Fundamentals of Human Nutrition/Lactic acids

From Wikibooks, open books for an open world
Jump to navigation Jump to search

Here are ten things you should know about lactic acid <What this resource does not make clear is that lactic acid is not produced, lactate is produced.

1. Lactate 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. Lactate formation does not 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. Neither accumulated lactate nor lactic acid is present as a fluid in sore muscle tissue long after anaerobic exercise.

Delayed Onset Muscle Soreness or DOMS, the aching sensation felt in your muscles hours to days after unaccustomed or strenuous exercise, is caused by microtrauma to muscle fiber tissue, not by lactate. 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 relieve muscle soreness and cramping they errantly believe is due to agitation caused by accumulated lactate lingering long after exercise has ceased. While these techniques increase blood supply which accelerates the healing and growth of damaged muscle tissue, they cannot address a nonexistent fluid. Lactate is rapidly processed by the liver into pyruvate and then glycogen for energy during exercise and recovery and does not remain accumulated and trapped as a fluid within the muscles long after exercise has ceased.

3. The body produces lactate whenever it breaks down carbohydrates for energy.

The faster you break down glucose and glycogen the greater the formation of lactate. 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 lactate you produce.

4. Lactate 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 lactate. 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 lactate.

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

Lactate 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 lactate production. Temporarily, lactate 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 lactate in metabolism in his "Lactate Shuttle Theory." This theory describes the central role of lactate in carbohydrate metabolism and it's importance as a fuel for metabolism.

6. The body uses lactate 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 lactate rather than directly from blood glucose.

Scientists call the process of making liver glycogen from lactate 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 lactate levels stabilize even though lactate production increases.

This occurs because your capacity to produce lactate 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 lactate, which also causes an increase in blood lactate/lactic acid - due to threshold effect.

As your body directs blood to your working muscles, you can shuttle the lactate to other tissues and use it as fuel. This reduces lactate levels in your muscles and blood, even though you continue to produce great quantities of lactate. 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. Lactate is a very fast fuel that can be used to athletes' advantage during exercise. The concentration of both glucose and lactate 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 lactate so important in metabolic regulation? The exact answer is unknown, but there do appear to be several physiological reasons. Lactate- 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.