By William Misner Ph.D.
Preface by Steve Born: One of the more controversial articles in The Endurance Athlete's Guide to Success is "The Pre-Race Meal" where, among other things, we suggest that athletes not consume food any sooner than three hours prior to the start of workout sessions or races. The information provided in the article seems to generate the most resistance and skepticism from athletes, probably because it's a concept that they've never heard of before. However, both Dr. Bill and I can say that we've yet to have one athlete tell us that the principles outlined in the article didn't work.
This article exists to provide a scientific view on the subject, supplying even more information on this somewhat controversial topic.
What advantages are achieved by consuming carbohydrates sooner than 3 hours as opposed to later than 3 hours?
What disadvantages occur by elevating insulin in sedentary resting state and how does that affect glucose disposal rate during exercise?
The 3-hour pre-exercise carbohydrate meal position is defendable from 3 principles:
- Optimal repletion liver glycogen (only) repletion with minimal depleting effect on muscle glycogen stores
- Favorable hormone environment including insulin homeostasis affecting glucose disposal rate
- Muscle glycogen oxidation rate 3-hour postprandial mechanisms (postponed glycogen depletion)
Pre-exercise carbohydrates consumed upon waking prior to a morning event are replacement donors for topping off liver glycogen levels lost to sustain PM metabolism. None of the AM carbohydrates find their way into muscle glycogen stores. Once liver glycogen stores are topped off, excess blood glucose levels needlessly raise hormones to initiate fat store mechanics. It takes only a small amount of AM carbohydrates to top liver glycogen stores, and the blood glucose disposal hormone profile process easily returns to pre-meal homeostasis-state within the 3 hour pre-meal period recommended. Athletes who follow this protocol report improved carbohydrate utilization relative for performance. We do not tell athletes they cannot take carbohydrates right before the start of the event, but we do encourage them to limit the amount. When insulin levels are high carbohydrate oxidation rate is also high and timing this affect postpones glycogen oxidations rate.
I have taken the liberty to share a few excerpts from research papers that address this discussion:
"Ingestion of a high carbohydrate meal 3-4 hours prior to exercise ensures adequate carbohydrate availability and enhances exercise performance. Although hyperinsulinaemia associated with carbohydrate ingestion in the hour prior to exercise may result in some metabolic alterations during exercise, it may not necessarily impair exercise performance and may, in some cases, enhance performance. Carbohydrate ingestion during prolonged, strenuous exercise, where performance is often limited by carbohydrate availability, delays fatigue. This is due to maintenance of blood glucose levels and a high rate of carbohydrate oxidation, rather than a slowing of muscle glycogen utilisation, although liver glycogen reserves may be spared." (Costill & Hargreaves 1992)
"Since fatigue rarely results from hypoglycaemia alone, the effectiveness of carbohydrate feeding should be judged by its potential for muscle glycogen sparing. Carbohydrate feeding during moderate intensity exercise postpones the development of fatigue by approximately 15 to 30 minutes, yet it does not prevent fatigue. This observation agrees with data suggesting that carbohydrate supplementation reduces muscle glycogen depletion. It is not certain whether carbohydrate feeding increases muscle glucose uptake throughout moderate exercise or if glucose uptake is higher only during the latter stages of exercise. In contrast to moderate intensity exercise, carbohydrate feeding during low intensity exercise (i.e. less than 45% of VO2 max) results in hyperinsulinaemia. Consequently, muscle glucose uptake and total carbohydrate oxidation are increased by approximately the same amount. The amount of ingested glucose which is oxidised is greater than the increase in total carbohydrate oxidation and therefore endogenous carbohydrate is spared. The majority of sparing appears to occur in the liver, which is reasonable since muscle glycogen is not utilised to a large extent during mild exercise. Although carbohydrate feedings prevent hypoglycaemia and are readily used for energy during mild exercise, there is little data indicating that feedings improve endurance during low intensity exercise. When the reliance on carbohydrate for fuel is greater, as during moderate intensity exercise, carbohydrate feedings delay fatigue by apparently slowing the depletion of muscle glycogen." (Coyle & Coggan 1984)
