Impaired lipid handling after oral fat ingestion results in increased circulating lipids and associated metabolic stress for prolonged periods. This postprandial characteristic is often reported in physical inactivity, obesity, and type 2 diabetes and is strongly associated with atherosclerosis (32). Acute endothelial dysfunction, increased inflammation, and oxidative stress occur during postprandial lipemia and may contribute to an atherogenic environment (9,37). Furthermore, elevated circulating postprandial lipids likely increase the propensity for oxidation of lipids, such as LDL, which are key protagonists of atherosclerosis (17). Attenuation of the postprandial triglyceride (TAG) response, total and oxidized LDL (oxLDL), is therefore likely to be beneficial for optimizing long-term cardiovascular and metabolic health, particularly in overweight or obese individuals.
Exercise performed acutely before a high-fat meal (typically 4–24 h before meal ingestion) reduces postprandial TAG (for a recent review, see ). Many studies have investigated the effects of continuous moderate-intensity exercise, with most showing favorable postprandial responses after exercise. These studies have been reviewed in detail elsewhere (13). Interval exercise involving several bursts of high-intensity exercise (lasting 6–240 s) interspersed with light exercise is also an effective strategy to reduce postprandial lipemia, but few studies have been conducted (for a recent review, see ). Burns et al. (3) identified that most studies reported significant reductions in postprandial TAG for both submaximal and supramaximal high-intensity interval exercise modes (defined relative to V˙O2max) compared with no exercise conditions. When compared with continuous moderate-intensity exercise, submaximal high-intensity interval exercise has been shown to be similar (12), or more effective (36), at reducing postprandial TAG. Supramaximal high-intensity exercise has the added benefit of reducing the time required to complete a fixed amount of work compared with exercise of lower intensities (24). Although this is appealing, because lack of time to exercise is a common reason for people not performing exercise (3,24), the practicality (24) and the safety (11) of supramaximal exercise are not fully understood in sedentary populations. As such, the use of submaximal high-intensity interval exercise to lower postprandial lipemia may be warranted. However, few studies have investigated this mode of exercise on modifying postprandial lipemia within adults at higher metabolic risk (3).
Having a healthy diet is inversely related to cardiovascular disease and all-cause mortality (40). Consuming sufficient portions of fruit and vegetables each day is an important component of a healthy diet, according to international guidelines (21). In addition to being rich in dietary fiber and essential nutrients, many fruits and vegetables are functional foods; that is, those that provide health benefits in addition to basic nutrition (1). Strawberry is considered to be a functional food because of its antioxidant, anti-inflammatory, antihypertensive, and lipid-lowering effects (for a recent review, see ). The high content of phenols (which include anthocyanins, catechins, ellagitannins, perlargonidins, and quercetin) within strawberries are proposed to be important for modifying circulating lipids and lipid oxidation in the postprandial period (4). Consumption of 10 g freeze-dried strawberries (equivalent to 110 g fresh weight strawberries) with a moderate fat (31 g) high carbohydrate (135 g) meal compared with a placebo acutely reduced postprandial TAG, oxLDL, and markers of inflammation (C-reactive protein, interleukin 6) in overweight men and women (4,10). However, the acute effects of strawberries on the postprandial responses to a high-fat, low-carbohydrate meal have, to our knowledge, not been investigated. This is important to understand more comprehensively the potential use of strawberry intake in reducing postprandial cardiometabolic stresses associated with fat ingestion.
Prior submaximal high-intensity interval exercise and strawberry consumption appear to be independently beneficial in acutely reducing lipid-induced metabolic dysregulation after moderate or high-fat meal ingestion. However, the combined effect of these lifestyle interventions has not been investigated to date. We aimed to investigate the separate and combined effects of prior acute exercise and strawberry consumption on reducing postprandial TAG responses and oxidative stress after an oral fat tolerance test (OFTT) in inactive overweight and obese adult males. We hypothesized that exercise and strawberry interventions would independently reduce postprandial TAG and that we would observe an interaction effect for strawberry and exercise in reducing postprandial TAG.
Overweight and obese adult males (body mass index [BMI] > 25 kg·m−2, waist circumference > 94 cm) with no known cardiometabolic disorders were recruited. Participants were excluded if they smoked, had known cardiometabolic disease, were taking lipid-lowering medication, had poorly controlled blood pressure, or had abnormalities identified by the cardiopulmonary exercise test during the screening visit that would increase the risk of performing the subsequent exercise trials. This study was conducted according to the Declaration of Helsinki and approved by the Department of Sport, Health and Exercise Science Ethics Committee, University of Hull. Written informed consent was given by all participants before study commencement.
