Sports nutrition and diabetes
There remains to date little specific research into Type 1 diabetes and sports nutrition, however the principles for non-diabetic athletes remain largely transferable provided the differences in physiology are recognised. Research and practice have pushed established boundaries, where medicine and society regarded Diabetes as a disability. With insulin therapy approaching its centenary there are emerging generations of ever more informed, motivated, physically fit and accomplished athletes with diabetes who now compete at all levels including elite and professional.
A common misconception in practice is the need to eat before exercise; in reality, even the leanest of athletes have fuel reserves in the forms of creatine phosphate, glucose, glycogen and fat; varying intensities and duration of exercise utilise different energy oxidation pathways and require different nutritional approaches.
The advice for the general population and particularly people with Type 2 diabetes, is to exercise regularly, not only to ‘burn’ calories and promote weight loss, but activity at least every other day prolongs expression of GLUT4 insulin-independent receptors, thereby reducing insulin resistance and basal requirements.
This reduction in requirements is also true of Type 1 and has been associated with an extended ‘honeymoon’ period in some active individuals. As insulin is not under homeostatic control in Type 1, athletes must be vigilant as to the effects of increasing training level. More frequent blood glucose monitoring is necessary to monitor the effects of exercise and devise appropriate strategies.
General rules for moderate intensity exercise 45-240minutes :-
|Initial BG level mmol/l||Grams of carbohydrate pre-exercise|
Reducing rapid acting insulin by 30-50% with carbohydrate pre-exercise (within the 4 hour action profile) is recommended if activity is planned. Insulin pumps allow temporary basal rates to be applied instead.
Hypoglycaemia in the day before activity makes further episodes more probable as glycogen stores are depleted, significantly more carbohydrate may be required before and during exercise than if the athlete had been normoglycaemic prior.
Exercising with BG above 13mmol/l, (if not due to a recent meal and insulin dose), is discouraged as it reflects insufficient insulin and a catabolic state. Whilst some may argue thisis advantageous for exercise, if the ability for glucose to enter cells, either via insulin mediated receptors or by GLUT 4 channel is exceeded, poor fuel oxidation by the muscles and early fatigue result.The presence of ketones in Type 1 diabetes illustrates a severe lack of insulin and a rise in metabolic rate during exercise accelerates ketone production, hence advising patients to exercise to reduce blood glucose may prove dangerous and the osmotic effect of high BG and ketones may contribute to dehydration.
High intensity/competitive exercise evokes a counter-regulatory hormone (Adrenaline, glucagon, growth hormone and cortisol)response, resulting in a significant rise in blood glucose level with the potential to cause ketosis in Type 1 athletes It is important for both clinicians and athletes to be aware of this effect and that training and competition produce varying physiological responses.
Not eating immediately prior to an event or giving significantly increased doses of insulin may be necessary. In practice,trebling morning insulin to carbohydrate ratio and injecting 10units of rapid acting analogue insulin with no food immediately prior to a game of football to prevent a counter-regulatory response have been observed, yet reductions in doses are needed for training as the absence of competition and adrenaline pre-disposes the athlete to hypoglycaemia. Athletes may be insulin sensitive with good day to day blood glucose control but may have unexpectedly high HbA1c levels where these are raised during their competition season due to adrenaline effects and are lower in the ‘off season’ .
The intensity and duration of the activity determine which fuel is used when and in what proportion. A 100m sprint only really uses creatine phosphate. A day hiking utilises predominantly fats as the intensity is lower and fatty acid oxidation has time to be activated and be useful. Carbohydrates are the main fuel at 75% VO2max; above this adrenaline protects from hypos, at lower intensities less carbohydrate is used. Human ‘metabolism’ is very adept at ‘learning’ and fat oxidation is activated sooner in trained than in untrained subjects.
Carbohydrate loading prior to an event was popular  but eating consistent amounts daily, appropriate to training demands, has proven equally successful at maximising glycogen storage. A small taper of training and increase in carbohydrate in conjunction with higher insulin dose may be helpful.
|Training hours per day||Grams of carbohydrates per KgBW/day||Grams of protein per KgBW/day||Estimated total energy requirements Kcal/KgBW/day|
|1-3 of combination aerobic/strength e.g. rugby/football||5-7||1.4-2.0||44-50|
|1-3 of endurance exercise e.g. running, cycling||7-10||1.2-1.4||37-41|
|>4-6 of ultra-endurance e.g. Iron man||10-12||1.2-1.4|
Readily digestible, well tolerated foods and suitable timing prevent exercising with a full stomach. Meals should contain complex carbohydrates and protein for satiety and to provide amino acids for repair to active tissues via blood flow-mediated delivery. Options include porridge and sweet potato, basmati rice, seeded bread or pasta with lean protein such as chicken, tuna, eggs or cottage cheese 2-3 hours or more before exercise. As these are low glycaemic index, they continue to be absorbed and affect blood glucose levels at the start of activity. Avoiding insulin spikes associated with high carbohydrate loads shortly before exercise enables more efficient oxidation of nutrients. The decision to inject insulin with food can be made, or not, with diabetes so athletes may be able to ensure adequate blood glucose without the natural anabolic reaction of insulin, though this has not been well researched.
