Prebiotic supplements and high-protein (HP) diets reduce body weight and modulate intestinal microbiota. Our aim was to elucidate the combined effect of an inulin/oligofructose (FOS) and HP diet on body weight gain, energy metabolism and faecal microbiota. Forty male C57BL/6NCrl mice were fed a control (C) diet for 2 weeks and allocated to a C or HP (40 % protein) diet including no or 10 % inulin/FOS (C + I and HP + I) for 4 weeks. Inulin/FOS was added in place of starch and cellulose. Body weight, food intake, faecal energy and nitrogen were determined. Indirect calorimetry and faecal microbiota analysis were performed after 3 weeks on diets. Body weight gain of HP-fed mice was 36 % lower than HP + I- and C-fed mice (P < 0⋅05). Diet digestibility and food conversion efficiency were higher in HP + I- than HP-fed mice (P < 0⋅01), while food intake was comparable between groups. Total energy expenditure (heat production) was 25 % lower in HP + I- than in C-, HP- and C + I-fed mice (P < 0⋅001). Carbohydrate oxidation tended to be 24 % higher in HP- than in HP + I-fed mice (P < 0⋅05). Faecal nitrogen excretion was 31–45 % lower in C-, C + I- and HP + I- than in HP-fed mice (P < 0⋅05). Faecal Bacteroides–Prevotella DNA was 2⋅3-fold higher in C + I- and HP + I- relative to C-fed mice (P < 0⋅05), but Clostridium leptum DNA abundances was 79 % lower in HP + I- than in HP-fed mice (P < 0⋅05). We suggest that the higher conversion efficiency of dietary energy of HP + I but not C + I-fed mice is caused by higher digestibility and lower heat production, resulting in increased body mass.

The results of our investigations indicate that dietary treatment with inulin or oligofructose incorporated in the basal diet for experimental animals: (i) reduced the incidence of mammary tumors induced in Sprague-Dawley rats by methylnitrosourea; (ii) inhibited the growth of transplantable malignant tumors in mice; and (iii) decreased the incidence of lung metastases of a malignant tumor implanted intramuscularily in mice. Moreover, besides such cancer risk reduction effects, the dietary treatment with inulin or oligofructose significantly potentiated the effects of subtherapeutic doses of six different cytotoxic drugs commonly utilized in human cancer treatment. If confirmed, such dietary treatment with inulin or oligofructose potentiating cancer therapy might become an interesting approach to complement classical protocols of human cancer treatment without any additional risk for the patients.