There is a current trend in endurance sports to move athletes towards a low-carbohydrate diet or use periods of low carbohydrate consumption to increase both health and performance. As a result, a market is developing for sports supplements to provide nutritional support during training and racing for athletes who follow a low-carbohydrate lifestyle. PHAT FIBRE (PF) is a powdered sports supplement that includes medium-chain triglycerides suspended in a digestion-resistant carbohydrate and is tailored to the needs of low-carb athletes. Eleven healthy participants were administered 25 g of PF after an overnight fast. After 30 minutes, median blood glucose increased by 6 mg/dl from 94 mg/dl to 100 mg/dl (
Over the last decade, there has been a large-scale re-examination, in both the popular and scientific literatures, about how to best fuel the human body.
The fat-adapted athlete is able to rely on body adipose tissue, including intramuscular triglycerides, to provide fatty acids for aerobic metabolism for longer periods and at higher exercise intensities.
Anecdotally, the GI effects of MCTs can be reduced when they are provided as a powder. However, most commercially available MCT-based powders include high glycaemic index carbohydrates (i.e. maltodextrin or highly branched cyclic dextrin),
Here, we report on an open-label pilot trial examining the glucose, insulin and BHB responses to an MCT-based sports supplement powder using a digestion-resistant carbohydrate as the sole powder component. The goal is to develop a powdered MCT source that overcomes the potential downsides of products currently available on the market.
Healthy participants (
Participant demographics.
Participant no. | Sex | Age | Height (cm) | Weight (kg) | BMI (kg/m2) |
---|---|---|---|---|---|
1 | M | 34 | 189.0 | 81.8 | 22.9 |
2 | F | 25 | 171.0 | 61.0 | 20.9 |
3 | M | 45 | 170.2 | 65.9 | 22.7 |
4 | M | 40 | 175.3 | 72.7 | 22.5 |
5 | M | 46 | 185.4 | 71.8 | 20.9 |
6 | M | 41 | 180.3 | 73.5 | 22.6 |
7 | M | 42 | 160.0 | 61.4 | 24.0 |
8 | M | 29 | 182.0 | 79.0 | 23.8 |
9 | M | 43 | 190.5 | 88.6 | 24.4 |
10 | M | 34 | 190.5 | 106.8 | 29.4 |
11 | F | 24 | 162.6 | 43.2 | 16.3 |
BMI, Body Mass Index; M, male; F, female.
Participants received the experimental protocol and a kit containing the study materials through the mail. An electronic study protocol was also provided via email. The study kit included a Precision Xtra handheld glucose and ketone meter (Abbott Diabetes Care Inc., Almeda, CA) with 10 ketone strips to measure blood levels of BHB. A Meridian Valley Glucose Tolerance Insulin Response (Blood Spot) measurement kit (#8069, Meridian Valley Lab, Tukwila, WA) was also included, as was a 25 g pre-weighed pouch of PHAT FIBRE (PF) (Nourish Balance Thrive, Redding, CA). PF is a newly-developed proprietary sports supplement powder consisting of MCT oil suspended in digestion-resistant maltodextrin. To minimise the likelihood of adverse reactions to PF, it is packaged in a gluten-free, dairy-free and peanut-free facility. A 25 g serving of PF contains 9.4 g carbohydrate (0.2 g sugars, 8.8 g fibre) and 15.2 g fat. The MCT oil-derived fat consisted of C6:0 (caproic acid, up to 20%), C8:0 (caprylic acid, minimum 50%) and C10:0 (capric acid, minimum 30%).
After an overnight fast of at least 8 h (but not more than 16 h), participants were instructed to follow the manufacturer’s instructions for the Meridian Valley Kraft Assay, using 25 g of PF dissolved in 250–300 ml of water instead of the 100 g of dextrose normally used for this test (not included in the study kit). This assay measures insulin and glucose responses over 4 h, determining levels from blood spots on filter paper using enzymatic (glucose) and immuno (insulin) assays. For this study, blood spots were taken on filter paper at baseline, as well as 30 min, 1 h, 2 h, 3 h and 4 h after consuming 25 g of PF. Participants were advised to avoid strenuous exercise and any other caloric intake during the study period. At each time point, participants also measured and documented their own blood BHB measurements using the Precision Xtra meter.
