Why and when we need carbohydrates in Sport

Why are carbohydrate’s important for athletes?

Endurance athletes rely upon both carbohydrates and fat as fuel sources during exercise. Carbohydrate becomes the predominant source over fat, the longer the duration and the higher the intensity. Carbohydrate is stored in both muscle (~350-700g) and liver (~100g) as glycogen (1).

A Carbohydrate rich diet has been proven to increase the body’s glycogen levels as it increases the glucose in circulation, provides fuel for muscles, stimulates insulin production from the pancreas and improves the conditions upon which muscle glycogen production can take place (1).

An athlete’s glycogen stores are depleted during exercise, firstly from muscle glycogen, then liver glycogen, before blood glucose drops and the risk of hypoglycemia develops without additional carbohydrate feeding. If an athlete has sufficiently maximized their glycogen stores pre exercise, they will deplete most of their muscle glycogen stores after 2hours of moderate to intense exercise. This can be extended if an athlete has undertaken a period of fat adaption. There is a strong correlation between the amount of muscle glycogen content at the commencement of exercise and the extent of optimal endurance performance that can be undertaken (1).

Fatigue and Carbohydrates

Additional carbohydrate is required from the onset of endurance exercise to fuel exercise over 2hours, and prevent fatigue. Fatigue is defined as the inability to maintain the required or expected level of power output and is common in endurance events of significant intensity over 2hours. We now know that this is often the result of depleted glycogen stores and reduced concentrations of blood glucose, along with inadequate carbohydrate intake (1).

Carbs for the Immune System

The impact of inadequate carbohydrate intake on the immune system is well researched and a high incidence of upper respiratory tract infections is common amongst endurance athletes (2). Athletes exercising between 3-72 hours experience temporary immune depression and a higher risk of infection as a result of extreme physiological stress. Acute intense exercise induces various responses in the immune system such as increased cortisone hormones, increased neutrophils and monocytes, lymphocytosis and increases in anti-inflammatory and pro-inflammatory cytokines and proteins. Sufficient carbohydrate intake during intense and prolonged training sessions lessens this impact.

Additional side effects of inadequate carbohydrate intake include reduced mood and motivation, early onset of fatigue and reduced performance.  This is because endurance athletes who consume inadequate carbohydrates can also struggle to maintain a positive and motivated mood state due insufficient fuel for the central nervous system, which requires carbohydrates, during exercise (1).

Carbohydrate Requirements 24 hours before Prolonged Exercise

Multiple studies have shown that carbohydrate loading can now be achieved over a 24 hour period with sufficient rest, compared to the traditional 3 day deplete and replenish approaches (1). This approach is recommended for intense training or competition with exercise over 2.5 hours duration. To maximize glycogen stores, moderate carbohydrate intake of 8-10g/kg- weight/day is recommended, if coupled with adequate exercise rest. Higher carbohydrate intake of 10-12g/kg-weight/day may be required if rest is not achievable (1).

Carbohydrate Requirements 1-4 Hours Pre Exercise

The pre-race meal, 1-4hours before exercise is imperative to top up glycogen stores. Recommend intake of 1-4g/kg-weight of carbohydrate (1). Choosing carbohydrate foods low in fiber, fat or pure fructose can assist in avoiding gastrointestinal (GI) discomfort during exercise (1).

Carbohydrate Requirements During Exercise

For exercise over 2.5 hours, Consume 60g/hour of carbohydrate, from the commencement of exercise, to avoid fatigue (1). Choose High-GI, Low-Fibre and preferably a glucose+Fructose or Glucose+maltodextrose combination for better absorption and lower GI issues. Add protein if 60g/hour is not achievable. Ideally, consume 1x600ml sports drink per hour (44g carbs) along with an additional energy bar or banana.

If an athlete is fat adapted they may be able to consume in the vicinity of 30-60grams per hour of carbohydrate as their fat stores will be a source of energy for longer compared to non-fat adapted athletes. If an athlete is training for less than 2.5 hours they should aim for 30g per hour of carbohydrates, prefereably in the form of an energy drink meeting hydration and electrolyte requirements.

Avoid flavor fatigue by mixing up carbohydrate sources. Sports drinks should contain 6-8% carbohydrate, supplied at room temperature to avoid GI issues (1). Mixed carbohydrates such as fructose+glucose versus glucose alone are more effective during exercise for maximum carbohydrate absorption and reduced gastrointestinal issues.

Post exercise Carbohydrate Requirements

After exercise athletes need to restore glycogen levels to prepare for subsequent exercise (1). The most rapid rates of glycogen re-synthesis occur in the first hour after exercise, due to the increased permeability of muscle cell membranes. During this time carbohydrate feeding, preferably high-GI for quicker absorption, is recommended. This should be consumed along with 21g high quality protein to aid muscle recovery. A large chocolate milk is an easy option.

• If no further Exercise within 24hours: 1.0-1.2g of carbohydrate per kg-weight per hour is recommended within the first hour followed by daily carbohydrate requirements of 6-10g/kg-weight/day
• If exercise within 8hours: 1.0-1.2g of carbohydrate per kg-weight per hour is recommended for first 4hrs (55-66g for a 55kg athlete), the first consumption being within the first hour, followed by daily carbohydrate requirements of 6-10g/kg- weight/day

References

1. Burke, Louise M., et al. “Carbohydrates for training and competition.” Journal of sports sciences 29.sup1 (2011): S17-S27.
2. Nieman, David C., and Bente K. Pedersen. “Exercise and immune function.” Sports Medicine2 (1999): 73-80.