This prospective single-blind, randomized, crossover study investigated the separate and combined effects of acute prior exercise and acute strawberry consumption on postprandial lipemic responses (serum TAG concentrations) and oxidative stress responses (serum oxLDL and lipid hydroperoxides). There were four experimental conditions, which included an abbreviated OFTT meal containing whole milk (257.5 g, Tesco, UK), double cream (117.5 g, Tesco, UK), and either strawberry milkshake mix (placebo) (20 g, Tesco, UK) or freeze-dried strawberries (intervention) (25 g, European Freeze Dry Ltd., Preston, UK) (detailed in the next section). The OFTT meals were preceded by either rest or submaximal high-intensity interval exercise (detailed in the next section) conducted on the day before OFTT. Each participant completed all experimental conditions. These were 1) placebo OFTT rest condition (R-P), 2) strawberry OFTT rest condition (R-S), 3) placebo OFTT exercise condition (Ex-P), and 4) strawberry OFTT exercise condition (Ex-S). Participants attended the research laboratory before 10:00 AM on four separate occasions, separated by at least 72 h. During the acute exercise conditions, participants attended the laboratory after 3:30 PM, 16 to 18 h before the scheduled OFTT. The order in which the trial conditions were performed was randomized a priori for each participant using the Research Randomizer software (39). Participants refrained from alcohol and exercise (other than that prescribed within the experimental protocol) for 24 h before each OFTT visit and attended the research laboratory having fasted overnight. All tests were completed within 8 wk of the screening visit. See Figure 1 for a depiction of the study design.
Participants fasted for 2 h before the screening visit. After providing their written informed consent, baseline stature (Harpenden Stadiometer; Holtain Limited, Crymych, Pembrokeshire, UK), body mass (Seca Balance Scales; Seca, Hamburg, Germany), and waist and hip circumferences (Seca 201 ergonomic circumference measuring tape) were measured in line with the American College of Sports Medicine’s Guidelines for Exercise Testing and Prescription (31). Body fat content (percentage) was estimated using bioimpedance (BF900 Maltron Body Composition Analyser; Maltron International Ltd, Essex, UK). Blood pressure (Omron M6; Omron Healthcare Ltd., Milton Keynes, UK) and resting ECG measurements (GE CASE system; GE Healthcare, Freiburg, Germany) were taken, and this was followed by a symptom-limited maximal cardiopulmonary exercise test (CPET) to volitional exhaustion (detailed in the next section).
Participants randomized to the exercise condition attended the laboratory the afternoon before the OFTT having refrained from exercise that day. Participants randomized to the rest condition refrained from exercise 24 h before OFTT and did not attend the laboratory. All participants were provided with a commercial “ready meal” (detailed in the next section) to consume as their only nutritional intake that evening and were asked to consume the same meal at a similar time before every OFTT study visit. Participants attended the laboratory before 10 a.m. the following morning having fasted overnight (>10 h). After 10 min of rest, three blood pressure measurements were taken for a period of 10 min. A cannula was inserted in to a vein in the antecubital fossa, and a blood sample was drawn. Once the participant was provided with an OFTT meal, they were invited to consume it within 5 min. The OFTT meal contained either freeze-dried strawberries (intervention) or strawberry flavoring (placebo). A blood sample was drawn on the hour for 4 h after OFTT meal ingestion.
The 4-h abbreviated OFTT has been validated against the standard 8 h test (41), and we have demonstrated the repeatability of this test within our laboratory (28). The OFTT meal (Table 1) was designed specifically for this investigation and was made primarily with dairy products and flavored with 20 g commercially available strawberry milkshake powder (placebo) or 25 g freeze-dried strawberries (European Freeze Dry Ltd.). The high-fat meal was designed for participant palatability and in accordance with OFTT expert statement guidelines, which recommended 75 g fat, 25 g carbohydrates, and 10 g protein (23).
Participants performed an incremental ramp-based CPET to volitional exhaustion on an electronically braked cycle ergometer (eBike ergometer, GE Healthcare) with online breath-by-breath expired gas analysis (Cortex Metalyzer 3B; Cortex Medical, Leipzig, Germany) and 12 lead ECG (GE CASE system, GE Healthcare) recorded throughout. CPET was performed and analyzed for peak oxygen consumption (V˙O2peak, mL·kg−1·min−1) and oxygen consumption at the anaerobic threshold (AT, mL·kg−1·min−1) in accordance with our previously described methods (28).