Throughout the study, as well as the following day, subjective data were collected on satiety, cognitive function, alertness and GI symptoms, using questions that were answered on an analogue scale ranging from 1 (not at all) to 5 (very much). A sample of the questions from the study protocol is provided in
Subjective measures questionnaire.
Once blood spot data had been collected, it became apparent that blood glucose measurements taken from the filter paper were lower than expected, with a median (range) fasting glucose measurement of 70 mg/dl (49 mg/dl–88 mg/dl) across the 11 subjects. During the study, two subjects had taken simultaneous blood glucose measurements using a handheld glucometer (Precision Xtra; Abbott Diabetes Care Inc., Almeda, CA), with the glucose level on the handheld glucometer being on average (median with 95% CI) 26 mg/dl (14 mg/dl–36 mg/dl,
As part of ongoing pilot experiments, one participant (#4) repeated the study using an updated formulation of PHAT FIBRE (PFv2), measuring both blood glucose and BHB using a handheld glucose and ketone meter, as described above. PFv2 was formulated to include 70% C8:0 (caprylic acid) triglycerides by weight, suspended in 30% digestion-resistant maltodextrin.
Statistical analyses were performed using SPSS software version 22 (SPSS Inc., Chicago, IL) and GraphPad Prism version 7 (GraphPad Software, La Jolla, CA). For subjective measures, answers were converted to their numerical equivalent for graphical representation and statistical analysis. Metabolic data did not consistently follow a normal distribution and were therefore analysed using nonparametric methods. Within-subject pairwise comparisons of dependent variables (i.e. blood glucose, insulin and BHB) at certain time points were compared to the individual’s fasting baseline measurement using the Wilcoxon signed-rank test. A
All participants completed the study in full. Based on the electronic time stamps, mean (standard deviation) time between hourly time points was 60.7 (± 6.2) min, suggesting good adherence to the study protocol. For the duration of the study, all participants documented normal household and work activities that would not be expected to result in large changes in insulin, glucose or BHB. One male participant (#10) had their blood spot card inappropriately handled by the laboratory, and their glucose and insulin levels were not available. Metabolic parameters from this participant were therefore excluded from the analysis. Two other participants (#1 and #9) provided blood spots that were considered low quality by the laboratory due to small blood volumes. However, the samples were still large enough to provide data for analysis. In addition, nine participants had at least one insulin level below the detection limit of the immunoassay (< 2 μU/ml). These samples were assigned an insulin level of 1 μU/ml to allow for statistical analysis.
None of the participants reported a significant effect of PF on irritability or negative affect: shakiness, sleepiness, distractibility, anxiety, loose stools, abdominal pain or nausea. Median hunger score was 1 (‘not at all’ hungry) for the first 2 h of the study, increasing to a median score of 2 (‘a little bit’ hungry) at 3 h and 4 h (
Subjective measures of mood, satiety and hunger: (a) score for hunger, (b) satiety, (c) feeling more alert and (d) focus.
