Dr Catherine Dunnett BSc, PhD, R.Nutr
Most trainers will be familiar with lactic acid, although misconceptions might exist as to whether its production is good, bad or indifferent. Lactic acid is produced in muscle as a consequence of anaerobic energy generation and whilst it can be metabolised or reprocessed within muscle, it can and does accumulate during acute periods of maximal exercise when peak power output and speed is required such as the closing stages of a race, or during the effort of repetitive jumping. Accumulated lactic acid dissociates or separates to form lactate- and a source of hydrogen ions (H+). Accumulation of H+ in muscle during exercise leads to a drop in pH and muscle acidosis, which contributes ultimately to muscle fatigue limiting performance. Lactic acid can also be produced as consequence of fermentation in the digestive tract, particularly the hindgut but this is a very different scenario to that which occurs in muscle. The level of lactic acid produced in the gut, pales into insignificance when contrasted with that generated in the horse’s skeletal muscles during exercise.
Research in horses and humans has shown how dietary intervention can be used to help tackle muscle acidosis, a key element in the process of muscle fatigue.
Lactic acid in muscle is a biochemical necessity for intense exercise
The scope for lactic acid production in horse muscles is enormous with levels in excess of 200mmol/kg (dry muscle) being attainable during maximal high intensity exercise. All horses produce lactic acid, whether they are sprinters, stayers, hurdlers, or chasers, eventers or showjumpers. Even dressage horses can potentially accumulate lactic acid in some muscle groups during execution of advanced movements. Lactic acid is produced as a biochemical consequence of the anaerobic metabolism of glucose and glycogen to produce energy (ATP) needed to fuel muscle contraction and it is both useful and necessary. A low level of lactic acid is produced in muscles continuously at rest and during low intensity exercise, but it is during high intensity exercise where its production increases significantly and it starts to accumulate within muscle, which is reflected in the plasma lactate concentration (table 1). Increased muscle lactic acid production is necessary to deliver energy (ATP) at a faster rate and under conditions where oxygen delivery to the muscles is limited. All of this is essential biochemically to deliver the sustained speed and power required to win races, or execute repetitive jumping at speed. The accumulating lactic acid rapidly separates or disassociates to form an anion lactate- and a cation or proton (H+). It is the accumulation in muscle of acidic H+ species, from lactic acid accumulation and other biochemical reactions during exercise, that if allowed to go unchecked, results in muscle acidosis, characterised by a fall in pH.
Table 1 – Comparison of plasma lactate following treadmill exercise or racing
Plasma Lactate (mMol/L)
Adapted from Harris et al 1991 and Sewell et al 1992
Significant muscle acidosis ultimately contributes to muscle fatigue. As muscle acidity increases (pH declines) it reaches a level where it can interfere with normal muscle contraction and muscle energy generation pathways. Practically speaking, horses will slow down when experiencing muscle fatigue, or where jumping is involved they may make crucial mistakes as they cannot sustain the required muscular effort. We have all felt that ‘burn’ which we associate with muscle fatigue, even if it was only running for the train rather than around the track. Nature is, however, a clever architect and has given horses like humans various mechanisms to counteract the potential negative impact of muscle acidosis on muscle function.
Efficient buffering is crucial to intense exercise
Once a significant amount of H+ has been formed in muscle, a proportion will be pushed out into the blood, where it can be buffered or rendered harmless by the bicarbonate buffering system. Bicarbonate is able to accept or buffer H+, eliminating its acidity. This then allows more H+ to pass from muscle into the blood, helping to control muscle acidosis and control the fall in muscle pH.
‘Milkshaking’, which is illegal under FEI rules and the rules of racing, involves the invasive administration of large quantities of bicarbonate via nasogastric tube on race day giving an acute but transient increase in the buffering capacity of blood. Whilst milkshaking has been shown to be effective biochemically in horses, it is strictly outlawed all over the world. The tell tale increase in the concentration of carbon dioxide in blood provides a highly effective testing procedure for regulators to help eliminate its use. Whilst unlikely in isolation to trigger a post race positive in PCO2, small additions of bicarbonate to the diet on a daily basis will not improve bicarbonate buffering appreciably and larger amounts may result in scouring.
Tackling the problem at the source – Muscle buffering
As well as being buffered in blood, the H+ arising from accumulating lactic acid and other metabolic pathways can be buffered at the source in muscle. There are a number of processes or elements that contribute to reducing the free H+ in muscle including:
- Conversion of phosphocreatine to creatine
- Conversion of ammonia to ammonium
- ATP breakdown
- Presence of intramuscular buffers including bicarbonate, hydrogen phosphate, histidine and carnosine.
One of the most interesting aspects to muscle buffering due to the significant research carried out in horses is the presence of carnosine. Carnosine, which should not be confused with L-carnitine, is an important peptide or ‘small protein’ like compound found in muscle at a high level in horses, Man and other athletic animals. Due to its chemical structure, carnosine is able to accept, or buffer hydrogen ions (H+) to help stabilise muscle pH. It can be thought of as a sort of biological sponge that holds onto the H+ ions allowing muscle to continue to function optimally.
Fig 1 – Increase in H+ buffering with increasing muscle carnosine content.
Variation in muscle carnosine
Carnosine concentration is highest in fast twitch muscle fibres (IIb and IIa) and lowest in slow twitch type 1 fibres. Considerable variation in muscle carnosine concentration exists both between horse breed and between individual horses within breeds, which helps explain some of the variation in innate talent of individuals. Muscle carnosine also increases with age in horses until maturity and then slowly declines. This is interesting given the increased use of older horses in certain equestrian disciplines such as eventing and showjumping. Research in humans indicates that muscle carnosine is also lower in females than males. A small adaptive increase in muscle carnosine content is seen with anaerobic training in horses.
Building blocks for carnosine
Carnosine is made from two key amino acids namely histidine and b-alanine, both of which are found in the horse’s diet naturally. It is the histidine component within carnosine that is able to accept a H+ and so gives carnosine its buffering capacity. The second amino acid, is needed to prevent the incorporation of the histidine molecule into protein, where its buffering ability would be reduced significantly.
Drs Mark Dunnett and Roger Harris uncovered the muscle carnosine story in horses whilst at the world famous Animal Health Trust, in Newmarket and they went on to show how the synthesis of muscle carnosine can be manipulated through the diet. Interestingly, in this area of research, horses led and humans followed. This equine research initiated an explosion of studies in humans exploring the beneficial effect of increasing muscle carnosine in a variety of human sports.
STORM® available now from Mitavite delivers a sustained and bioavailable source of key building blocks for muscle carnosine (ProCarnosine®) as well as important co-factors for carnosine synthesis.
The STORM® formulation and feeding guidance ensures an elevated and steady level of active ingredients within the blood, allowing efficient uptake into muscle for carnosine synthesis. The level of muscle carnosine increases slowly reaching a significant level after 4-6 weeks of supplementation.