"As carbohydrate ingestion does not slow the rate of glycogen utilization in working muscle, it is also advisable for endurance athletes to start exercise with an adequate supply of muscle glycogen, irrespective of whether or not they ingest carbohydrate during exercise. While carbohydrate ingestion 'spares' conversion of liver glycogen to plasma glucose and prevents hypoglycemia, it does not delay the fatigue associated with a low (approximately 20 mmol kg-1) glycogen content in working muscle. Conversely, increases in glycogen content of working muscle at the start of exercise have no effect on the rates of plasma glucose oxidation. Higher initial rates of glycogen utilization by active muscles in 'carbohydrate-loaded' subjects decrease the indirect oxidation (via lactate) of non-working muscle glycogen, rather than the conversion of liver glycogen to plasma glucose. Hence, athletes should ingest carbohydrate during endurance exercise even if they have 'carbohydrate-loaded' before exercise." (Dennis et al., 1997)
"Whereas in the earlier studies, estimates of CHO oxidation were made using respiratory gas exchange measurements, investigations since the early 1970s have employed stable 13C and radioactive 14C isotope techniques to determine the amount of ingested CHO that is oxidised during exercise. Most of the early interest was in glucose ingestion during exercise. These studies showed that significant quantities of ingested glucose can be oxidised during exercise. Peak rates of glucose oxidation occur approximately 75-90 minutes after ingestion and are unaffected by the time of glucose ingestion during exercise. Rates of oxidation also appear not to be influenced to a major extent by the use of different feeding schedules." (Hawley et al., 1992)
"Ingestion of CHO 3-4 hr prior to exercise can increase liver and muscle glycogen stores and has been associated with enhanced endurance exercise performance. The metabolic effects of CHO ingestion persist for at least 6 hr. Although an increase in plasma insulin following CHO ingestion in the hour prior to exercise inhibits lipolysis and liver glucose output, and can lead to transient hypoglycemia during subsequent exercise, there is no convincing evidence that this is always associated with impaired exercise performance. Having said that, individual experience should inform individual practice. Interventions to increase plasma FFA availability prior to exercise have been shown to reduce CHO utilisation during exercise, but do not appear to have major ergogenic benefits. It is more difficult to hyperhydrate prior to exercise and although there has been interest in glycerol ingestion, to date research results have been equivocal. At the very least, athletes should ensure hydration prior to exercise." (Hargreaves 2001)
"Increased dietary carbohydrate intake in the days before competition increases muscle glycogen levels and enhances exercise performance in endurance events lasting 90 min or more. Ingestion of carbohydrate 3-4 hours before exercise increases liver and muscle glycogen and enhances subsequent endurance exercise performance. The effects of carbohydrate ingestion on blood glucose and free fatty acid concentrations and carbohydrate oxidation during exercise persist for at least 6 h. Although an increase in plasma insulin following carbohydrate ingestion in the hour before exercise inhibits lipolysis and liver glucose output, and can lead to transient hypoglycaemia during subsequent exercise in susceptible individuals, there is no convincing evidence that this is always associated with impaired exercise performance." (Hargreaves et al., 2004)
"The aim of this study was to compare the effect of preexercise breakfast containing high- and low-glycemic index (GI) carbohydrate (CHO) (2.5g CHO/kg body mass) on muscle glycogen metabolism. On two occasions, 14 days apart, seven trained men ran at 71% maximal oxygen uptake for 30 min on a treadmill. Three hours before exercise, in a randomized order, subjects consumed either isoenergetic high- (HGI) or low-GI (LGI) CHO breakfasts that provided (per 70 kg body mass) 3.43 MJ energy, 175 g CHO, 21 g protein, and 4 g fat. The incremental areas under the 3-h plasma glucose and serum insulin response curves after the HGI meal were 3.9- (P < 0.05) and 1.4-fold greater (P < 0.001), respectively, than those after the LGI meal. During the 3-h postprandial period, muscle glycogen concentration increased by 15% (P < 0.05) after the HGI meal but remained unchanged after the LGI meal. Muscle glycogen utilization during exercise was greater in the HGI (129.1 - 16.1 mmol/kg dry mass) compared with the LGI (87.9 - 15.1 mmol/kg dry mass; P < 0.01) trial. Although the LGI meal contributed less CHO to muscle glycogen synthesis in the 3-h postprandial period compared with the HGI meal, a sparing of muscle glycogen utilization during subsequent exercise was observed in the LGI trial, most likely as a result of better maintained fat oxidation."