Submaximal High-Intensity Interval Exercise
Submaximal high-intensity interval exercise was performed on a cycle ergometer (eBike ergometer, GE Healthcare) using individualized protocols during each of the two exercise sessions. Before interval exercise, there was 6 min of exercise at 20 W immediately followed by 6 min of exercise at a work rate selected at 90% of the oxygen consumption at the AT, performed as a warm-up. The low-intensity interval exercise was set at 50% of the work rate at the AT. The high-intensity interval exercise was set at 50% of the difference between work rates at AT and V˙O2peak. The high to low-intensity exercise ratio was 1 min high-intensity exercise to 1 min low-intensity exercise for 40 min. Work rates were calculated from CPET with adjustment for oxygen kinetics and ramp rate as described previously (28).
The nutritional composition of the meal consumed on the evening before OFTT influences the postprandial response to OFTT (34). To control for this, participants were provided with a standardized commercial meal. Participants chose one of two meals, and the same meal was consumed by the participant on the evening before all OFTT. The mean ± SD nutritional contents of the meals were as follows: calories, 755.5 ± 13.4 kcal; protein, 34.7 ± 1.1 g; carbohydrates, 77.9 ± 5.0 g; fat, 32.5 ± 0.3 g; saturated fat, 14.8 ± 4.0 g.
Blood Sampling and Analysis
Blood samples were drawn from a 20-gauge peripheral venous cannula (Braun Introcan Safety 20G Closed Catheter; Braun, Bethlehem, PA) inserted in to a vein in the antecubital fossa. The cannula was kept patent between blood draws with a mandarin stylet (Braun Vasofix Stylet). Up to 25 mL of blood was drawn at each time point. Fluoride/oxalate blood collection tubes were spun immediately at 2383g for 15 min at 4°C. SST II blood collection tubes were stored at room temperature for 30 min to allow blood to clot and then spun at 1992g for 10 min at 4°C. Serum and plasma samples were aliquoted and stored at −80°C until analyses.
The ABX Pentra 400 biochemistry autoanalyzer (Horiba, Montpellier, France) was used to analyze serum TAG, total cholesterol, HDL cholesterol (HDL-c), and plasma glucose. Calibration and quality controls were performed before use in accordance with manufacturer’s guidelines and samples were measured in duplicate. LDL cholesterol (LDL-c) was estimated from the Friedewald equation (14). Serum oxLDL was determined by using an enzyme-linked immunosorbent assay performed in accordance with the manufacturer’s guidelines (Mercodia Inc., Upsala, Sweden); each sample was measured in duplicate. Serum lipid peroxidation was estimated by using the ferrous oxidation in xylenol orange (FOX1) assay in line with established methods (42).
Antioxidant Capacity of Strawberry Product
The Folin–Ciocalteau assay was performed on the freeze-dried strawberry product and on the placebo product in keeping with established methods but using epicatechin equivalents in place of gallic acid equivalents (35). Briefly, the strawberry/placebo product was mixed with 100% dimethyl sulfoxide to make a 50-mg·mL−1 sample concentration. Then 15 μL of this sample, 170-μL double-distilled water, 12-μL Folin–Ciocalteau reagent, and 30-μL sodium carbonate solution (concentration, 200 g·L−1) were added to each well of a 96-well plate. This was incubated in the dark for 1 h at 21°C, and then 73-μL double-distilled water was added to each well. Absorbance was then measured at 765 nm.
The primary outcome was TAG area under the curve (AUC) during OFTT. Secondary outcome measures were TAG iAUC, oxLDL, and lipid peroxidation (FOX1 assay).
