A diet containing complex carbs – read 'lots of dietary fibre' – makes rats with a dodgy pancreas slimmer and more muscled, we wrote a few days ago. A similar diet has the same effect on mice with a normal pancreas, researchers at the Children's Hospital Boston discovered. What's more, carbohydrates that are difficult for the body to absorb made the animals more physically active.
The researchers divided a group of male mice into two groups. Both groups were given a diet that consisted for 60 percent by weight of starch for a period of 40 weeks. In the control group the starch used consisted entirely of amylopectin; and in the experimental group it consisted of 40 percent amylopectin and 60 percent less easily absorbed amylose.
The difference between amylose and amylopectin lies in their structure. Amylopectin is a branched chain. The digestive enzymes can loosen the glucose molecules from the amylopectin at different locations at the same time and absorb them. This process goes pretty fast. Amylose, on the other hand, is a straightforward chain of glucose units.
To be digested the units have to be loosened one by one, and that takes time. Most of the glucose molecules are not absorbed by the body.
The sugar chains that are not absorbed form an ideal medium for beneficial micro-organisms in the large intestine to feed off. These convert glucose into short-chain free fatty acids. Researchers suspect that the positive effects of fibre – fibre is a collective noun for all carbohydrate chains that your small intestine can't digest, and which are partially or wholly fermented in the large intestine by micro-organisms – are partly a result of the work of the short-chain fatty acids. These are thought to boost the metabolism a little.
The starch combination had no effect on body weight, but it did affect fat mass. The figure below shows that the mice that ate slow carbs [SAC] built up noticeably less fat than the mice that ate fast carbs [RAC]. That means that they built up more lean body mass.
The researchers came up with two partial reasons for the positive body recompositioning effect of the slow carbs. The respiratory quotient [RQ] of the mice that ate slow carbs was lower. That means that they burned a little more fat and less carbs. But perhaps a more important factor was that the mice moved more.
The researchers registered the mice's movement by placing light-sensitive sensors in their cage, and the mice that ate slow carbs moved more. So they were more physically active. "Higher levels of physical activity are characteristically associated with greater lean body mass", the researchers suggest cautiously. Between the lines you can see that they don't think that that little bit of extra exercise in the SAC mice doesn't explain their different body composition. Nevertheless, the increase in physical activity is in itself interesting.
"A low glycemic diet could increase spontaneous physical activity level, a possibility that might have important implications for the prevention and treatment of obesity and promotion of physical fitness", the researchers conclude.
Am J Physiol Endocrinol Metab. 2008 Nov;295(5):E1126-31.