"The aim of this study was to compare the effect of preexercise breakfast containing high- and low-glycemic index (GI) carbohydrate (CHO) (2.5g CHO/kg body mass) on muscle glycogen metabolism. On two occasions, 14 days apart, seven trained men ran at 71% maximal oxygen uptake for 30 min on a treadmill. Three hours before exercise, in a randomized order, subjects consumed either isoenergetic high- (HGI) or low-GI (LGI) CHO breakfasts that provided (per 70 kg body mass) 3.43 MJ energy, 175 g CHO, 21 g protein, and 4 g fat. The incremental areas under the 3-h plasma glucose and serum insulin response curves after the HGI meal were 3.9- (P < 0.05) and 1.4-fold greater (P < 0.001), respectively, than those after the LGI meal. During the 3-h postprandial period, muscle glycogen concentration increased by 15% (P < 0.05) after the HGI meal but remained unchanged after the LGI meal. Muscle glycogen utilization during exercise was greater in the HGI (129.1 - 16.1 mmol/kg dry mass) compared with the LGI (87.9 - 15.1 mmol/kg dry mass; P < 0.01) trial. Although the LGI meal contributed less CHO to muscle glycogen synthesis in the 3-hour postprandial period compared with the HGI meal, a sparing of muscle glycogen utilization during subsequent exercise was observed in the LGI trial, most likely as a result of better maintained fat oxidation." (Wee et al., 2005)
"The present study was designed to examine the effects of mixed high-carbohydrate meals with different glycaemic indices (GI) on substrate utilization during subsequent exercise. Nine healthy male recreational runners (age 26.8 (sem 1.1) years, body mass 74.7 (sem 2.4) kg, VO2max 58.1 (sem 1.7) ml/kg per min) completed three trials: high-glycaemic-index meal (HGI), low-glycaemic-index meal (LGI) and fasting (FAST), separated by 7 d. The test meals contained 2 g carbohydrate/kg body mass, they were isoenergetic and the GI values were 77.4, 36.9 and 0.0 respectively. In each trial, subjects consumed the test meal 3 h before performing a 60 min run at 65 % VO2max on a motorized treadmill. Ingestion of the HGI and LGI resulted in hyperglycaemia and hyperinsulinaemia during the postprandial period compared with the FAST (P<0.05). The incremental area under the curve for plasma glucose was 2-fold higher for HGI compared with LGI (108.7 v. 48.9 mmol/l per min). In contrast, plasma non-esterified fatty acid concentrations were significantly lower following HGI and LGI compared with FAST (P<0.05). During the subsequent submaximal exercise, plasma glucose declined to below the fasting value in HGI compared with LGI and FAST (P<0.05). The estimated total fat oxidation was significantly higher for the LGI than the HGI during exercise (P<0.05). In summary, both pre-exercise carbohydrate meals resulted in lower rates of fat oxidation during subsequent exercise than when subjects performed exercise in the fasting state. However, the LGI resulted in a higher rate of fat oxidation during exercise than following the consumption of the HGI." (wu et al., 2003)
"The present study investigated the effects of mixed high-carbohydrate (CHO) meals (breakfast and lunch) with different glycaemic indices (GI) on substrate metabolism during rest throughout the postprandial periods and during subsequent exercise. Nine recreationally active males completed two trials, high glycaemic index (HGI) and low glycaemic index (LGI), separated by 7 d in a randomised crossover design. In each trial, participants consumed breakfast and lunch, both of which were followed by a 3 h resting postprandial period. Following this, participants completed a 60 min run at 70 % of V O2max. The plasma glucose and serum insulin concentrations following both meals were significantly higher in the HGI trial than in the LGI trial (P<0.05). Serum insulin concentrations remained higher throughout the postprandial period following lunch in the HGI trial compared with the LGI trial (P<0.05). The total amount of fat oxidised was higher during the 3 h rest following lunch in the LGI trial than in the HGI trial (P<0.01) and subsequently CHO oxidation was lower (P<0.005). No significant differences in substrate utilisation were observed throughout the subsequent run. At 45 and 60 min, plasma glucose concentrations were higher in the LGI trial v. the HGI trial (P<0.05). The results of the present study provide further support that the GI concept can be successfully applied to mixed meals. The results also suggest that meals composed of LGI CHO may be more beneficial for maintaining a favourable metabolic milieu during the postprandial periods. Furthermore, during subsequent exercise, plasma glucose concentrations were better maintained following the LGI CHO meals." (Stevenson et al., 2005)
The major effects of insulin in tissues sedentary or muscles during exercise include:
- It increases the rate of transport of glucose across the cell membrane in adipose tissue and muscle,
- it increases the rate of glycolysis in muscle and adipose tissue,
- it stimulates the rate of glycogen synthesis in a number of tissues, including adipose tissue, muscle, and liver. It also decreases the rate of glycogen breakdown in muscle and liver.
- it inhibits the rate of glycogenolysis and gluconeogenesis in the liver.
- It decreases the rate of lipolysis in adipose tissue and hence lowers the plasma fatty acid level,
- it stimulates fatty acid and triacylglycerol synthesis in tissues, although only to a minor extent in humans,
- it increases the rate of very-low-density lipoprotein formation in the liver, (d) it increases the uptake of triglyceride from the blood into adipose tissue and muscle,
- it decreases the rate of fatty acid oxidation in muscle and liver,
- it increases the rate of cholesterol synthesis in liver.
