Carbs Are Bad? Think Again.

Obesity and obesity-related metabolic diseases such as type 2 diabetes and cardiovascular disease are a substantial threat to public health. Since 1980, the prevalence of obesity in the United States has doubled, and two-thirds of the adult population is now considered overweight or obese (Flegal et al., 2010). Even though activity level and other lifestyle changes may be contributors to the obesity outbreak, the core factor appears to be an increasing caloric intake during this period (Hill et al., 1998; Caballero, 2007; Swinburn, 2009). What has caused this increase is still debated, while the more immediate concern of consumers is to understand diets that will maintain lower body weight or promote weight loss.

Notably, dietary carbohydrates are often considered unhealthy, and it is a belief by many that eating a low-carb diet is a good strategy to lose body weight or simply to become healthier. But  is this necessarily true? It is the point of this article to show that “carbohydrates,” meaning the ones the body can utilize directly for energy like digestible starch (not dietary fiber), have a degree of complexity when they are received by the body in terms of how it senses them and systems that are triggered to affect appetite, rate of nutrient delivery to the body, and even food intake.

Let’s first think for a moment about the low-carb diet. Low-carb diets only reflect the quantity of carbohydrate in food and not its quality. “Quality” in the scientific and medical community is often associated with the concept of glycemic index (GI), a parameter to evaluate glycemic carbohydrates that is based on blood glucose response following consumption of such foods (Jenkins et al., 1981). However, GI is only one aspect relating carbohydrates to health and, in light of other physiologic effects related to location of digestion in the small intestine, perhaps a  more sophisticated way to gauge quality of carbohydrates is needed.

Recent research in our laboratory builds on an old idea, though not well recognized today, of the importance of locational delivery of sugars from digestion in the gastrointestinal tract to effect changes related to the food-driven gut-brain axis. While more often attributed to dietary fat, the small intestine also appears sensitive to location of carbohydrate digestion and sugar release.

More to the point, where glucose delivery occurs in the 15- to 20-foot-long small intestine sends hormonal signals that control stomach emptying, appetite, and food intake. When glucose is  generated in the duodenum, the proximal part of the small intestine, cholecystokinin (CCK) is released, whereas glucagon-like peptide-1 (GLP-1) and peptide YY (PYY) are triggered by glucose in the ileum, the distal part of the small intestine. GLP-1 and PYY are known to suppress food intake through modulating the brain signal and gastrointestinal motility, called the “ileal brake” (Spiller et al., 1984; Read et al., 1984). Delayed stomach (gastric) emptying has been shown to correlate with increased satiety as well as reduced food intake (Sturm et al., 2004; Janssen et al., 2011). Carbohydrate that reaches the ileum causes retarded upper gastrointestinal mobility (Layer et al., 1995; Groger et al., 1997) and moderates and extends the glycemic response (Venkatachalam et al., 2009).

New findings in our group suggest that the ileal portion of the small intestine has a high sensitivity to dietary glucose in controlling gut motility and food intake. This effect is exhibited in a dose-dependent fashion and requires relatively small amounts of glucose delivered distally to activate it. Moreover, we recently found that rats fed a diet including ileal-targeted carbohydrates had reduced food intake compared to rats fed a diet with proximal-targeted carbohydrates. Inhibition of food intake in the rats administrated with the distal-targeted carbohydrates correlated to a suppression of a prominent appetite-stimulating neuropeptide signal in the brain. Some foods that, for example, slow down starch digestion due to complex matrices or for other reasons have this property, and moreover, it seems possible to make natural ingredients that produce this effect. Yet, at this point, more studies need to be done to see if humans respond as laboratory animals, and to find ways to identify or design foods that have location-specific utilization in order to achieve intestinal-targeted carbohydrates.

Bruce R. Hamaker, Ph.D.,
is Roy L. Whistler Chair Professor, Dept. of Food Science, Purdue University, West Lafayette, Ind., and
Director, Whistler Center for Carbohydrate Research at Purdue ([email protected]).
Choon Young Kim, Ph.D.,
is a postdoctoral researcher at the Whistler Center.
Like Hasek is a Ph.D.
student, working on research projects at the Whistler Center.

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