2.1. Epidemiological Studies
Data from nine large epidemiological studies, two cross-sectional [19
] and seven longitudinal studies [21
] consistently demonstrate that fibre intake is favourably associated with outcomes related to weight management (i.e., BMI, body weight, percentage of body fat, and waist circumference). Longitudinal studies clearly demonstrate that high consumption of dietary fibre, generally more than 10 g/1000 kcal/day or about 20 g/day in most studies, is associated with a body weight loss ranging from 1.2 to 3.6 kg in 8–12 years of follow-up [21
]. In the EPIC-Potsdam cohort, beneficial effects of fibres on body weight also persisted over time as shown by the fact that individuals with the greatest intake of dietary fibre (13.5 g/1000 kcal/day) were more able to maintain body weight or to prevent excess weight gain over 4 years of follow-up than to those with the smallest intake (8.8 g/1.000 kcal/day) [24
]. Beneficial effects of fibres on body weight were observed in all cohorts irrespective of baseline body weight status (normal-weight, overweight or obese) of the study participants.
Observational studies also show that a higher intake of dietary fibres is associated with a healthier distribution of body fat. In a cross-sectional observation [19
], individuals in the highest quintile of fibre intake (11.3 g/1000 kcal) compared with those in the lowest quintile (6.6 g/1000 kcal) had a smaller waist circumference (82.9 vs. 86.1 cm). In a longitudinal study, overweight youth who had decreased dietary fibre intake from year 1 to year 2 (mean decrease 3 g/1000 kcal) compared with participants who had increased dietary fibre intake (mean increase 3 g/1000 kcal) had a significant increase in visceral adipose tissue (+21% vs. −4%) [25
]. Similarly, an inverse association between cereal fibre intake (highest quintile vs.
lowest quintile) with percent body fat (34.7% (32.8–36.6) vs. 31.5% (29.4–33.5); Ptrend
= 0.004) and trunk fat mass (42.8% (40.2–45.4) vs. 37.8% (35.0–40.6); Ptrend
= 0.001) was shown by McKeown et al. [20
Epidemiological evidence also shows a beneficial effect of fibre on weight management considering total dietary fibre or cereal fibre intake. In a 6.5-year follow-up large European study, 10 g/day of total fibre intake was associated with a weight reduction of 39 g/year and a waist circumference reduction of 0.08 cm/year, while 10 g/day in whole grain fibre intake was associated with a weight reduction of 77 g/year and a waist circumference reduction of 0.10 cm/year [26
]. In The Health Professionals Follow-up Study, over an 8-year follow-up period, for each 20 g/day increase in total dietary fibres, weight gain was reduced by 1.2 kg, while for each 20 g/day increase in cereal fibres weight gain was reduced by 0.81 kg [27
In all the epidemiological studies reported above, the adjustment for confounders, including other dietary components and energy intake, did not influence or only modestly attenuated the association between dietary fibre intake and weight outcomes.
2.2. Randomized Controlled Trials
In the last decades, a large amount of clinical trials have been conducted to investigate the possible effects of fibre on outcomes related to body weight changes. These trials focused on the effects of different types of fibre—i.e., fibres from whole grain, pulses, fruit and vegetables, and fibre supplements—and some of these trials have been included in previously published meta-analyses reported in Table 2
In contrast with epidemiological evidence, the results of clinical trials evaluating the effect of fibres from whole grain on weight reduction are limited and not consistent. In a meta-analysis of 26 studies, Pol et al. [28
] showed that whole grain intake compared to refined grain had no effect on body weight, although it slightly reduced the percentage of body fat. Seven of these trials compared whole with refined grain in weight-loss diets, while the other 19 studies were performed on isoenergetic conditions. The daily whole grain intake ranged from 18.2 to 150 g/day and the trials’ duration ranged from 2 to 16 weeks. In a “review of meta-analyses” [32
], the reductions in body weight, BMI, and abdominal obesity observed in individuals with the highest dietary whole grain intake were not significantly different from individuals consuming non-whole grain foods or refined grain. In a more recent RCT, not included in the previous meta-analysis, Kirwan et al. [33
] showed that an 8-week whole grain vs. refined grain diet (29 vs. 21 g/day fibre intake) in 40 overweight/obese adults induced a similar effect on weight loss (−3%), fat loss (−6%), and waist circumference (−2 cm) with no significant difference between diets. The discrepancy between epidemiological studies and RCTs could be related to differences in study design, short duration of trials, selected populations, and the type and amount of whole grain foods consumed.