Normal (Gaussian) distribution of data was verified using the Shapiro–Wilk test. Tests for skewness and kurtosis of distributions and visual inspection of histogram charts were conducted. Data are presented as mean and SD for normal data, and nonnormally distributed data (age, V˙O2max, V˙O2 AT, fasting glucose, and glucose AUC) are presented as median and quartiles 1 and 3 (Q1 and Q3, respectively). Differences between peak heart rate responses for each exercise session were compared using paired t-tests, and Cohen’s d was used to demonstrate effect size. Total AUC and incremental AUC (iAUC) for TAG, cholesterol, HDL-c, and glucose were determined by the trapezoidal method (25). oxLDL and lipid hydroperoxides were measured at baseline and at 4 h, and the difference between baseline and 4 h was calculated. To assess the differences between outcome measures for each trial condition, 2 × 2 repeated-measures ANOVA was used for normally distributed (TAG, cholesterol, HDL-c, LDL-c, oxLDL, and lipid hydroperoxides) and nonnormally distributed (glucose) AUC and oxidative stress data. Specifically, activity (exercise/no exercise) was treated as a study condition and nutritional content (strawberry/no strawberry) was treated as a study condition. Each activity/nutritional intervention and placebo appeared twice across the study trials; therefore, the 2 × 2 repeated-measures ANOVA enabled the exercise and strawberry interventions to be assessed independently across the study, and the interaction revealed whether a combination of the study conditions influenced postprandial responses. Mean difference with 95% confidence intervals (CI), P values, and effect sizes using partial eta squared (ηp2) are reported. The alpha level was set at 0.05, and ηp2 was used to determine the effect size with small, medium, and large effects set at 0.01, 0.06, and 0.14, respectively (8). Where significance was reached, post hoc pairwise comparisons were made with Bonferroni adjustment and reported as mean difference, 95% CI, P values, and ηp2. Microsoft Excel (2013; Microsoft Corp., Redmond, WA) and Statistical Package for the Social Sciences (Version 22; SPSS Inc., Chicago, IL) were used for all statistical analyses.
The complexity of the 2 × 2 repeated-measures ANOVA with two within factors makes sample size estimation for this design challenging (33). As such, we estimated the sample size required to detect differences between the main effects for the diet condition and the exercise condition using a one-way repeated-measures ANOVA design with two measures for each condition. On the basis of previous data (41), we expected that the repeatability of our primary outcome, TAG AUC, would be high (intraclass correlation coefficient = 0.83). Using a more conservative estimate of rho = 0.7, an effect size of 0.7, an alpha value of 0.05, and 80% power, we obtained a sample size of 10 participants.
Ten of 11 males (median age = 31.5 Q1, 28.5 Q3, 46.3 yr; mean ± SD, BMI = 29.9 ± 1.8 kg·m−2, waist circumference = 1.05 ± 0.05 m) completed all study visits. Demographics for these participants are reported in Table 2. One participant dropped out of the study after the screening visit for personal reasons. Six participants were overweight (BMI = 25–30 kg·m−2), four were obese (BMI > 30 kg·m−2), and all were inactive (defined by self-reported exercise <150 min·wk−1). All participants completed the two submaximal high-intensity interval exercise protocols, which lasted 1 h in total. The peak heart rates achieved during exercise were 93% ± 4% of peak heart rates measured in CPET. There were no differences in peak heart rates between the two interventions: exercise session 1 = 154 ± 14 bpm and exercise session 2 = 153 ± 11 bpm (95% CI = −2 to 5 bpm, P = 0.504, Cohen’s d = 0.09). The mean ± SD work rate (W) for the low and high-intensity intervals were 48 ± 16 and 181 ± 49 W, respectively. Participants performed the same preprogrammed tailored exercise protocol for both exercise sessions on an electromagnetically braked cycle ergometer. The Folin–Ciocalteau assay identified that freeze-dried strawberry had 4.5-fold greater phenolic capacity compared with the placebo (895 vs 194 mg). There were no adverse effects during or after the exercise interventions or high-fat meal ingestion.
Serum TAG responses to OFTT
Mean ± SD TAG responses at each time point for each condition are presented in Figure 2. TAG increased from baseline in all conditions and peaked at 3–4 h.
TAG AUC was 1.5 mmol per 4 h−1·L−1 lower (95% CI = −2.3 to −0.8 mmol per 4 h−1·L−1, P = 0.001, ηp2 = 0.71) for the two exercise conditions compared with the two resting conditions. Post hoc pairwise comparisons with Bonferroni adjustment identified that TAG AUC was 1.6 mmol per 4 h−1·L−1 lower in the exercise condition compared with rest condition for the placebo OFTT (95% CI = −2.5 to −0.5 mmol per 4 h−1·L−1, P = 0.009, ηp2 = 0.55) and by 1.5 mmol per 4 h−1·L−1 for the strawberry OFTT (95% CI = −2.9 to −0.2 mmol per 4 h−1·L−1, P = 0.033, ηp2 = 0.41). There were no differences in TAG AUC between the strawberry OFTT and the placebo OFTT (mean difference = −0.3 mmol per 4 h−1·L−1, 95% CI = −1.3 to 0.7 mmol per 4 h−1·L−1, P = 0.475, ηp2 = 0.06). There was no exercise and strawberry interaction (P = 0.970, ηp2 < 0.001).
