Although claims of improved GI function are widely implied in the marketing of probiotic and synbiotic dairy products, the roles of these products in improving GI function in healthy individuals requires further investigation. Furthermore, there are few reports of typical GTT in healthy Canadians. To our knowledge, this study is the first to examine the effects of a synbiotic yogurt on measures of GI function and dietary intake in healthy Canadian adults.
The average PRE-treatment GTT in this study was in the range of 30 h, based on all participants. This is slightly lower than the 41.0 ± 18.9 h reported for a sample of healthy Canadian adults consuming their habitual diet and determined using fecal x-ray monitoring for passage of radiopaque pellets . Consuming a synbiotic yogurt did not change GTT or DOC from baseline and was not different from the control yogurt, for either the STT or LTT groups. Past research demonstrates that DOC can vary even among individuals with the same frequency of bowel movements, particularly in those with longer transit times . In this study, high variability in DOC was observed in both STT and LTT PRE- and POST-treatment.
Our STT and LTT groups had baseline GTT 25.2 ± 0.4 h and 49.5 ± 4.3 h, respectively. In a recent study, GTT, measured using the dye marker technique, was lowered in women, especially those with functional constipation (transit time ≥48 h), whom consumed two 125 g servings of synbiotic yogurt containing 0.625 g inulin and oligofructose with Bifidobacterium lactis Bb12 (109-1010 CFU/g) and Lactobacillus casei CRL (1 × 109- 6 × 109 CFU/g) daily for 15 days . Similarly, consumption of 2 daily servings for 14 days of a commercial yogurt containing B. animalis DN-173 010 (108 CFU/g) and fructoligosaccharide by women with and without functional constipation improved measures of bowel evacuation . Sairanen (2007) used radio-opaque marker detection in feces and, similar to the present study, found no improvements in transit time in healthy men and women whom, for 3 weeks, consumed 600 mL fermented milk with 12 g inulin and probiotics (Bifidobacterium longum BB536, Bifidobacterium spp. 420 and Lactobacillus acidophilus 145) versus fermented milk with the probiotics or a fermented milk control. In that study, average baseline transit times for the treatment groups ranged from 48.6 ± 25.6 to 53.5 ± 22.4 h.
There is consistent evidence to support the benefits of probiotic-enriched dairy products in improving GTT, even with relatively short nutrition interventions, if GTT is initially slow [9, 10, 12, 13]. However, in healthy individuals, there is minimal room for improvement in GTT, the measurements of which can vary from 20 to 30%, even with strict dietary controls [47, 51, 52]. Therefore, the relatively short initial GTT of the healthy and relatively young (28.6 ± 11.3 years) participants in the present study may help to explain the finding of no improvements in GTT. Of note, the physiological significance of small changes in GTT potentially induced in healthy individuals has not been confirmed. Conceivably, it would be undesirable for a product to excessively shorten a GTT such that it had laxative effects .
Differences between the results of the current study and those previously reported may be related to differences in probiotic strain and dosage, prebiotic type and concentration, and food matrix. The use of different techniques to determine GI transit may also explain apparent inconsistencies between studies. In the present study, use of the dye marker technique, while relatively non-invasive, inexpensive and conducive to studies with a relatively large number of participants, relies on participant self-reports of timing for dye ingestion and fecal appearance. It also cannot reveal transit times within specific segments of the GI-tract, where differences may exist .
Probiotic-containing foods are generally well tolerated, with minimal adverse side effects or undesirable GI symptoms . However, the tolerability of products containing both probiotics and prebiotics is understudied and expected to be product-specific. In the current study, supplementing the daily diet with probiotic- and inulin-enriched yogurt was well tolerated by healthy adults. Consumption of the same probiotic strains in yogurt also did not result in incidence of adverse GI symptoms beyond that observed with the placebo yogurt . Similarly consumption of 108 to 1011 CFU of Bifidobacterium animalis and Lactobacillus paracasei per day did not induce adverse effects and was generally well tolerated in healthy adults . In comparison to a milk control, milk fermented with Bifidobacterium lactis DN173010 was found to improve overall GI symptoms in women who reported minor digestive symptoms at baseline, mainly due to improvements in gas-related symptoms . In contrast, intestinal discomfort from gas production has been implicated as a side effect of prebiotic consumption . One study found that healthy adults experienced a significant increase in GI symptoms when consuming probiotic fermented milk with 12 g of inulin per day (4 g per 200 ml serving) compared to the probiotic and control fermented milks . However, inulin consumed in doses of 5 and 10 g is generally well tolerated . Healthy adults whom consumed a supplement containing probiotics which included Bb12 and La5, as in the present study, reported beneficial effects on bowel habits . A prebiotic dose of 5 g/day is reportedly sufficient to elicit a positive effect on the GI microbial population . The current study delivered 4 g of inulin per day (2 g per 100 g serving) and was well tolerated by healthy adults. Collectively, these results suggest that consumption of fermented dairy products, including or in conjunction with, a moderate amount of inulin (≤10 g per day) does not induce undesirable GI symptoms in healthy adults [34, 54].
Analysis of dietary intake data revealed that energy intake was decreased in the STT and trended towards a decrease in the LTT during consumption of the synbiotic yogurt. The consumption of prebiotic fibres, including inulin-type fructans, can impact appetite-related endpoints and may play a role in appetite regulation and energy intake [55, 56]. Reductions in daily energy intake by 100 kcal, similar to the 120 kcal observed in this study can impact energy balance sufficiently to prevent weight gain . This indicates that regular consumption of the synbiotic could translate into measurable and beneficial changes in dietary intake in free-living, healthy adults. In the LTT group, no differences in dietary fiber intake were observed between baseline and POST-treatment. However, in the STT group dietary fibre intake significantly decreased in the control group, but was maintained with consumption of the synbiotic yogurt. This indicates that consuming two daily servings of a synbiotic dairy product with 2 g inulin may reduce energy intake, while contributing dietary fibre. Since the average dietary fibre intake of adults in Canada and the United States fails to meet Adequate Intakes of 38 g/day for males and 25 g/day for females , food-based strategies that support this aim are warranted. The current study is unique in that dietary intake was measured in healthy Canadian adults after 15 days of supplementing their diets with a synbiotic yogurt, whereas other studies have measured short-term hunger and satiety in laboratory settings [43–45]. Limitations of the study include the reliance on self-reported nutrient intake by participants and the fact that the significant reduction in energy intake was observed in the STT, but not the LTT group.