With regard to fibre from pulses, a meta-analysis of 21 trials showed a significant weight reduction of 0.34 kg in diets containing dietary pulses (median intake of 132 g/day or about 1 serving/day) compared with diets without dietary pulse, over a median duration of 6 weeks [29
]. The effect on body weight reduction was observed in trials testing both low-calories and no calorie restricted diets; in 6 trials, a reduction in body fat percentage (−0.34%) was also observed [29
With respect to fruit and vegetables, the meta-analysis by Mytton et al. [30
], including 8 randomized trials, showed a significant weight reduction of 0.68 kg with diets rich in fruit and vegetables (in the absence of specific advice to decrease consumption of other foods) compared to diets poor in fruit and vegetables. The study duration ranged from 4 to 52 weeks and the mean difference in high vs. low fruit and vegetables intake for trial’s arms was 133 g/day (ranging from 50 to 456 g).
Concerning the effects on body weight of fibre from pulses, and even more from fruits and vegetables, the influence of other components such as polyphenols should be considered.
Studies on the effects of soluble fibre supplementation seem to support a per se
beneficial effect of this type of fibre. Thompson et al. [31
] in a meta-analysis of 12 RCTs in overweight and obese adults, reported that soluble fibre supplementation, compared to placebo, reduced BMI by 0.84 kg/m2
, body weight by 2.52 kg, and body fat by 0.41%. The soluble fibres were used as supplements in the form of manno-oligosaccharides, galacto-oligosaccharides, fructo-oligosaccharides, β-glucan, flaxseed mucilage, mannans and dextrin. The mean soluble fibre dose was 18.5 g/day (range: 3–34 g/day) and the trials’ duration ranged from 2 to 17 weeks (Table 2
In conclusion, evidence from RCTs indicates that low-fermentable fibres intake, mostly from whole grain, does not affect body weight; this could be related to the heterogeneity of experimental settings in which whole grains have been tested. Fermentable fibres from other sources (pulses, fruits and vegetables) induced a statistically significant but clinically irrelevant reduction in body weight that, however, was prevalently obtained in isocaloric conditions.
2.3. Possible Mechanisms of Fibre Effects on Body Weight Regulation
In the small intestine, by creating a mechanical barrier and increasing intraluminal viscosity, soluble and insoluble fibre delay intestinal transit and reduce glucose and free fatty acid absorption with a consequent increment in fat oxidation and reduction in fat storage. Furthermore, the reduction in glucose absorption can also decrease insulin secretion, preventing the risk of reactive hypoglycemia during the post-absorption period and reducing hunger [34
]. In the small intestine, the role of dietary fibre on gastrointestinal hormone secretion should be also considered. In particular, a fibre-rich meal may favour the release of cholecystokinin, a peptide involved in gastric emptying regulation and hypothalamic satiety nucleus stimulation [36
Dietary fibre also increases glucagon-like peptide-1, a gut hormone involved in satiety control, gastric emptying and small intestine transit [37
]. Finally, in the large intestine, fibre is fermented by intestinal bacteria and influences microbiota composition [38
]. Short-chain fatty acids (SCFAs) derived from intestinal bacteria (acetic, propionic and butyric acids) could positively influence body weight regulation by different mechanisms: decreasing gastric emptying and prolonging satiety, improving insulin sensitivity and modulating glucose and lipid oxidation [40
]. With respect to microbiota composition, dietary fibre can increase Bacteroidetes
, which are predominant in lean individuals, and decrease the prevalence of Firmicutes
, which are dominant in obese individuals [43
]; the beneficial impact on microbiota could explain the possible fibre effect on body weight regulation mediated by increasing caloric extraction from food [44