There was a large effect size for lower TAG iAUC (mean difference = −0.4 mmol per 4 h−1·L−1, 95% CI = −1.1 to 0.2 mmol per 4 h−1·L−1, P = 0.175, ηp2 = 0.19) in the exercise conditions compared with the resting conditions. TAG iAUC was 0.5 mmol per 4 h−1·L−1 lower in the placebo conditions than the strawberry conditions (95% CI = −1.0 to −0.1 mmol per 4 h−1·L−1, P = 0.021, ηp2 = 0.47). Post hoc analyses identified that TAG iAUC was 0.7 mmol per 4 h−1·L−1 lower for the placebo condition compared with strawberry condition with exercise (95% CI = −1.1 to −0.3 mmol per 4 h−1·L−1, P = 0.005, ηp2 = 0.61) but not with rest (mean difference = −0.4 mmol per 4 h−1·L−1, 95% CI = −1.2 to 0.5 mmol per 4 h−1·L−1, P = 0.331, ηp2 = 0.11). There was no interaction between conditions (P = 0.516, ηp2 = 0.05).
Baseline TAG was 0.3 mmol·L−1 lower (95% CI = −0.4 to −0.2 mmol·L−1, P = 0.001, ηp2 = 0.74) in the exercise conditions compared with the resting conditions. Post hoc analyses identified that baseline TAG was 0.2 mmol·L−1 lower with exercise compared with rest condition with the placebo (95% CI = −0.4 to −0.1 mmol·L−1, P = 0.011, ηp2 = 0.53) and 0.3 mmol·L−1 lower with the strawberry condition (95% CI = −0.5 to −0.1 mmol·L−1, P = 0.014, ηp2 = 0.50). There were no differences in baseline TAG in the strawberry conditions compared with the placebo conditions (mean difference = 0.1 mmol·L−1, 95% CI = −0.1 to 0.2 mmol·L−1, P = 0.484, ηp2 = 0.06). There was no interaction effect between conditions (P = 0.660, ηp2 = 0.02).
Oxidative stress responses to OFTT
Mean ± SD change (Δ) in oxLDL and lipid hydroperoxides from baseline to 4 h are reported in Table 3. There were no differences in oxLDL for the exercise (mean difference = −3.6 mU·L−1, 95% CI = −14.3 to 7.0 mU·L−1, P = 0.45, ηp2 = 0.06) or strawberry (mean difference = −2.9 mU·L−1, 95% CI = −9.6 to 3.7 mU·L−1, P = 0.34, ηp2 = 0.10) conditions. However, there was a large interaction effect size between conditions (P = 0.16, ηp2 = 0.21). There were no differences in lipid hydroperoxides for the exercise (mean difference = 0.8 μmol·L−1, 95% CI = −8.0 to 9.6 μmol·L−1, P = 0.84, ηp2 = 0.01) or strawberry (mean difference = −2.8 μmol·L−1, 95% CI = −11.1 to 5.6 μmol·L−1, P = 0.47 μmol·L−1, ηp2 = 0.06) conditions. However, there was a large interaction effect size between the conditions (P = 0.13, ηp2 = 0.24).
Cholesterol, HDL, LDL, and glucose responses to OFTT
Cholesterol, HDL, LDL, and glucose AUC in response to OFTT are presented in Table 3. Cholesterol AUC was 0.7 mmol per 4 h−1·L−1 lower in the exercise conditions compared with the rest conditions (95% CI = −1.1 to −0.2 mmol per 4 h−1·L−1, P = 0.01, ηp2 = 0.58). There was no effect for exercise (mean difference = 0.01 mmol per 4 h−1·L−1, 95% CI = −0.13 to 0.14 mmol per 4 h−1·L−1, P = 0.94, ηp2 = 0.001) or strawberry (mean difference = 0.03 mmol per 4 h−1·L−1, 95% CI = −0.06 to 0.14 mmol per 4 h−1·L−1, P = 0.43, ηp2 = 0.07) conditions on HDL responses to OFTT. There was no effect for exercise (mean difference = −0.05 mmol per 4 h−1·L−1, 95% CI = −0.58 to 0.49 mmol per 4 h−1·L−1, P = 0.85, ηp2 = 0.004) or strawberry (mean difference = 0.39 mmol per 4 h−1·L−1, 95% CI = −0.74 to 1.52 mmol per 4 h−1·L−1, P = 0.46, ηp2 = 0.06) conditions on LDL responses to OFTT. There was no effect for exercise (mean difference = 0.29 mmol per 4 h−1·L−1, 95% CI = −1.04 to 0.43 mmol per 4 h−1·L−1, P = 0.387, ηp2 = 0.08) or strawberry (mean difference = 0.14 mmol per 4 h−1·L−1, 95% CI = −0.55 to 0.83 mmol per 4 h−1·L−1, P = 0.655, ηp2 = 0.02) on glucose responses to OFTT.
