Scientists have studied the advantages of lactate burning during exercise, but there has been limited research on the role of lactate during rest or after eating. Exercise physiologists conducted a study on fasting individuals who were given a carbohydrate meal, and then observed the levels of lactate in their blood as well as their fat and carbohydrate metabolism. They discovered that lactate helps to regulate glucose levels after a meal, and is used for energy in a similar way to glucose.
Lactate acid was once thought to be the cause of muscle fatigue and soreness in athletes during intense training. However, a kinesiology student at Fresno State was given an out-of-print textbook that debunked this myth, explaining that lactate is actually a normal byproduct of metabolic activity required to fuel muscles during sustained exercise. This student is now a graduate student in the laboratory of the scientist who wrote that textbook at the University of California, Berkeley.In a recent study, researchers have gained a better understanding of the role of lactate in the body, challenging the belief that it is a sign of oxygen deprivation in the muscles. The findings, published in the journal Nature Metabolism, demonstrate that lactate is produced naturally in the body after consuming carbohydrates. It enters the bloodstream quickly, even before glucose. This suggests that lactate is not a harmful byproduct to be removed during intense exercise, but rather is produced alongside glucose as a key component.Carbon-containing energy carriers are present in the body. The research findings indicate that the swift transformation of glucose into lactate, which begins in the intestines, is a method used by the body to handle a sudden intake of carbohydrates. Lactate, in conjunction with insulin, helps to regulate the appearance of dietary glucose in the bloodstream. According to Brooks, a professor of integrative biology at UC Berkeley, “Instead of experiencing a sharp increase in glucose levels, we see a surge in lactate and glucose after eating. The more glucose that is converted into lactate, the better it is for managing glucose levels. Lactate serves as a buffer for carbohydrates.” Brooks and his team had previously demonstrated this concept.
The recent study has confirmed that lactate previously thought to only be produced during intense exercise actually plays a similar role during normal non-exercise activity and during resting.
“This is evidence that shows lactate is not just associated with anaerobic metabolism, which is oxygen-limited metabolism. It is a normal response to consuming carbohydrates or to exercising,” Leija explained. “During exercise, lactate is used as the main fuel source. This is why blood lactate levels increase as you exercise more intensely. It is not just a waste product, but rather it is released into the blood because tissues in need of it require it.”
The study was conducted on 15 healthy, physically active young adults — eight women and seven men — as part of a larger NIH-funded study to determine how well people switch from fat to carbohydrate metabolism as they age. The volunteers were asked to fast overnight (12 hours) to deplete their carbohydrate and glycogen stores so that they were getting energy primarily by breaking down fats into fatty acids and using them to power basic bodily functions. They then drank 75 grams of glucose, a rapidly absorbed sugar, to stimulate a switchover for continuing their physiological performance.
Glucose tolerance
The study by Brooks focused on the transition from fatty acid to carbohydrate metabolism, which is comparable to the glucose tolerance test used for diagnosing diabetes. This test is commonly administered to pregnant women to detect gestational diabetes. Unlike previous studies, Brooks and his colleagues closely monitored the volunteers’ blood lactate levels for two hours after they ingested glucose. They also periodically measured the ratio of oxygen to carbon dioxide in their breath, which indicates the proportion of fatty acids versus carbohydrates being burned. This allowed them to calculate the amount of lactate that entered the blood.Compared to using glucose alone, the researchers infused both lactate and glucose tracers labeled with stable isotopes for 90 minutes. This allowed them to bring the levels of labeled lactate and glucose in the blood to between 1% and 2%. By diluting the labeled lactate and glucose with unlabeled dietary glucose, they were able to study the appearance, disappearance, and clearance of blood lactate and glucose. This is a departure from most experiments that only measure static venous blood concentration, which does not provide much information about the kinetics of glucose and lactate.
According to Leija, the study’s success hinged on sampling arterialized blood to observe the effects in the gut. Unlike the usual method of sampling blood from a forearm vein 30 minutes after a glucose challenge, this approach provided clearer results.
