The effect of training intensity, age and diet on aerobic capacity and metabolic risk factors: testing procedures and training interventions
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- Ecology 
BACKGROUND: Low cardiorespiratory fitness (CRF) with accompanying metabolic risk factors such as overweight/obesity, reduced blood glucose control, elevated blood pressure, and dyslipidemia, is one of the most important predictors of all-cause mortality. Maximal oxygen uptake (VO2max) is considered the single best indicator of CRF. VO2max gradually decreases with increasing age, partly because of a reduced activity level, with concurrent increase of body weight (BW), reduced ability of fat oxidation (FatOx) and increased prevalence of T2D. High-intensity aerobic interval training (HAIT) has been found to effectively increase VO2max. HAIT may thus have the potential to counteract both age, -and inactivity related declines in VO2max and concomitant metabolic risk factors. Still, previous studies indicate blunted VO2max adaptations among older compared to younger individuals. However, no previous studies have systematically compared the VO2max responses after HAIT in different age groups. T2D most often occurs above the age of 30 and is associated with metabolic disturbances in both glucose- and FatOx. T2D is also associated with elevated blood pressure (BP), overweight and impaired blood lipid profile (BLP). Aerobic exercise has been associated with improvements in all these metabolic risk factors, but few studies have investigated the effects of HAIT compared to work matched moderate-intensity continuous training (MIT) on these variables among T2D. When assessing FatOx adaptations to exercise, it is crucial to have a strict diet control since changes in diet will interfere with the FatOx results. Test – retest reliability should also be considered when interpreting FatOx measurements, since day to day variations in FatOx will occur also during equal diet conditions. OBJECTIVES: To investigate a) the effects of HAIT on metabolic risk factors among individuals diagnosed with T2D (paper I), b) the effects of age on VO2max adaptations (paper II),- and c) to evaluate the impact of diet and testing reliability on FatOx during exercise (paper III). METHODS: Paper I and II are both training interventions, investigating VO2max adaptations using HAIT; 4 x 4 minutes at an intensity between 85-95% of maximum heart rate (HRmax). The main outcome in paper II was to explore if there are differences in VO2max response between six decade-cohorts from 20-70+ years with a training status typical for their age group. In paper I, the main outcome was to explore VO2max- and glycosylated hemoglobin level (HbA1c) adaptations to HAIT. Responses in lactate threshold (LT), FatOx, BP, body composition and (BLP) was also investigated, and the results were compared to adaptations after work matched MIT (70-75% HRmax). 32 and 94 individuals participated in paper I and paper II, respectively, and all exercise sessions were supervised. In paper III, day to day variations in FatOx during equal diet conditions, as well as the influence of either a high-fat diet or high-carbohydrate diet on FatOx during moderate-intensity exercise (60% VO2max) was examined among 9 healthy female adults. Due to the diets influence on FatOx, thorough diet registrations were performed also in paper I. In all papers, an incremental protocol was used to measure VO2max, with the ergo spirometry metabolic test system Sensor Medics Vmax Spectra (Sensor Medics 229, Yorba Linda, Calif., USA). In paper I and III indirect calorimetry based on respiratory exchange ratio was used to estimate FatOx during exercise at 60% VO2max. Three 5-min submaximal work periods with increasing watt or speed, together with the VO2max measurement represents a linear regression which was used to calculate each persons workload at 60% VO2max. RESULTS AND CONCLUSIONS: In paper I, 12 weeks of HAIT led to a 21% (ml·kg-1·min-1) - and 19% (L·min-1) increase in VO2max, while no change in VO2max was found in MIT. Velocity at LT improved in both groups. LT expressed as %VO2max did not change in either of the groups. A significant improvement was found in HbA1c in HAIT compared to MIT, with a 0.58% points reduction in HbA1c, while no change was found in MIT. A significant correlation was found between change in VO2max and change in HbA1c (r= -0.52, p<0.01). There was a tendency towards an improved FatOx at 60% VO2max in HAIT (p= 0.065) with a significant difference in change between HAIT and MIT. No change in FatOx was found in MIT. HAIT reduced BW, BMI, %BF, waist circumference and hip circumference. In MIT, there were no changes in BW or BMI, but improvements were discovered in waist circumference, hip circumference and %BF. Only the changes in BW and BMI were significant different between HAIT and MIT, with the greatest improvement in HAIT. HAIT reduced diastolic BP but not systolic BP. No changes were found in any of the BLP measurements in HAIT. In MIT, improvements were found in both systolic BP and diastolic BP, as well as reduced triglycerides and increased HDL. The changes in BP and BLP were not significantly different between the groups. Paper I thus showed HAIT to be an effective strategy to reduce important risk factors associated with T2D, -and a more effective one than MIT in improving VO2max and lowering HbA1c. In paper II, we showed that 8 weeks of HAIT increased VO2max with 9-13% with no differences between the six age groups. The percentage improvements were inversely related to baseline training status (r= 0.66, p<0.001). HRmax was not changed within the respective age cohorts, but the two oldest cohorts exhibited a tendency to increase HRmax in contrast to a training-induced decrease in the younger cohorts (p=0.07). This paper revealed that for people with an aerobic capacity typical for what is observed in the population, the VO2max adaptations are likely not affected by age in a short-term training intervention, but may rather be affected by their baseline training status. These results indicate HAIT to be an excellent training strategy to counteract the decrease in VO2max associated with aging. In paper III, the reliability test revealed no differences in FatOx, respiratory exchange ratio (RER), oxygen uptake, carbon dioxide production, heart rate, blood lactate concentration, or blood glucose between 2 habitual diet days with equivalent kilo kalories (KCAL) intake and macronutrition composition. However, FatOx was reduced by 31% after the CHO-rich diet compared with the fat-rich diet. FatOx was also decreased after the CHO-rich diet compared with the habitual day 2. No difference was found in FatOx between fat-rich diet and the 2 habitual diet days. The results from this study show the use of RER data to assess FatOx during moderate exercise to be a reliable method as long as the diet is strictly controlled. The importance of diet control is emphasized through the finding in this study of a huge change in FatOx after only one day change in macronutrient composition.
Består avArticle I: Støa, E.M., Meling, S.V., Nyhus, L.-K., Strømstad, G., Mangerud, K.M., Helgerud, J., Bratland-Sanda, S. & Støren, Ø.: "High-intensity aerobic interval training improves aerobic fitness and HbA1c among persons diagnosed with type 2 diabetes". European Journal of Applied Physiology 117(3), (2017), 455-467. http://dx.doi.org/10.1007/s00421-017-3540-1
Article II: Støren, Ø., Helgerud, J., Sæbø, M., Støa, E.M., Bratland-Sanda, S., Unhjem, R.J., Hoff, J. & Wang, E.: "The effect of age on the VO2max response to high-intensity interval training". Medicine & Science in Sports & Exercise 49(1), (2017), 78-85. http://dx.doi.org/10.1249/MSS.0000000000001070
Article III: Støa, E.M., Nyhus, L.-K., Børresen, S.C., Nygaard, C., Hovet, Å.M., Bratland-Sanda, S., Helgerud, J. & Støren, Ø.: "Day to day variability in fat oxidation and the effect after only 1 day of change in diet composition". Applied Physiology, Nutrition and Metabolism 41(4), (2016), 397-404. Not available due to publisher restrictions. The published version is available at http://dx.doi.org/10.1139/apnm-2015-0334