We investigated the separate and combined effects of acute submaximal high-intensity interval exercise and strawberry consumption on postprandial responses to OFTT among overweight and obese adult males. We have demonstrated that acute submaximal high-intensity interval exercise was effective in reducing TAG AUC after OFTT. This significant effect of acute exercise in lowering postprandial TAG was evident both with and without strawberry consumption. However, contrary to our hypotheses, strawberry consumption with OFTT did not alter TAG AUC, and there was no interaction between strawberry consumption and submaximal high-intensity interval exercise. Our secondary findings indicate that there was a large effect size observed for acute submaximal high-intensity interval exercise reducing TAG iAUC. However, TAG iAUC was increased with strawberry consumption. There were no significant changes in lipid related oxidative stress responses between conditions.
Exercise and postprandial TAG
We observed a reduction in TAG AUC in response to the OFTT by approximately 20% in the submaximal high-intensity interval exercise conditions compared with the control conditions. Prior acute exercise significantly lowered baseline TAG, and there was a large effect size for lower TAG iAUC, which contributed to the reduction in total AUC. Reductions in TAG AUC of a similar magnitude have been reported in response to moderate continuous exercise (13) and high-intensity interval exercise (12,36). We selected an individualized submaximal high-intensity interval exercise protocol consistent with exercise intensity domains identified by analysis of expired ventilatory gasses measured during a CPET (29). Other submaximal high-intensity interval exercise interventions that have successfully reduced postprandial lipemia lasted approximately 40 min and were stopped when participants had expended 500 kcal (12) or 660 kcal (36). We recruited an older, more overweight, and less active population with higher mean fasting TAG concentrations compared with these studies. For practical reasons (i.e., to avoid unrealistic length of exercise sessions) and real-life application, we predefined the 40-min duration of high-intensity interval exercise (rather than a target energy expenditure) and investigated the effects of individualized interventions at clearly defined exercise intensities. We believe this to be important because participants with lower levels of cardiorespiratory fitness, exercising at the same relative intensity, will need to exercise for longer compared with a fitter individual to attain the same overall energy expenditure. A high total energy expenditure (>500 kcal) would typically require exercise sessions in excess of 1 h for a less fit individual. Such study designs may not be ecologically valid because >1 h of exercise is above the recommended target exercise guidelines, which are seldom met (38). Furthermore, standard equations used for calculating energy expenditure from expired oxygen and carbon dioxide are inaccurate during interval exercise that involves exercise intensities above the AT. Carbon dioxide (V˙CO2) is produced from nonoxidative processes and oxidative metabolism during exercise at an intensity above the AT (2). The respiratory exchange ratio (V˙CO2/V˙O2), used to infer substrate-specific oxidative metabolism, is increased by nonoxidative carbon dioxide production from anaerobic metabolism (2). This overestimates oxidative glucose and underestimates oxidative fat metabolism and will therefore reduce the accuracy calculations to estimate energy expended (19). Furthermore, in our protocol, exercise intervals at an intensity above the AT lasted 1 min. Oxygen kinetics during exercise take longer than 1 min to reach a steady state (and may never reach a steady state at higher intensities). Attaining steady state is a requirement of energy expenditure estimation from expired gasses. Therefore, the validity of high-intensity interval exercise interventions that use predefined estimated energy expenditure targets based on expired oxygen and carbon dioxide calculations could be questioned. Finally, as considered later, total energy expenditure may not be the key mechanism involved in reducing postprandial TAG with high-intensity interval exercise (3).