Based on the data reported by the laboratory, median (95% CI) glucose at baseline was 70 mg/dl (49 mg/dl–80 mg/dl). Blood glucose increased by 9 mg/dl 30 min after consuming PF to 79 mg/dl (61 mg/dl–103 mg/dl), which was statistically significant (
Blood glucose calibration. Linear regression of 18 data points where data were available for both the blood spot enzymatic assay and a handheld glucometer. The single point at 200 mg/dl is from one participant (#4) who had data using the same assay after a 100 g glucose load. A significant correlation between the two (
For the adjusted blood glucose results, median (95% CI) glucose at baseline was 94 mg/dl (79 mg/dl–101 mg/dl). This increased by 6 mg/dl to 100 mg/dl (87 mg/dl–116 mg/dl), which was statistically significant (
Blood glucose responses to PF. (a) Adjusted individual blood glucose changes (from baseline) at 30 min and 60 min. The magnitude of blood glucose change at 30 min ranged from -2.8 mg/dl to +16.0 mg/dl. In 70% of participants, glucose had returned to baseline by 60 min. (b) Scatter plot (line at median) of blood glucose at baseline (fasting) and across the 4-h study period. Median (95% CI) blood glucose after 30 min had increased from 94 mg/dl (79 mg/dl–101 mg/dl) to 100 mg/dl (87 mg/dl–116 mg/dl), which was statistically significant (
Median (95% CI) insulin level was 2.1 μU/ml (1.0 μU/ml–7.6 μU/ml) at baseline, which increased by 4.6 μU/ml to 6.7 μU/ml (1.0 μU/ml–8.9 μU/ml) after 30 min. However, this difference was not statistically significant. Median insulin fell to 1 μU/ml (1.0 μU/ml–6.1 μU/ml) by 2 h and remained at 1 μU/ml for the remainder of the study period. Individual insulin responses for the first hour, and aggregated data over the 4-h study period, are shown in
Insulin responses to PF. (a) Individual insulin changes (from baseline) at 30 min and 60 min. The magnitude of insulin change at 30 min ranged from -2.5 µU/ml to +11.0 µU/ml. (b) Scatter plot (line at median) of insulin levels at baseline (fasting) and across the 4-h study period. Median (95% CI) insulin after 30 min had increased from 2.1 µU/ml (1.0 µU/ml–7.6 µU/ml) to 6.7 µU/ml (1.0 µU/ml–8.9 µU/ml). However, this change was not significantly different (Wilcoxon signed rank test,
Median (95% CI) BHB at baseline was 0.3 mM (0.2 mM–0.5 mM), which increased by 0.2 mM to 0.5 mM (0.2 mM–0.8 mM) after 30 min. However, this difference was not statistically significant (
Beta-hydroxybutyrate responses to PF. (a) Individual BHB changes (from baseline) at 30 min and 60 min. The magnitude of BHB change at 30 min ranged from -0.2 mmol/L to +0.6 mmol/L. (b) Scatter plot (line at median) of insulin levels at baseline (fasting) and across the 4-h study period. Median (95% CI) BHB after 30 min had increased from 0.3 mmol/L (0.2 mmol/L–0.5 mmol/L) to 0.5 mmol/L (0.2 mmol/L–0.8 mmol/L). At 4 h, median BHB was 0.45 mM (0.4 mM–0.6 mM). #Indicates significantly increased BHB (
In a single participant (#4), 25 g of PFv2 did not elevate blood glucose from a baseline of 88 mg/dl. This is in contrast to a large 25 mg/dl glucose response seen in this participant after 25 g of PFv1, from a baseline of 89 mg/dl to 114 mg/dl at 30 min (
Glucose and BHB responses to PFv2. (a) In participant #4, who had displayed the greatest blood glucose response to PFv1, 25 g of PFv2 did not elevate blood glucose from a baseline of 88 mg/dl. (b) A greater BHB response was seen with PFv2 compared to PFv1. After 25 g of PFv2, BHB increased by 0.5 mmol/L from 0.2 mmol/L at baseline to 0.7 mmol/L after 30 min, reached a peak of 0.8 mmol/L at 2 h and remained above 0.5 mmol/L for the whole study period.
In the field of endurance sports, the nutritional approaches used to optimally fuel performance are undergoing a paradigm shift. In dramatic opposition to the high-carbohydrate diets and frequent ‘carb-loading’ strategies previously used by endurance athletes, many elite long-distance runners, triathletes and cyclists are now employing regular carbohydrate restriction, including during races, to improve performance.
During prolonged endurance exercise, especially events of moderate intensity (i.e. up to ≈50% of VO2Max), aerobic fat metabolism provides at least 50% of total energy expenditure (TEE).
Despite a greater contribution of fatty acids to TEE during aerobic exercise, keto-adapted athletes deplete muscle glycogen at a similar rate to athletes consuming a carbohydrate-based diet.
