Journal Title (Medline/Pubmed accepted abbreviation): Appl Physiol Nutr Metab
Page numbers: 541-547
doi (if applicable):
Summary of Background and Research Design
Hypothesis: A short-term, low-CHO diet, consumed after glycogen-depleting exercise, would increase sympathetic nervous system (SNS) activity, which could be indentified by increments in heart rate variability (HRV) indexes, such low frequency (LF) and LF/HF (HF = high frequency) ratio and plasma catecholamine levels.
Subjects: Twelve nonsmoking, physically active, healthy males (mena age 28.5 y, mean body mass = 73.8 kg, mean percentage body fat = 12.9%, mean VO2 max = 47.1 mg/kg/min)
Experimental design: Randomized crossover with 1 week washout between treatments
Treatments and protocol: Subjects first underwent a preliminary screening visit to the lab to determine body composition, VO2 max, and first and second lactate thresholds (LT1 and LT2, respectively). The second visit to the lab was for a control (no intervention) measurement of plasma catecholamines, heart rate and breathing frequency. The third visit consisted of the subjects performing glycogen depleting exercise (cycling for 90 min at 50% of the difference between LT1 and LT2, followed by six 1-min sprints) and then being assigned to either a high or low carbohydrate diet for the 48-h postexercise period. During the fourth visit, repeat measures of heart rate, catecholamines and breathing frequency. The fifth and sixth visits were the same as visits three and four, just with the subject consuming the other diet. Diet records indicated the the nutrient composition of the three diets was as follows: control (2252 kcal, 53.4% carbohydrate, 30.5% fat, 16.0% protein); low carbohydrate (1703 kcal, 28.3% carbohydrate, 39.7% fat, 29.5% protein); high carbohydrate (1791 kcal, 72.7% carbohydrate, 16.7% fat, 10.6% protein). Heart rate was recorded using a portable heart rate monitor and the data were transferred to a computer for further analysis.
Summary of research findings:
The different diets had no effects on heart rate, breathing frequency, R-R intervals, or plasma catecholamines. However, the low carbohydrate diet significantly increased the response in the LF domain and decreased the response in the HF domain relative to control. This also led to an increased LF/HF ratio for the low carbohydrate diet compared with control. The high carbohydrate diet had no effect on any of these parameters versus the control diet.
Interpretation of findings/Key practice applications:
The changes observed in LF and HF response as the result of the low carbohydrate diet were interpreted by the authors as suggestive of an increase in sympathetic HRV indices and a reciprocal decrease in vagal activity indices. This, in turn, could produce greater cardiac stress during exercise, potentially increasing risk of arrhythmias and impaired performance. The authors did not propose a mechanism for how the low carbohydrate diet could have affected the sympathetic response. They also noted that, despite these significant results for LF and HF, there were no changes in catecholamines or other variables such as heart rate and R-R interval. An additional point of note is that the low carbohydrate diet, by its nature, was also higher in fat and protein versus either the control or high carbohydrate diet. Certain types of fatty acids (e.g., omega-3 fats) may have effects on heart rate that would be independent of potential effects related to carbohydrate. The authors did not report the fat sources present in each diet.