The researchers discovered that the volunteers started converting dietary glucose into lactate before it even exited the intestines. Levels of lactate began to increase in arterial blood just five minutes after the meal, while glucose, typically considered the body’s main energy source, only appeared in the bloodstream 15 to 30 minutes after glucose.
Brooks has performed research on both humans and animals. He discovered that after consuming glucose, the first carbohydrate to enter the bloodstream is lactate. This is because intestinal cells convert most of the glucose into lactate before it is released into the blood for the muscles. The liver captures the majority of the glucose before it reaches the muscles. This process results in glucose being converted to lactate. Brooks observed lactate clearance and oxidation, and found that carbon-13 from the lactate tracer appeared in blood glucose. This indicates that lactate plays a major role in distributing carbohydrate energy.
The lactate shuttle
Brooks’ research has focused on the concept of the lactate shuttle in the body, which serves as a major energy highway for distributing carbohydrate carbon energy flux.
For over 50 years, research has been conducted to explore the role of lactate in the body. Each study has contributed more evidence to the understanding that lactate is not a harmful byproduct of oxygen-deprived anaerobic metabolism, which doesn’t occur in the human body. Despite this, the assumption has influenced how athletes and doctors have viewed lactate. Many doctors still see high levels of lactate, often mistakenly referred to as lactic acid, in the blood as a sign of illness that requires treatment with extra oxygen or medication.
“Measuring lactate is a significant aspect of the work done by sports medicine professionals. And now we comprehend what’s happening.”
“Athletes consistently produce lactate and are consistently able to clear it. However, when they reach a point where they are unable to clear it through oxidation and converting it into glucose, we know that their endurance will be limited,” Brooks explained.
He expressed his belief that these findings are groundbreaking, even though they may seem confusing to some. He also noted that previous understanding of this process may need to be reevaluated. Brooks referenced his own textbook, “Exercise Physiology: Human Bioenergetics and Its Applications,” which was first published in 1984. The book is currently in its 5th edition, with plans for a 6th edition in the works.
“When I readIn Dr. Brooks’ 1984 book, Leija expressed that he was truly amazed and had a change in perspective. He had always associated lactic acid with extreme exercise and the feeling of running out of breath, but he didn’t understand the physiological aspect until he read the book. After reading it, everything started to make more sense.
Brooks introduced the term “lactate shuttle” in his book to explain the body’s metabolic process, in which lactate plays a key role in sustaining most, if not all, tissues and organs.
He demonstrated that in many tissues, lactate is actually the preferred fuel over glucose. During intense physical activity, the muscle mitochondria preferentially burn lactate.The author of the article discusses the findings of a study that focused on the use of lactate as a fuel source for the human body. The study showed that human skeletal muscle, heart muscle, and the brain prefer lactate to glucose as fuel and operate more effectively on lactate. Additionally, lactate signals fat tissue to stop breaking down fat for fuel. The study aimed to fill the gap in understanding what happens during normal non-exercise activity and resting, and it supports the idea that high levels of lactate in the blood signal a disruption in the lactate shuttle cycle, rather than indicating that lactate itself is harmful to the body. The findings of the study provide valuable information for medical research.Lactate is a significant piece of the puzzle when it comes to performance and metabolic conditions,” Brooks said. “The recent study shows that it’s not just about muscle, but also about dietary carbohydrate. This was something we were missing before.”
The recent study is part of Leija’s Ph.D. thesis, and he plans to continue researching the metabolic role of lactate.
“Back in college, I used to read physiology books to improve my training, but I would often ignore the science terms because all I wanted to know was how to run faster and longer,” Leija said. “Now I realize how important those terms are and how they indirectly helped me.On this day, there is still a lot to be discovered about it.”
Additional contributors to the research include graduate students Casey Curl, Jose Arevalo, Adam Osmond, and Justin Duong, as well as Melvin Huie, MD, a UC Berkeley graduate associated with Brooks’ Exercise Physiology Laboratory, and Umesh Masharani, MD, an endocrinologist at UC San Francisco’s Diabetes Center.
