Measuring Lactate Threshold


Lactic Acid: Friend or Foe?

Lactic acid is an important fuel for the body during rest and exercise. It is used to synthesize liver glycogen and is one of our most important energy sources. Lactate is the preferred fuel source in highly oxidative tissues, such as heart muscle and slow-twitch skeletal muscle fibers. It is used rapidly by the body and is a valuable component in athletic fluid replacement beverages.

Lactic acid also is a powerful organic acid, and its accumulation can cause distress and fatigue during exercise. Athletes need both high intensity and over-distance training to improve the capacity to use lactic acid as a fuel during exercise and recovery. High intensity training develops cardiovascular capacity that reduces lactic acid transport to tissues that can use it as fuels. Over distance training causes tissue enzymes adaptations that increase use of fatty acids for energy. This helps slow lactic acid production from carbohydrates and to enhance tissues ability to use lactic acid as fuel.


Lactic acid has a bad reputation.

Many people blame it for fatigue, sore muscles, and cramps. They think of it as a waste product that should be avoided at all cost. Guess what? Scientists have discovered that lactic acid plays a critical role in generating energy during exercise. Far from being the bad boy of metabolism, lactic acid provides fuels for many tissues, helps use dietary carbohydrates, and serves as fuel for liver production of glucose and glycogen. In fact, lactic acid is nature’s way of helping you survive stressful situations.

Lactic acid has a dark side. When your body makes lactic acid, it splits into lactate ion (lactate) and hydrogen ion. Hydrogen ion is the acid in lactic acid. It interferes with electrical signals in your muscles and nerves, slows energy reactions, and impairs muscle contractions. The burn you feel in intense exercise is caused by hydrogen ion buildup. So, when you fatigue, don’t blame it on lactic acid. Rather, place the blame where it belongs-on hydrogen ion.

Lactate has been made guilty by association. Far from being a metabolic pariah, the body loves lactate. It is an extremely fast fuel that’s preferred by the heart and muscles during exercise. Lactate is vital for ensuring that your body gets a steady supply of carbohydrates, even during exercise that lasts for many hours. 

Lactate is so valuable, that taking it as part of a fluid replacement drink before, during, or after exercise improves performance and speeds recovery.

Lactate is a friend to triathletes, distance runners, swimmers, and cyclists. When you learn the facts about lactic acid, you will think of it in a whole new light. Harness the power of lactic acid and you will increase your energy level and stave off fatigue.


Ten things you should know about lactic acid:

1.  Lactic acid is formed from the breakdown of glucose: 

During this process the cells make ATP (adenosine triphosphate), which provides energy for most of the chemical reactions in the body. Lactic acid formation doesn’t use oxygen, so the process is often called anaerobic metabolism. Lactate-related ATP production is small but very fast. This makes it ideal for satisfying energy needs anytime exercise intensity exceeds 50% of maximum capacity.

2.  Lactic acid doesn’t cause muscle soreness and cramps:

Delayed onset muscle soreness, the achy sensation in your muscles the day after a tough workout, is caused by muscle damage and post-exercise tissue inflammation. Most muscle cramps are caused by muscle nervous receptors that become over excitable with muscle fatigue. 

Many athletes use massage, hot baths, and relaxation techniques to help them rid their muscles of lactic acid and thus relieve muscle soreness and cramping. While these techniques probably have other benefits, getting rid of lactic acid isn’t one of them. Lactate is used rapidly for fuel during exercise and recovery and doesn’t remain in the muscles like motor oil.

3.  The body produces lactic acid whenever it breaks down carbohydrates for energy:

The faster you break down glucose and glycogen the greater the formation of lactic acid. At rest and submaximal exercise, the body relies mainly on fats for fuel. However, when you reach 50% of maximum capacity, the threshold intensity for most recreational exercise programs, the body “crosses over” and used increasingly more carbohydrates to fuel exercise. The more you use carbohydrates as fuel, the more lactic acid you produce.

4.  Lactic acid can be formed in muscles that are receiving enough oxygen:

As you increase the intensity of exercise, you rely more and more on fast-twitch muscle fibers. These fibers use mainly carbohydrates to fuel their contractions. As discussed, whenever you break down carbohydrates for energy, your muscles produce lactic acid. The faster you go, the more fast-twitch muscles you use. Consequently, you use more carbohydrates as fuel and produce more lactic acid. Increased blood lactic acid means only that the rate of entry of lactic acid into the blood exceeds the removal rate. Oxygen has little to do with it.

5.  Many tissues, particularly skeletal muscles, continuously produce and use lactic acid:

Blood levels of lactic acid reflect the balance between lactic acid production and use. An increase in lactic acid concentration does not necessarily mean that the lactic acid production rate was increased. Lactic acid may increase because of a decreased rate of removal from blood or tissues.

Lactic acid production is proportional to the amount of carbohydrates broken down for energy in the tissues. Whenever you use carbohydrates, a significant portion is converted to lactate. This lactate is then used in the same tissues as fuel, or it is transported to other tissues via the blood stream and used for energy. 

Rapid use of carbohydrate for fuel, such as during intense exercise, accelerates lactic acid production. Temporarily, lactic acid builds up in your muscles and blood because it can’t be used as fuel fast enough. However, if you slow down the pace of exercise or stop exercising, the rate of lactate used for energy soon catches up with the rate of lactate production.

Dr. George Brooks, a Professor from the Department of Integrative Biology at University of California at Berkeley, described the dynamic production and use of lactic acid in metabolism in his “Lactate Shuttle Theory.” This theory describes the central role of lactic acid in carbohydrate metabolism and it’s importance as a fuel for metabolism.

