Over the years, research has consistently shown that a higher intake of dietary fibre correlates with a reduced risk of several chronic diseases, including cardiovascular diseases (CVDs), cancer, type 2 diabetes, and obesity . The health benefits provided by a higher dietary fibre intake are thought to occur via the following mechanisms: delaying the absorption of glucose; increased satiation and satiety effect which can reduce food intake, and may promote weight loss; gut microbe–induced production of short-chain fatty acids, which have immunomodulatory and anti-inflammatory properties; trapping of bile acids and carcinogenic substances; and increased intake of biologically active compounds, such as phytochemicals and antioxidants [28, 29]. However, it is still not well known how a higher fibre intake through supplementation affects micronutrients absorption in the gastrointestinal tract, especially in overweight and obese individuals. Therefore the aim of this study was to determine whether 3-months’ supplementation with psyllium or PGX® fibre affects micronutrient status of overweight and obese adults.
Current literature, although limited suggests that a higher fibre intake can either inhibit or promote bioavailability depending on the nutrient, the absorptive mechanism involved, and the composition of the food matrix as a whole . Factors affecting vitamin or mineral bioavailability include how it is absorbed, excreted, stored/distributed; whether its sequestered by fat or dispersed in tissue, metabolic processes (catabolic losses, possibly oxidative), increased physiologic requirements, or lower absolute total dietary intake [31, 32]. This may help explain the variability that can occur within serum micronutrient levels. The habitual diet of overweight and obese individual is often seen as energy-dense and nutrient-poor, so maintaining nutritional sufficiency can potentially be compromised. An emerging theory suggests that nutritional deficiency states in obese persons are not more prevalent, as they may innately consume more food to compensate for the poor nutritional quality of their diet to achieve sufficiency. However due to the satiating effects of fibre, if a therapeutic dose is consumed, less food consumption may occur. Therefore nutrient intake and nutrient status may be affected in the long term, if malabsorption from the combined effects of obesity on bioavailability and a poor diet are already in effect.
This study examined the micronutrients present in serum samples and self-reported dietary intake data of overweight and obese individuals at baseline and 3 months following consumption of 15 g per day of either a control (rice flour), psyllium or PGX® fibre supplement. According to the serum values (Table 1 and Table 2), there were no between group differences in micronutrient status compared to control, both at baseline as well as after 3 months of fibre supplementation. Self-reported dietary intake using the 3-day food diaries (Table 1) showed the baseline value for zinc was significantly different in the PGX® group compared to the control group. As this is a baseline value, its significance is probably not related to an increase in dietary fibre, and could be explained by simple misreporting or poor zinc status in overweight and obese [33, 34]. All other micronutrients for self-reported dietary intake data at baseline, showed no significant differences compared to control at p < 0.05.
There was no significant difference between groups in serum micronutrient levels following 3 months fibre supplementation (Table 2), however there was a notable increase or decrease for some key micronutrients from baseline to 3 months (within groups) (Table 1 and Table 2). Folate, sodium and potassium all showed changes from baseline, while not significant at p < 0.05 it shows the potential for serum levels to be further affected over a longer time period (greater than 3 months). For between group differences in self-reported dietary nutrient intake after three months of fibre supplementation, sodium intake was found to be significantly lower in the psyllium group compared to the control (Table 3). Dietary intake data for vitamin C was also significantly lower in the PGX® group at 3 months compared to the control group at p < 0.05. Other micronutrients for dietary intake data were also lower following 3 months dietary fibre supplementation but not statistically significant at p < 0.05. These included iodine and calcium in the PGX® group compared to the psyllium group and control group (Table 3). There appeared to be a downward trend for most dietary micronutrients in both fibre groups at 3 months, compared to the control (Table 3).