Interval exercise has the advantage of enabling a greater volume of work to be completed within a period (24,36), as well as varying the physiological challenge on the body when compared with continuous moderate-intensity exercise. High-intensity interval exercise has superior levels of enjoyment (18), lower perceived work (22), and increased likelihood of continuing regular exercise (18,22) in addition to the numerous cardiometabolic benefits (16,24) compared with continuous moderate-intensity exercise. Furthermore, the activity of lipoprotein lipase (LPL; a key enzyme involved with the removal of TAG) appears to be increased after high-intensity interval exercise training (3,36). This is important because TAG clearance appears to be the primary mechanism of reducing postprandial TAG after high-intensity interval exercise (3).
One mechanism by which high-intensity interval exercise elicits greater reductions in postprandial TAG compared with continuous moderate-intensity exercise could be explained by the regulation of LPL and its specificity to type 2 muscle fibers (3). A greater number of type 2 muscle fibers will likely be recruited during high-intensity interval exercise, and subsequently type 2 muscle fiber-specific LPL activity may be increased (36). Reductions in postprandial TAG with continuous moderate-intensity exercise may also occur via this mechanism because type 2 muscle fiber recruitment increases with prolonged moderate-intensity exercise. In addition to increased LPL activity, moderate-intensity exercise increases the affinity of VLDL1 for LPL, which will likely facilitate systemic removal of lipids (15). However, the effects of high-intensity interval exercise on altering VLDL1 affinity for LPL are unknown. Further mechanistic investigations in to the effects of acute high-intensity interval exercise-induced attenuation in postprandial TAG excursions, fiber-specific LPL activity, and VLDL1 affinity for LPL would help to identify optimal exercise interventions for those at risk of cardiometabolic disease.
Our data support the use of submaximal high-intensity interval exercise as a training modality to reduce postprandial TAG, which may favorably modify lipid-related cardiovascular risk in overweight and obese men.
Exercise and postprandial oxidative stress
We did not observe improvements in markers of oxidative stress with exercise in the present study. This could be due to the small sample size within our study and the variability within these markers. These were also secondary outcome measures, and therefore the study was not adequately powered to detect differences between interventions for these markers.
There were no changes in postprandial oxLDL concentrations or lipid hydroperoxides with prior acute submaximal high-intensity interval exercise. Reduced oxLDL with endurance cycling exercise (70% V˙O2max for approximately 47 min) performed 16 h before high-fat meal ingestion has been previously reported (20). Compared with the present study, the high-fat meal used in the study by Jenkins et al. (20) contained approximately 50 g more fat. The higher fat intake is likely to have contributed to a larger and prolonged lipemic response. Higher circulating lipids provide a greater capacity for postprandial LDL oxidation (17); therefore, there may have been a greater capacity for reduction in oxLDL with exercise compared with the present study.
A reduction in lipid hydroperoxides with the exercise session performed either immediately before OFTT or 1 h after OFTT has been demonstrated previously (7,26). However, to our knowledge, the effects of exercise performed 16 h before OFTT on lipid hydroperoxides, as in our protocol, have not been investigated. Of the studies that have investigated the effects of exercise in reducing postprandial oxidative stress, all used continuous endurance exercise lasting 47 min (20) or 60 min (7,26) at an intensity of 70% V˙O2max (20), 60% predicted maximum heart rate (7), or 60% maximum heart rate (26). The timing of exercise and perhaps the mode of exercise required to reduce oxidative stress may therefore be important.
Strawberry consumption and postprandial TAG
In contrast to previous research (4), strawberry consumption had no effect on TAG AUC. Interestingly, TAG iAUC was higher with strawberry consumption than with the placebo. In contrast to the beneficial effects of strawberry consumption on postprandial TAG that have been reported previously (4), the present findings suggest that strawberry consumption had no effect on postprandial TAG.
Our OFTT had a higher fat content (73 vs 31 g), and our carbohydrate content was considerably lower (33 vs 135 g) compared with a previous study which demonstrated reduced TAG after OFTT with strawberry consumption (4). In addition, our OFTT was composed of milk and cream as opposed to typical American breakfast foods. We propose that the differences in carbohydrate quantities of the OFTT and the amount of fructose relative to the total carbohydrate content may explain these findings. Approximately 20% of the carbohydrate content of our strawberry OFTT was fructose, with glucose the predominant carbohydrate source in the placebo high-fat meal (which did not contain fructose). It has been demonstrated previously that an OFTT containing fructose resulted in a higher postprandial TAG response compared with the same OFTT when the carbohydrate content was glucose (6). It was proposed by Chong et al. (2007) (6) that the lower insulin response to fructose compared with glucose may explain the greater postprandial TAG response. The fructose content in our strawberry OFTT may therefore have contributed to the greater incremental increase in postprandial TAG in our study compared with placebo. Given the relatively small fructose contribution to the high total carbohydrate in the test meals of Burton-Freeman et al. (4), the overall effect of fructose on the insulin response was likely minimal in this study. Further, strawberry polyphenols promote increased insulin sensitivity (10). This could potentially stimulate enhanced insulin-mediated TAG storage in adipose tissue and thus increase TAG clearance from the circulation, when carbohydrate is high as was the case in the study by Burton-Freeman et al. (4).
