Endurance athletes regularly suffer from GI distress during endurance events. (1) This article will discuss the nutritional causes and focus on strategies to help mitigate these.
GI distress is a highly individualised problem that many endurance athletes face when training and racing. (2) To maintain high levels of performance during endurance events, especially those of longer duration and higher intensity there is generally a need for significant volumes of fuel in the form of carbohydrate (CHO) and fluid. (3, 4) These requirements are in many instances the very cause of GI distress. Nutritional factors that promote GI distress usually result from strategies aimed at providing the necessary CHO and fluid to ensure a high level of performance. By implementing these strategies athletes can ingest large, highly concentrated volumes of CHO and inappropriate quantities of fluid which can slow Gastric Emptying (GE) and ultimately lead to GI distress. (5) Symptoms can be alleviated by executing specific individualised nutrition and hydration strategies. (6) As with many other nutritional issues, individual trialling is the best way to identify effective solutions. (7)
GI distress is common in endurance athletes as they strive to maintain energy balance and hydration status to perform at their peak during training and racing. (8) At worst it could mean not completing an event but more often leads to a sub-optimal performance. The causes are multi-factorial and can be broadly summarised as physiological, reduction in the blood flow to the gut; mechanical, movement of internal organs; and nutritional, made up of a number of factors. (2, 7) Symptoms are grouped into upper abdominal; reflux/heartburn, belching, stomach pain/cramps, vomiting, nausea, and lower abdominal; intestinal/lower abdominal cramps, flatulence, urge to defecate, diarrhoea and intestinal bleeding. (2) This article will focus on the nutritional causes of GI distress and strategies that can help reduce the likelihood of suffering.
As with day to day nutrition, one glove does not fit all in endurance sports. Experimenting to find the best nutritional strategy for an individual is key. Trialling different fuels and fluids at event intensity and duration is the best approach to understanding an individual’s most effective nutritional strategy. (7)
Slowed GE has been linked to GI distress. (7) It is one of the major causes and various factors can contribute. Consumption of hypertonic beverages (those with a higher concentration of solutes, e.g. sodium and carbohydrate, than fluids within the body) should be avoided. (9) This is because the higher concentrations of CHO slow GE, lead to residual contents in the gut and cause symptoms of GI distress. (10) Hypertonic beverages can also reduce fluid intake leading to dehydration. (9, 11) Dehydration in excess of ~4% of body weight can lead to slowed GE. (12-14) A solution is the use of isotonic carbohydrate and electrolyte containing beverages which have been shown to support more rapid and prolonged GE. (15) Ensuring adequate hydration early in events is also key in avoiding dehydration and its side effects. (14) Because of the amount of fuel required to maintain peak performance in endurance events it is important to maintain a constant supply of CHO to the working muscles. Due to this many athletes ingest quantities and concentrations above their upper limits of processing ability. Oxidation rates of CHO fuels utilising multiple transporters are greater than those using single transporters so are preferential when building strategies for CHO ingestion. (16, 17) By taking this approach oxidation efficiency and rates of GE are improved reducing the chances of slowed GE and residual CHO in the gut, a key causation of GI distress. (6, 18)
A steady supply of both fluid and CHO are keys to maintaining optimal performance in endurance events. (19) To enable best possible performance athletes should ingest levels relevant to sustained performance for themselves. Absorption is dependent on maximal rates of GE being achieved and maintained. It has been shown that maintaining a proper balance of fluid, sodium and potassium plays a role in optimal GE in an individual. (9, 20, 21) To identify an appropriate hydration strategy and supplementation levels of sodium and potassium individuals should experiment in various conditions to understand fluid and minerals lost through sweat and therefore replacement requirements. By implementing the correct replacement strategy GE will be maintained helping to avoid potential GI distress. (8)
If an athlete could utilise lipids as a fuel source at higher intensities during endurance events, it stands to reason that they would be less reliant on CHO. By building metabolic flexibility, athletes could up-regulate their lipid metabolism during exercise sparing CHO. This would mean less need for large quantities of CHO to maintain performance reducing the chance of slowed GE, residual CHO in the gut and the symptoms of GI distress related to this. (8)
To some extent, the gut is a trainable organ so training it to process higher concentrations and larger volumes of CHO and fluid during endurance events could be beneficial in reducing the instances of GI distress. (22, 23) By experimenting with specific volumes of CHO and fluid required during longer training sessions this adaptation could be achieved and ultimately help mitigate the possibility of GI distress. (23)
Foods high in fat, protein and fibre taken close to an event have all been proven to play a role in the onset of GI distress. (11) Avoiding foods high in dietary fibre, and limiting those higher in fat or protein prior to events should reduce the chances of suffering during-competition GI distress. (9) A good example of a pre-endurance event meal might be a small meal of rolled oats mixed with banana and a boiled egg. This is relatively low in dietary fibre while restricting fat and protein.
GI distress is common in athletes who compete in and train for endurance events. Several of the causal factors can be attributed to nutritional strategies put in place in an effort to supply the required levels of CHO and fluid to maintain peak performance throughout an event. By individual trialling athletes can identify nutritional practices and strategies to help mitigate the possibility of suffering.
Notable non-nutritional causes of GI distress include;
• Events longer in duration and higher in intensity lead to reduced splanchnic blood flow. This means the gut receives reduced amounts of blood which adversely impacts GE and can cause GI distress. (24)
• Training or racing in hotter environments causes blood flow to be directed away from the gut to aid in body temperature regulation. This, in turn, leads to slowed GE which is a key cause of GI distress. (12)
• A history of gastrointestinal issues such as irritable bowel syndrome could indicate an individual's predisposition. (4)
• Nonsteroidal Anti-Inflammatory Drugs (NSAIDS) used by those with a history of GI issues can induce GI distress during endurance events and should be avoided. (7, 9)
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