Although PF was originally intended to not include a significant source of glucose, a small glycaemic load may actually be of benefit to the low-carb endurance athlete if provided before or during exercise. One major use of glucose during exercise in athletes consuming a low-carbohydrate diet is the regeneration of oxaloacetate (OAA) in the mitochondrial Kreb’s cycle, a process known as anapleurosis. This occurs when mitochondrial acetyl-CoA accumulates, which increases the activity of pyruvate carboxylase. Pyruvate carboxylase diverts pyruvate from glycolysis to the production of OAA that can bind with acetyl-CoA to form citrate as the ‘first step’ of the Kreb’s cycle. In the low-carb athlete, mitochondrial acetyl-CoA may accumulate in the skeletal muscle due to increased lipolysis and beta-oxidation of fatty acids, or as the result of increased ketone availability (transesterification and cleavage of acetoacetate to two molecules of acetyl-CoA). To help drive aerobic production of ATP, this acetyl-CoA must then bind to OAA. Adequate production of OAA is therefore a rate-limiting aspect of the ability to use fatty acids and ketone bodies as fuel during exercise. This is thought to be the reason why exogenous ketones (given as a ketone ester) have a greater performance-boosting effect when given with a source of glucose (Clarke K 2016, personal communication, October 13).
As fat-adapted athletes do not appear to spare glycogen during exercise despite the majority of TEE coming from fatty acids, and exogenous ketone sources have a greater benefit when given with a source of carbohydrate, glucose availability in the muscle (either from glycogen stores or from an exogenous source) still has the potential to be a rate-limiting factor to performance in the low-carb athlete. One other product, a hydrothermally processed corn starch (UCAN superstarch), has been specifically marketed to athletes eating a low-carbohydrate diet for this reason.
To provide an optimal fuel source for low-carb or keto-adapted athletes during exercise, any supplement should have minimal effects on insulin levels, as endogenous lipolysis must still be allowed to contribute the majority of TEE. Consuming PF resulted in a small but non-significant increase in insulin after 30 min, with the response ranging from -2.5 μU/ml to +11.0 μU/ml. The degree of insulin response appeared to be associated with fasting insulin level, with minimal responses seen in those with a fasting insulin level above 5 μU/ml (but below 10 μU/ml). Aside from one participant (#1), insulin remained below 10 μU/ml throughout. This is the approximate level at which lipolysis is thought to be maximally suppressed in healthy individuals.
This study does have some limitations. Firstly, this was a series of open-label pilot studies, and therefore, the participants were not blinded. While it is likely that the acute metabolic effects were due to PF, the nature of the study may have prevented the participants from openly reporting negative subjective side effects. Although generally considered to be the standard measure of blood ketone levels, using blood BHB to determine the ketogenic capacity of an intervention gives an incomplete picture. Absolute BHB levels are a dynamic sum of ketone production (in the liver), usage (in the peripheral tissues) and wastage (i.e. in the urine and breath). Other factors such as the hepatic NAD+/NADH ratio will determine the relative abundances of BHB and acetoacetate (AcAc, which can also be decarboxylated to acetone),
Another potential limitation is the fact that a linear regression had to be constructed to adjust the reported blood glucose results, as the glucose levels from the blood spots were spuriously low. A similar method has previously been used to reduce the error in the delayed measurement of blood glucose from filter paper,
In summary, we show that 25 g of an MCT oil-based sports supplement powder increases average BHB levels by 0.2 mmol/L with only a small increase in blood glucose and without significantly increasing insulin levels. A revised formulation of the supplement resulted in twice the BHB response without affecting blood glucose. For the low-carb or keto-adapted athlete, both PF and PFv2 have the potential to support both ketone production and anapleurosis in order to help promote optimal substrate utilisation during exercise. Future studies will provide a larger dose of PFv2 and determine the metabolic responses during exercise compared to a placebo.
The authors would like to thank all the participants for their time and input to the study. They would also like to thank Ms. Amelia Luker for her assistance assembling study kits and collecting the study data, as well as Dr Elizabeth Nance for her input during the preparation of the manuscript.
Both T.R.W. and C.K. are employees of Nourish Balance Thrive and co-creators of PF and PFv2. All profits to date from the sales of PF were used to fund the current study.
C.K. recruited study participants, provided study materials and assisted in the preparation of the manuscript. T.R.W. designed the study protocol, performed the statistical analysis, and drafted and revised the manuscript.