Strawberry consumption and postprandial oxidative stress
There were no changes in oxLDL or lipid hydroperoxides between groups. Previous studies have demonstrated the benefits of strawberries on reducing postprandial oxLDL after lipid ingestion (4,30). We gave a dose of strawberries (25 g freeze-dried strawberries), which is similar to the optimal dose (20 g) for lowering postprandial TAG identified by Park et al. (2016) (30). We used a higher fat content and specifically a higher dairy fat content in our OFTT meal compared with that of other studies (4,30). Dairy products within our high-fat meal may have reduced circulating bioavailability of the strawberry polyphenols because milk proteins and fat may reduce bioavailability of berry polyphenols (5,43). However, despite the bioavailability of berry polyphenols being lower when combined with milk, this may not necessarily reduce the intestinal-blood transfer of berry polyphenols according to in vitro experiments (5). Notably, reduced circulating oxLDL and increased circulating strawberry polyphenols have been observed after consumption of a strawberry drink containing milk in humans (30). It is therefore unclear whether dairy products reduced the bioavailability of strawberry polyphenols and, therefore, the capacity to reduce oxLDL in the present study. Lipid hydroperoxides, which increase during postprandial lipemia (7,26,27), are reduced after anthocyanin intake from grapes (27). However, we did not observe this reduction in the present study involving assumed strawberry anthocyanin intake. As discussed, the potential for reduced bioavailability with dairy products may explain our findings. Differences in the agricultural and preparation processes of the strawberry products could also contribute to the discrepancies between the present study and previous studies (1).
We have eluded to some of the limitations that exist within the present study in the discussion. A further limitation is that only the evening meal on the day preceding the OFTT was standardized. Therefore, we cannot completely exclude the influence of food intake 24 h before OFTT. We gave strict instructions to participants to continue with their habitual diet and abstain from alcohol and caffeine. We trusted their adherence to our instructions as we did in our previous repeatability study (28). This was the same for restricting physical activity beyond their habitual levels (which were self-reported to be below standard guidelines); however, previous studies have attempted to measure activity levels during this period (36). Participants did not attend the laboratory on the day before OFTT for the resting conditions at the equivalent time to when they reported to the laboratory to perform exercise during the exercise conditions. This could have influenced the metabolic responses measured. The abbreviated 4-h OFTT has been shown to be predictive of the 8-h period (41) and is a repeatable test (28); however, it does not allow assessment of clearance of postprandial TAG (this is, chylomicrons and their remnants), which may have been beneficial to evaluate. Finally, the amount of fat in OFTT is not representative of typical western diets; therefore, although these findings are important, investigating this exercise protocol using ecologically valid meals is of interest.
Our findings support the use of acute submaximal high-intensity interval exercise as an effective intervention to reduce lipoprotein-related cardiovascular risk factors in overweight and obese adult men. This mode of structured exercise could be incorporated in to lifestyle management of overweight and obese adult males to reduce cardiovascular risk. However, freeze-dried strawberry supplementation within an OFTT containing dairy products did not improve postprandial TAG response, which may be related to the fructose and total carbohydrate content of meal. Nevertheless, this is an interesting finding that merits further investigation. We recommend that future studies 1) investigate the role of carbohydrate and polyphenols in reducing postprandial lipemia and 2) evaluate the effects of acute submaximal high-intensity exercise on reducing postprandial lipemia in dyslipidemic males and females.
European Freeze Dry Ltd. provided the freeze-dried strawberry product for this study at no cost. Horiba UK Ltd. provided the reagents for analysis of lipids and glucose at a reduced cost for this study.
The authors have no personal or financial conflicts of interest to declare with regard to the present study. The results of the present study do not constitute endorsement by the American College of Sports Medicine.
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