Digestible, high bioavailability protein like whey or soya (e.g. protein ‘shakes’, cottage cheese, milk) 30-40minutes pre-resistance exercise can be more effective than taking this after, as assimilation is then less specific to the muscles requiring repair. Commercial supplements should be used with caution to avoid excessive protein intake and renal load. Taking protein in divided doses during the day improves absorption and assimilation. Many supplements are carbohydrate-protein mixtures (usually in a ratio of 3 or 4:1 respectively) which are excellent for post- exercise recovery but may require insulin.
Taking carbohydrate regularly during exercise balances blood glucose levels, fuel demands, insulin dosing and gastrointestinal comfort. Initial suggestions are 30-60g carbohydrate per hour during moderate –high intensity exercise lasting 1-4hours .
Sweets are low in fat and protein and are absorbed quickly, 20g carbs is equivalent to 4 jelly babies or 10 jelly beans. As sugar starts being absorbed through the oral mucous membranes, sweets are effective hypo treatment agents.
There remains the need for hydration and replacement of electrolytes lost via sweat, hence isotonic drinks (5-8g carbs/100ml) are popular. Hypertonic drinks (higher carbohydrate) reduce gastric emptying speed and are inadvisable as they may induce nausea and produce a sharp rise in BG level. Hypotonic fluids may be useful for rehydration but can be too dilute to protect against hypoglycaemia.
For longer events or those with intervals, such as cycling, athletics or walking, relatively easy to eat carbohydrates such as bananas, trailmix, flapjacks and cereal bars may be taken. Running events usually preclude these foods, with more reliance on sweets or energy gels. Gels should be experimented with in training; they contain 20-30g carbs in a concentrated ‘shot’ and can cause gastrointestinal upset especially if not taken with fluids.
Exercise in extremes of heat, cold and at altitude increase carbohydrate oxidation rate and fluid requirements.
As BG levels may rise during adrenaline stimulating activities only water or small amounts of carbohydrate might be ingested during the event with the majority of the work being from stored fuels. Whilst fats do not need to be replenished, glycogen reserves in the liver (80-100g ) and the muscles (200-400g) do. The liver maintains normoglycaemia in response to glucagon in the fasting state (including overnight),in the ‘fight or flight’ response, in the event of hypoglycaemia and to some degree, during exercise. Liver glycogen is preserved to as great an extent as possible, as are intramuscular reserves however some will invariably be utilised and must be restored; taking insulin to correct a high blood glucose level immediately post-exercise invariably results in hypoglycaemia.
Testing blood glucose levels post exercise without intervention with insulin ascertains the timescale for the glycogenesis related fall in glucose levels. Hypoglycaemia often occurs around 2 hours post exercise. This is particularly true if insulin is given pre-exercise to prevent an excessive adrenaline- related rise in blood glucose level. The 20minutes post exercise is when muscles are at their most receptive to glucose as there is a time-lag when increased uptake continues but the oxidation rate slows after exercise has ended. High glycaemic index carbohydrates such as fruit juices or maltodextrin based carbohydrate and protein supplement drinks/ chocolate milk are ideal. Advice is for 1g carbohydrate per Kg body weight immediately or certainly within 2 hours post-exercise (usually through a mixed carbohydrate-protein meal like pasta and chicken) to optimise stores after prolonged exercise. After this time, glycogen can be supplemented but exercise mediated glucose channels close so more will be insulin dependent.
Hypoglycaemia can occur for up to 48 hours post-exercise, particularly if insufficient carbohydrate is consumed post event. For activities of long duration and over consecutive days, such as skiing or cycling holidays, a cumulative effect occurs. Energy expenditure, increased metabolic rate, enhanced insulin sensitivity and regeneration of glycogen stores reduce the requirement for insulin, with basal requirements often falling to less than 50% of initial doses.
Sport continues to be a circus of body image and extensions of abuse of insulin to manipulate this should be counselled against.
Athletes with diabetes must be aware that the use of insulin is permitted but needs to be disclosed to and recorded by WADA as it is on the controlled substances list.
^ DAFNE Study Group (2002). “Training in flexible, intensive insulin management to enable dietary freedom in people with type 1 diabetes: dose adjustment for normal eating (DAFNE) randomised controlled trial”. BMJ; 325: 746
^ Hornsby Jnr. W.G.(2005 ) “Management of competitive athletes with diabetes”. American Diabetes Association, Diabetes Spectrum.
^ ADA/ACSM (2009) “Nutrition and Athletic Performance”, Medicine & Science in Sports & Exercise, 41 (3) - pp 709-731
^ Burke L, Deakin V. (2006)”Clinical Sports Nutrition”. McGraw-Hill: Australia