TITLE: ENDURANCE TRAINING INCREASES GLUCONEOGENSIS DURING REST AND EXERCISE IN MEN
FIVE KEY IDEAS
Endurance training increases gluconeogenesis (GNG) at rest and during exercise in men, even when lactate levels in the arteries remain unchanged.
Participants in the training program experienced significant improvements in V˙o2 peak, decreased respiratory exchange ratio (RER), increased power output needed to reach the lactate threshold, decreased concentration of lactate in arteries during exercise, and increased resting muscle glycogen concentration.
GNG rates vary depending on fasting duration and pre-exercise meals, with longer fasting periods generally increasing GNG rates.
After endurance training, there is an increase in the percentage and rate of glucose derived from GNG during rest, although GNG accounts for less than 10% of overall glucose production.
Enhanced lactate metabolic clearance rate and muscle lactate uptake contribute to lower lactate concentrations after training.
TERMINOLOGY
Acetate: A two-carbon compound that can be metabolized in the liver and contribute to gluconeogenesis.
Glucagon: A hormone that increases blood glucose levels by stimulating glycogen breakdown and gluconeogenesis.
Glucose turnover: The rate at which glucose is produced, utilized, and cleared from the bloodstream.
Glycemic index: A measure of how quickly a carbohydrate-containing food raises blood glucose levels.
Glycogen: The storage form of glucose in the body, primarily found in the liver and muscles.
Glycerol: A component of triglycerides that can be released during lipolysis and used as a substrate for gluconeogenesis.
Gluconeogenesis (GNG): The process by which glucose is synthesized from non-carbohydrate sources, such as lactate, amino acids, or glycerol.
Insulin: A hormone that lowers blood glucose levels by promoting glucose uptake and storage.
Lactate: A byproduct of anaerobic metabolism that can be converted back into glucose through gluconeogenesis.
Lactate threshold: The exercise intensity at which lactate production exceeds lactate clearance, indicating a shift toward anaerobic metabolism.
Oxaloacetate (OAA): An intermediate compound involved in the tricarboxylic acid (TCA) cycle and gluconeogenesis.
Respiratory exchange ratio (RER): The ratio of carbon dioxide produced to oxygen consumed during metabolism, used to assess the relative contribution of different energy substrates.
Tricarboxylic acid (TCA) cycle: Also known as the citric acid cycle or Krebs cycle, a series of chemical reactions that generates energy through the oxidation of acetyl-CoA.
VO2 peak: Maximum oxygen consumption, a measure of aerobic fitness and endurance capacity.
ABSTRACT
We wanted to test the idea that endurance training can increase the process of gluconeogenesis (GNG) in both rest and exercise. To do this, we measured how glucose is produced and used in the body using a special form of glucose called [6,6-(2)H]glucose, and how lactate, a byproduct of exercise, is converted into glucose using [3-(13)C]lactate. We conducted the study during one hour of cycling exercise at two different intensities (45% and 65% of maximum oxygen consumption) before and after the training period. The workload was adjusted to be the same before and after training based on each participant's individual fitness level.
Nine male participants, who were around 27 years old and had an average height of 178.1 cm and weight of 81.8 kg, underwent 9 weeks of training on a cycle ergometer. They trained for 1 hour, 5 times per week, at 75% of their maximum oxygen consumption. The exercise intensity that previously corresponded to 66.0% of their maximum oxygen consumption was reduced to 54.0% after the training period.
We measured the concentration of glucose in the blood at rest and during exercise before and after training. The concentration of lactate in the blood during exercise was higher than at rest, both before and after training. After training, the concentration of lactate decreased during exercise at the same absolute workload and relative intensity compared to before training.
At rest after training, we observed that the percentage of glucose produced through gluconeogenesis more than doubled compared to before training, and the rate of gluconeogenesis also increased significantly. During exercise after training, the percentage of glucose produced through gluconeogenesis increased significantly at the same absolute workload and relative intensity. Similarly, the rate of gluconeogenesis nearly tripled at the same exercise intensities after training.
Based on these findings, we can conclude that endurance training increases gluconeogenesis by two times at rest and three times during exercise at specific absolute and relative exercise intensities.