, reflected by the strong correlation between the weight or water content of the food and SI, i.e. foods with the highest water content, and hence “bulk” were associated with increased feelings of fullness, potatoes being the best example.

This forms the basis of work conducted Dr Barbara Rolls, a US nutritionist, who recommends the use of food volume in weight loss, based on the higher SI of bulky, “watery” food. However, it should be noted that mechanical distention of the gut wall (via mechano-receptors) is only one contributing factor to feelings of fullness. Less obvious characteristics of bulky foods are also important. These include a low glycemic index (GIs), low fat content (and hence low energy density) and high fibre level.

On this note, let us talk about potatoes and GI. Scientists think glucose is a key player in the hunger and satiety tug-of-war via stimulation and inhibiton of “appetite centres” in the brain’s hypothalamus. Potatoes are well known for their high GI and high GI foods are often associated with less fullness (satiation). A blood sugar spike followed by a rapid fall in blood glucose levels can stimulate feelings of hunger. Yet potatoes were the most filling food despite their high GI. Why? Well, one reason may be the fact that the subjects did not rate their hunger beyond two hours. Perhaps potatoes are one of those foods that make us feel only temporarily full. But more important, when we compare high and low GI foods, it is necessary to compare like with like (bread with bread, breakfast cereal with breakfast cereal). With any luck, the potato breeders will come up with a low GI, even more filling potato in the near future.

Previous work has ranked fat as the least, and protein the most, satiating macronutrient, findings supported by the SI, but has often omitted detail on type of carbohydrate and satiety, which was highlighted in Brand-Miller’s study. For example, food with a mix of slowly digestible and indigestible carbohydrate (fibre), such as porridge, scored high on the satiety scale. Oats have high levels of soluble fibre, which forms a gel in the stomach, slowing gastric emptying and hence delivery and absorption of nutrients to and from the gut. Such a viscous mix also “sticks” to the walls of the stomach and upper small intestine, activating mechano- and chemo-receptors and enteroendocrine cells which relay information to the brain via gut-brain signals (such as glucagon-like peptide-1).

These hormones signal the presence of food still in the gut, reminding us that it can take only so much food. In addition, the carbohydrate “inside” a porridge oat is generally harder for the body to process than, for example, white bread. The physical fibrous barrier encasing the oat makes it harder for digestive enzymes to access the interior. White bread in comparison contains starch that is readily available, as all the “tough” outer layers of the original wheat grain have been removed during processing. Indigestible fibre that reaches the lower climes of the small intestine has further effects of stimulating the release of a wider array of gut hormones that signal satiety (for example, peptide YY from the ileum). Similarly, anti-nutrients found in some protein-rich foods, such as baked beans (for example, trypsin inhibitors), may result in some undigested protein reaching these areas and increasing fullness.