Residue, Fiber, and Subjectivity

Dear Editor:

I was very interested to read the review by Vanhauwaert et al. (1), and I agree that in a number of pathological conditions there is a need for a temporary suspension of fiber intake. However, for the most part, it is important to have fiber at every meal to limit microbial growth and promote useful fermentation (1–5). Food moves rapidly through the small intestine, from minutes to one-half hour under healthy conditions. This velocity, combined with high absorption, protects the small intestine from bacterial growth (4–6). The colon also needs to limit bacterial replication. Provision of slowly fermentable material at each meal is essential to this goal (5). This fermentable material varies in biochemical composition and related pathologic and physiologic effects, which remain poorly understood. The review by Vanhauwaert et al. (1) should stimulate further investigation. Intestinal microflora (the microbiome) is regulated by intake of indigestible fibers and residues. Cellulose, hemicellulose, and lignins are insoluble fibers found in whole-grain cereals. Fruit and vegetables are rich in soluble fiber (pectin) (2). In the colon, insoluble components are partially (30–50%) fermented and soluble components are more completely fermented (85%) by 100 trillion bacteria. Fiber fermentation takes 6–8 h after meals and yields acetic, propionic, and butyric acids. These SCFAs flow slowly into the blood, prolonging energy availability and preventing depressed blood glucose (7). A meal rich in nonstarchy vegetables also delays hunger and allows subsequent intake to be planned more easily (2–4, 8, 9). Fiber also contributes to lower energy intake by increasing gastric volume, giving a sensation of fullness (2–4, 7, 8). At the sight of food, a will to eat a certain amount of energy develops, more or less consciously. The will to eat decreases at the sight of food with lower energy concentration. The will may always be corrected by the attention to the experience from previous meals. In our experience with carbohydrates, preference is directly proportional to the glycemic index—the area under the blood glucose curve produced by 100 grams of food divided by the area produced by 100 grams of white bread.

Each meal remains completely subjective. The experience provides the association between amount of energy intake and length of time without food (10). We train children to recognize initial hunger 3–4 times/d to yield a subjective, reproducible limit in energy intake. Observed decreases in energy intake were attributed to higher intake of nonstarchy vegetables. At the age of ≥8 y, 0.5–1 kg of fruit/vegetables per day should be consumed (1–3). In some cases, adding these vegetables to the diet of an insulin-resistant person produces excessive fermentation and induces bowel symptoms. However, nonstarchy vegetable intake is needed to reduce energy intake and to recover insulin sensitivity. We find that training in the recognition of initial hunger is a way to tolerate these vegetables without symptom relapse (4). Healthy nutrition requires balancing these interdependent factors, along with other recommended changes, such as regular exercise.