6.  The body uses lactic acid as a biochemical “middleman” for metabolizing carbohydrates:

Carbohydrates in the diet are digested and enter the circulation form the intestines to the liver mainly in the form of glucose (blood sugar). However, instead of entering the liver as glucose and being converted directly to glycogen, most glucose from dietary carbohydrate bypasses the liver, enters the general circulation and reaches your muscles and converts into lactic acid. Lactic acid then goes back into the blood and travels back to the liver where it is used as building blocks for making liver glycogen. Your body produces much of its liver glycogen indirectly from lactic acid rather than directly from blood glucose.

Scientists call the process of making liver glycogen from lactic acid the “Glucose Paradox”. The theory was formulated by famous biochemist Dr. J.D. McGarry and his associates. It shows the importance of lactic acid in carbohydrate metabolism.

7.  During endurance races, such as marathons and triathlons, blood lactic acid levels stabilize even though lactic acid production increases:

This occurs because your capacity to produce lactic acid is matched by your ability to use it as fuel. Early during a race, there is a tremendous increase in the rates that muscle uptake and use glucose and breakdown glycogen. The increased rate of carbohydrate metabolism steps up production of muscle lactic acid, which also causes an increase in blood lactic acid.

As your body directs blood to your working muscles, you can shuttle the lactate to other tissues and use it as fuel. This reduces lactic acid levels in your muscles and blood, even though you continue to produce great quantities of lactic acid. However, you often feel better during the race or training. This relief is sometimes called “second wind”. 

Scientists use radioactive tracers to follow the use pattern of fuels in your blood and muscles. Their studies show that during exercise, lactic acid production and removal continue at 300-500 percent of resting rates, even though oxygen consumption has stabilized at submaximal levels.

8.  The heart, slow-twitch muscle fibers, and breathing muscles prefer lactate as a fuel during exercise:

In the heart, for example, the uptake of lactate increases many fold as the intensity of exercise increases while uptake of glucose remains unchanged. These tissues suck up lactate at a fast rate to satisfy their energy needs.

9.  Lactic acid is a very fast fuel that can be used to athletes’ advantage during exercise:

The concentration of both glucose and lactic acid rise in the blood after a carbohydrate-rich meal, but the blood lactic acid concentration does not rise much because it is removed so rapidly. The body converts glucose, a substance removed from the blood only sluggishly, to lactate, a substance removed and used rapidly. Using lactic acid as a carbohydrate “middleman” helps you get rid of carbohydrates from your diet, without increasing insulin or stimulating fat synthesis. During exercise, you won’t want an increase in insulin because it decreases the availability of carbohydrates that are vital to high performance metabolism.

Why is lactic acid so important in metabolic regulation? The exact answer is unknown, but there do appear to be several physiological reasons. Lactic acid– in contrast to glucose and other fuels– is smaller and better exchanged between tissues. It moves across cell membranes by a rapid process called facilitated transport. Other fuels need slower carrier systems such as insulin. Also, lactate is made rapidly in large quantities in muscle and released into general circulation. Muscle cells with large glycogen reserves cannot release significant amounts of this potential energy source as glucose because muscle lacks a key enzyme required to produce free glucose that can be released to the blood.

Including lactate as part of a fluid replacement beverage provides a rapid fuel that can help provide energy during intense exercise. The rationale for including lactate in athletic drinks is simple- since the body breaks down so much of dietary carbohydrates to lactate anyway, why not start with lactate in the first place? Lactate in the drink can be used rapidly by most tissues in the body and serves as readily available building blocks for restoring liver glycogen during recovery.

10. Proper training programs can speed lactic acid removal from your muscles:

This can be achieved by combining high intensity, interval, and over-distance training. Athletes and coaches must learn to deal effectively with lactic acid. Fortunately, most training programs incorporate elements necessary to speed lactate removal. Training programs should build your capacity to remove lactic acid during competition.

Lactic acid formation and removal rates increase as you run, bike or swim faster. To improve your capacity to use lactate as a fuel during exercise, you must increase the lactic acid load very high during training. Training with a lot of lactic acid in your system stimulates your body to produce enzymes that speed the use of lactic acid as a fuel.

High intensity interval training will cause cardiovascular adaptations that increase oxygen delivery to your muscles and tissues. Consequently, you have less need to breakdown carbohydrate to lactic acid. Also, better circulation helps speed the transport of lactic acid to tissues that can remove it from the blood.

Over distance training causes muscular adaptations that speed the rate of lactate removal. Over distance training in running, swimming, or cycling increases muscle blood supply and the mitochondrial capacity. Mitochondria are structures within the cells that process fuels, consume oxygen, and produce large amounts of ATP. A larger muscle mitochondrial capacity increases the use of fatty acids as fuel, which decreases lactate formation and speeds its removal.

Nutrition is also important, strenuous training depletes glycogen reserves in the muscle and liver. A diet high in carbohydrates is essential for all endurance athletes. Carbohydrates supply an immediate source of glucose so the athlete has a feeling of well-being and a source of quick energy. Further, glucose is used to restore muscle glycogen from exercise. When the blood glucose and muscle glycogen reserves are renewed, glucose provides a source of lactate that helps replenish liver glycogen.


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Credits / References:
T. Fahey, Ed.D., Professor of Exercise Physiology, California State University at Chico
B. Wiesner. 10-things you should know about lactic acid. Delano academic/science department
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