Fibre is the non digestible part of food, usually a carbohydrate that our bodies cannot break down and absorb. They pass through our system largely unchanged. There are 2 types of fiber – soluble and insoluble based on whether it can be dissolved in water. Plant foods often contain varying amounts of both types of fibre.
Fibre has several purported health benefits – bulking, viscosity, and fermentation. The increased bulk may perhaps fill our stomachs and therefore make us eat less. It may also slow gastric emptying time. In the colon, the fibre increases stool bulk and thus decreases the transit time. It was hypothesized this would decrease rates of colon cancer by ‘cleansing’ the bowels so to speak as the food is sped through the colon.
Soluble fibre can often increase viscosity in the gut and therefore change the absorption of other nutrients. Some fibre containing plant foods may contain phytic acid which may act as an anti-nutrient. It produces benefits by reducing absorption. Reduced absorption of sugar and reduced sugar response may be some of the benefits here.
Increasing viscosity slows movement of food through the stomach and small bowel. It may also interfere with the mixing of food and digestive enzymes, disrupts micelle formation, and alters diffusion and interaction of nutrients with mucosal surface. The combination of these events results in slowed fat and carbohydrate absorption, which may induce satiety. Fermentation of the fibre may produce beneficial compounds. There is a thought that fibre feeds the ‘good bacteria’ in our guts and therefore provides benefit. Short chain fatty acids may also be produced that may have benefit.
Fibre came to public attention in the 1970s. From missionary work and comparison with native populations, it became clear that the so called Diseases of Civilization (heart disease, obesity, diabetes, stroke, cancer etc) were quite rare amongst natives that followed a traditional diet and lifestyle. In urban areas, as the lifestyle of the natives became Westernized, these disease started to appear.
Some researchers, notably Dr. Cleave believed that these diseases were caused by the increased consumption of refined carbohydrates – particularly sugar and white flour. These could be stored at room temperature without fear of spoiling – a large advantage. This enabled them to be carried in great quantity and great distances. Through the 1960s and early 1970s, debate about the main dietary villain raged back and forth between dietary fat and refined carbohydrates. The pendulum finally swung in favor of lowering dietary fat, which necessitated an increase in carbohydrate intake. Since most of the carbohydrates in the Western diet are refined, the goal of reducing fat was incompatible with lowering refined carbohydrates.
As a result, the carbohydrate hypothesis quickly fell into disfavor. Another possibility presented itself. Perhaps, it was not the carbohydrate that was the culprit but the lack of fibre that was the problem. The hypothesis had the large advantage of being compatible with the dietary fat hypothesis and quickly gained ground. Traditional unrefined foods that included carbohydrates tended to be very high in fibre.
Denis Burkitt, a missionary surgeon who spent much time in Africa, was one of the leading proponents of the dietary fibre hypothesis. By the time the Dietary Guidelines for Americans was released in 1977, there was a recommendation to “Eat foods with adequate starch and fiber”. These became enshrined in the pantheon of Dietary Myths. Fibre was good for you. There was a problem, though. It was difficult to show exactly how it was good for you. One of Dr. Burkitt’s thoughts was that the fibre decreased risk of colon cancer by speeding passage of the stool through the colon.
Large trials were undertaken to supplement patients with fiber. 88,757 women in the prosepctive Nurse’s Health Study were studied to identify whether increased fiber intake would reduce the risk of colorectal cancer. Despite the 16 year follow up period, there was no significant reduction in risk of colon cancer as the fiber intake increased. There seemed to be a flat response. Taking more and more fibre seemed to confer no additional benefits.
A randomized controlled trial attempted to show the benefits of fiber on a precancerous lesion called an adenoma. Colorectal cancers often arise from an adenoma but it usually takes between 5 and 10 years to develop. 1303 patients were randomized to high fibre intake or usual intake. With a median 34 month follow up, it was impossible to demonstrate a benefit.
Well, fibre supplementation by itself seemed to have very little benefit in reducing cancer risk. Perhaps we may fare better in heart disease. So, the Diet and Reinfarction trial was undertaken with the goal of answering the question whether a low fat or a high fibre diet would be beneficial for heart disease. In the DART study, 2033 men were randomized to various diets after a heart attacks to see if any particular diet would reduce the risk of a second heart attack. To their astonishment, the American Heart Association low fat diet did not seem to reduce risk at all. The Mediterranean diet (a high fat diet) seemed to have benefit. This was what Dr. Ancel Keys had shown years ago. Recent trials have also seems to confirm the benefits of eating more natural fats such as nuts and olive oil in prevention of heart disease. So eating more fat may be beneficial. What about fibre?
Those that received fibre supplements did not seem to benefit. Not only that, but increasing fibre may actually have increased the risk of dying! Yowzers. That came out of the blue. Despite the publics infatuation with roughage, it was difficult to answer the question “Where was the benefit of fibre?”
The use of fibre for weight loss has had mixed results. While there was initial enthusiasm for this approach, it has mostly died away due to lack of success.
Most of the time we think about foods, we think about their nutrient content. The vitamins and minerals they contain. The key to understanding fibre is to realize that fibre’s benefit is not as a nutrient, but as an anti-nutrient. We think of foods and vitamins adding good things to the body. Fibre has the ability to reduce absorption and digestion. Fibre has the ability to subtract bad things away from the body. In the case of sugars and insulin, this is a good thing.
Continue to Hormonal Obesity XVII
Begin here with Calories I
See the entire lecture here – The Aetiology of Obesity 2/6 – The New Science of Diabesity