Food a fact of life eseminar: ENERGY REQUIREMENTS FOR SPORT. Dr Sarah Schenker British Nutrition Foundation

Transcription

1 Food a fact of life eseminar: ENERGY REQUIREMENTS FOR SPORT Dr Sarah Schenker British Nutrition Foundation

2 Energy systems in the body Skeletal muscle is powered by ATP (adenosine triphosphate) Stores in cells are very small so ATP must be resynthesized on a constant basis

3 Energy systems in the body Several energy substrates are used for the production of ATP Creatine phosphate - fastest Carbohydrate Fat - slowest

4 Contribution of energy systems

5 In endurance events the aerobic metabolism of glucose plus a contribution from the metabolism of fat are the major sources of energy The depletion of muscle glycogen is a factor that can limit sustained high performance Glycogen stores in liver and muscle can generally be depleted after 2 hours of vigorous exercise Depletion leads to fatigue often termed hitting the wall Training increases muscle glycogen stores and increases the aerobic capacity of muscles enabling them to use fatty acids more effectively (sparing glycogen)

6 As the duration of exercise increases so the contribution made by fatty acids to the energy supply of the muscles increases Once glycogen stores are used up running speed is limited by the speed at which fatty acids can be metabolised aerobically Maximizing the glycogen stores in muscle and liver are key to preparation The maximum duration that heavy exercise (75%VO2max) can be sustained is directly proportional to glycogen content of the muscle at the start

7 High carb diets (70%en) during training increases muscle glycogen content so increases duration Recommended that the diet of athletes should be 20-25%en fat and 75-80% from carb and protein (15%en protein) No benefits from lowering fat to less than 20%en Popular carb loading technique is one week prior to event is normal high carb diet during training, in week of event 3 days 50%en carb diet then 75%en carb diet as training tapers down to complete rest just before

8 Typical Training Day 9.30/10am 11.30am 1pm 3pm 5pm 7pm pm 10.30/11pm Training Bowl of breakfast cereal Mandarin oranges Glass of fresh orange juice 1 slice of toast Banana or toasted muffin with jam Pint of diluted fruit juice or fruit squash Jacket potato with prawns and cheese (e.g. cottage cheese) Pint of diluted fruit juice or fruit squash 2 bananas Handful of grapes Pint of diluted fruit juice or fruit squash Pasta with a chicken, broccoli and tomato sauce Low-fat yoghurt Pint of diluted fruit juice or fruit squash Bowl of breakfast cereal or banana and a packet of raisins Glass of water 2 slices of toast and jam Cereal and fruit Pint of diluted fruit juice or fruit squash

9 Typical match Day (2pm kick-off) 9.30/10am 11am 1pm Up to match Half-time After match 5/6pm Pint of water Cereal and fruit 2 slices of toast and scrambled eggs, tomatoes Fresh orange juice Sports drink Banana Sports drink Sports drink Sports drink Banana or cereal bar Soup and bread Chicken, pasta, vegetables in BBQ sauce Bananas and custard Pint of diluted fruit juice or fruit squash 8pm 2 slices of toast and jam Pint of diluted fruit juice or fruit squash

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13 Figure 1 Proposed phases of the nutrition cycle of athletes as identified for description of glycemic index effects in sport nutrition.

14 Figure 2 Comparison of typical blood glucose variations after high GI or low GI CHO ingestion. After high GI CHO ingestion, blood glucose concentration shows a steeper increase and reaches higher values than after low GI CHO ingestion.

15 Which foods have a low GI? Low GI foods can be hard to identify as most are unlabelled and published values limited. The GI is affected by food processing, preparation, ripeness, other foods eaten with meal / in preceding meal, fat content etc. High GI Low GI

16 First phase recovery The activation of glucose uptake and glycogen synthase by muscle contractions is very short lived Studies have shown that there is no difference in muscle glycogen synthesis following high or low GI post exercise meals or with delayed feeding providing recovery time is 24 hours and sufficient CHO is taken during this time If recovery time is short (less than 8 hours) the choice of high GI is warranted

17 Second phase recovery Higher insulin secretion could be advantageous so high GI may be preferable Studies have shown substantial differences between high and low GI feeding on the effect of second phase recovery High GI increased muscle glycogen concentration by 50% compared with low GI (Wee at al J Appl Physiol 99, )

18 Figure 3 Low GI meal(s) before exercise: effects of metabolism and exercise performance. Proposed rationale for choosing low GI meal(s) before exercise: biochemical and functional levels of evidence.

19 Last meal before exercise Low GI for the last meal may increase contribution of lipid oxidation to energy production and extend availability of glucose In the first few minutes of exercise blood glucose levels can drop dramatically rebound hypoglycaemia Therefore insulin levels before exercise are important and low GI is a better choice (Wu and Williams 2006 Int J Sport Nutr Exerc Metab 16, )

20 Figure 4 Typical behavior of blood glucose levels in reactive hypoglycemia and their correlations with symptoms in health and disease.

21 Insulin and free fatty acids Once exercise starts insulin levels fall independently of the pre-exercise meal Plasma FFA gradually increase most evidence shows a greater increase after a low GI meal Plasma FFA can be used as energy substrates in direct proportion to their concentration The greater availability of FFA following a low GI meal therefore supports energy metabolism

22 Muscle glycogen The goal is to have as much CHO in the body as possible during the latter stages of prolonged exercise Febbraio et al found glycogen degradation higher during exercise tests following high GI (2000 J Appl Physiol 89, )

23 Exercise performance The effects on exercise have been inconsistent. Improvement following low Gi meals No improvement following low Gi meals Wu and Williams 2006 Int J Sport Exerc Metab 16, Wong et al 2008 Eur J sport 8, Thomas et al 1991 Int J sports Med12, Febbraio et al 2000 J Appl Physiol 89, Sparks et al 1998 Med Sci Sports Exerc 30, Wee et al 1999 Med Sci Sports Exerc 31, DeMarco et al 1999Med Sci Sports Exerc 31, Stevenson et al 2005 Int J sport Nutr Exerc Metab 15, Jentjens and Jeukendrup 2003 Eur J Appl Physiol 88, Chen et al 2008 Int J Sport Nutr Exerc Metab 18, Chen et al 2008 Int J Sports Med 29, Stannard et al 2000 Int J Sport Nutr Exerc Metab 10, Febbraio and Stewart 1996 J Appl Physiol 81, Stevenson et al 2006 Am J Clin Nutr 84,

24 Conclusions GI manipulation has been evelauated to improve many aspects of sports nutrition including recovery and glycogen load High GI are most effective choice during first phase recovery Blood glucose levels support insulin action in second phase recovery and in glycogen loading Insulin levels inhibit lipolysis and lower rates of lipid oxidation, therefore muscle glycogen sparing may be better after a low GI meal

25 Figure 5 The relative value of high GI and low GI feedings in pursuing glycogen recovery and load or sparing of glucose sources and availability during exercise.

26 Thank you for listening!