While these results may reflect differences or changes in dietary intake (respectively), it may potentially be the result of inaccurate self-reporting . It is well known that overweight and obese participants frequently under-report dietary intake in self-reported food diaries , therefore these results may be due to simple misreporting of 3-day food diaries completed by participants as opposed to any actual changes. Another possibility of reduced nutrient intake data after 3 months dietary fibre supplementation of 15 g per day is due to the satiating effect of dietary fibre. Participants physically consumed less food as they were simply not as hungry, meaning over all, recorded dietary macro and micronutrient intake was reduced. This theory was supported by the significant reductions in energy, fat, carbohydrate and protein and the significant increase in fibre intake per day in the self-recorded dietary macronutrient intake following 3 months fibre supplementation (Table 4).
The fact that there were no between group differences in the serum micronutrients following three months of PGX® or psyllium fibre supplementation is a positive outcome for the present study. Dietary fibre supplementation is becoming recognised as an adjunct to weight management programs for overweight and obese individuals, and potentially complements many existing weight loss programs. However, as with any new treatment, it is essential to eliminate any negative effects that a new method or product may have on patients’ health, in this instance their micronutrient status. Recent research promotes the safety and efficacy of the novel fibre supplement PGX®, as well as showing positive effects for metabolic syndrome (MS) and associated risk factors such as CVD, blood pressure and blood lipids [13, 27]. However the effect of PGX® or psyllium fibre on micronutrient status, when used as a part of a weight loss treatment in overweight and obese individuals, has not been well researched to date. The micronutrients examined in this study play an essential role in regulating biochemical pathways associated with metabolism. As obesity is a multifaceted and complex disease, the daily intake of 15 g of supplemented dietary fibre to help weight loss, needed to be ruled out as a possible confounder to attaining a better health outcome for this subpopulation.
Strengths and limitations
This study is unique in that there has been very limited research examining the micronutrient status of overweight and obese individuals taking high fibre doses to assist with weight loss. While there is still much to learn about how fibre and micronutrients interact within the digestive system, the results of this study add to the body of knowledge in this area.
One of the limitations of this study was the time frame examined (baseline to 3 months only). It is possible that while no significant differences were found in micronutrient status following 3 months of high supplementary fibre (15 g per day), longer term fibre treatment or higher fibre dosages may have produced different results. It would be advisable for any future studies in this area to be conducted over a greater duration than 3 months as well as vary the dosage of dietary fibre supplement. A second limitation in this study was the use of self-reported dietary intakes. With any self- reporting nutritional data, potential bias is introduced as overweight and obese populations are most likely to under report their dietary intake . While food diaries are a useful tool in the examination of dietary intake in weight management trials, the fluctuations in folate, zinc, magnesium sodium and vitamin C data found in this study may not represent changes to dietary intake, but rather inconsistent food consumption reporting. Thirdly the fibre dosage itself needs to be considered. Participants were consuming an extra 15 g of fibre each day in supplement form prior to breakfast, lunch and dinner. If this fibre amount was to increase to attempt to maximize or encourage further weight loss, this may produce more significant effects on serum micronutrient status. A further point to consider for any future studies is whether any significant results or differences between fibre groups from baseline to 3 months is due to the increase in PGX® or Psyllium fibre supplementation and not due to possible weight loss. High dietary fibre promotes satiety, so participants physically consumed less food and fewer kilojoules. Over a longer time period this possibly may have led to more significant differences in serum micronutrients. Another limitation was the lack of intention-to-treat analysis as the exclusion of dropouts from the study may have introduced bias.
As high fibre diets have been linked to deficiencies of calcium, iron, trace metals, and certain vitamins in previous studies, it is important to update the body of knowledge in determining the possible health implications of therapeutic doses of fibre. If fibre is to be recommended as a potential adjunct to weight loss treatments for overweight and obese individuals, it is important to identify any possible limiting factors. The results from this research trial provide further evidence as to the physiological and biochemical effects high intakes (15 g daily) of psyllium or PGX® fibre supplementation have on the micronutrient status of an overweight or obese individual.