- It increases the rate of transport of some amino acids into tissues,
- it increases the rate of protein synthesis in muscle, adipose tissue, liver, and other tissues,
- it decreases the rate of protein degradation in muscle (and perhaps other tissues),
- it decreases the rate of urea formation.--These insulin effects serve to encourage the synthesis of carbohydrate, fat and protein." (Newsholme & Dimitriadis 2001)
Research advises lower glycemic carbohydrate meals above high glycemic meals prior to prolonged exercise. Several of these papers systematically examine the 3-hour correlate concluding that muscle glycogen carbohydrate oxidation rates were favorably affected by low glycemic carbohydrate intake above the high glycemic meals, I contend that eating sooner than 3 hours and or eating excess high glycemic carbohydrate sources sooner than immediately before the event (5 minutes - 10') will not produce the best physiological muscle environment for optimizing prolonged endurance performance. Insulin as the regulatory hormone that determines the fate of exogenouos carbohydrate fate for the energy cycle should only be recruited 3 hours before or a few minutes prior to to exercise. This action practiced will enable the endurance athlete to increase performance by making muscle glycogen stores available longer resulting in improved performances. Needlessly taking large amounts or insulin provoking high glycemic carbohdyrates sooner than 3 hours before the event only imposes a hormone-environment that spends glycogen stores faster than optimal potential.
When I first wrote this suggestion based on the rationale stated above, several well-informed endurance athletes questioned this recommendation including some of our own staff. From those subjects (including several elite athletes) who tried this methodology of pre-exercise nutrition 3 hours prior, 100% reported improved performance. I find that during both sedentary or exercise state, endurance athletes and non-athletes habitually addict themselves to high glycemic calorie overdose. This is one of the reasons why syndrome X, diabetes, and cardiovascular pathology are on the increase in Americans.
Costill DL, Hargreaves M. Carbohydrate nutrition and fatigue. Sports Med. 1992 Feb;13(2):86-92. Review. PMID: 1561511 [PubMed - indexed for MEDLINE]
Coyle EF, Coggan AR. Effectiveness of carbohydrate feeding in delaying fatigue during prolonged exercise. Sports Med. 1984 Nov-Dec;1(6):446-58. Review. PMID: 6390613 [PubMed - indexed for MEDLINE]
Dennis SC, Noakes TD, Hawley JA. Nutritional strategies to minimize fatigue during prolonged exercise: fluid, electrolyte and energy replacement. J Sports Sci. 1997 Jun;15(3):305-13. PMID: 9232556 [PubMed - indexed for MEDLINE]
Hawley JA, Dennis SC, Noakes TD. Oxidation of carbohydrate ingested during prolonged endurance exercise. Sports Med. 1992 Jul;14(1):27-42. Review. PMID: 1641541 [PubMed - indexed for MEDLINE]
Hargreaves M. Pre-exercise nutritional strategies: effects on metabolism and performance. Can J Appl Physiol. 2001;26 Suppl:S64-70. Review. PMID: 11897884 [PubMed - indexed for MEDLINE]
Hargreaves M, Hawley JA, Jeukendrup A. Pre-exercise carbohydrate and fat ingestion: effects on metabolism and performance. J Sports Sci. 2004 Jan;22(1):31-8. Review. PMID: 14971431 [PubMed - indexed for MEDLINE]
Wee SL, Williams C, Tsintzas K, Boobis L. Ingestion of a high-glycemic index meal increases muscle glycogen storage at rest but augments its utilization during subsequent exercise. J Appl Physiol. 2005 Aug;99(2):707-14. Epub 2005 Apr 14. PMID: 15831796 [PubMed - indexed for MEDLINE]
Wu CL, Nicholas C, Williams C, Took A, Hardy L. The influence of high-carbohydrate meals with different glycaemic indices on substrate utilisation during subsequent exercise. Br J Nutr. 2003 Dec;90(6):1049-56. PMID: 14641964 [PubMed - indexed for MEDLINE]
Stevenson E, Williams C, Nute M. The influence of the glycaemic index of breakfast and lunch on substrate utilisation during the postprandial periods and subsequent exercise. Br J Nutr. 2005 Jun;93(6):885-93. PMID: 16022758 [PubMed - indexed for MEDLINE]
Newsholme EA, Dimitriadis G. Integration of biochemical and physiologic effects of insulin on glucose metabolism. Exp Clin Endocrinol Diabetes. 2001;109 Suppl 2:S122-34. Review. PMID: 11460564 [PubMed - indexed for MEDLINE]