Endurance exercise
9.4 Effects of nutritional status
9.4.3 Pre-exercise and during-exercise CHO ingestion
When compared with exercise after overnight fasting, ingestion of carbohydrate-rich meals within the hours before exercise (as well as carbohydrate ingestion during exercise) has been shown to enhance endurance performance (Wright et al., 1991). Consequently, it is common practice for athletes to adopt such dietary approaches to competition. However, it is now well documented that pre- and during-exercise CHO ingestion is
one of the most potent ways to alter the pattern of substrate utilization during exercise through a number of control points.
One of the main responses to CHO feeding is to attenuate plasma FFA availability and lipid oxida- tion, while simultaneously increasing CHO oxida- tion rates. The reduced plasma FFA availability is due to an attenuation of lipolysis which, in turn, is regulated by increased circulating insulin concen- trations caused by CHO feeding. The anti-lipolytic effect of insulin is mediated through its ability to activate the enzymephosphodiesterase, which degradescAMPand thereby attenuates activation of protein kinase A and, eventually, hormone- sensitive lipase.
Convincing data confirming that lipolysis limits fat oxidation following CHO feeding is provided by Horowitz et al. (1997). In this study, male subjects completed 60 minutes of exercise at 45%
VO2max in fasted conditions or one hour after consuming 0.8 g/kg of glucose (to induce a high insulin response), 0.8 g/kg fructose (to induce a low insulin response) or an additional glucose trial during which intralipid and heparin were infused so as to maintain plasma FFA availability in the face of high insulin (see Figure 9.22). In accordance with the insulin response, lipolysis (as indicated by rate of appearance of glycerol) was reduced with CHO feeding and plasma FFA availability was reduced in these conditions (see Figure 9.23). In addition, rates of lipolysis exceeded lipid oxidation rates during fasted exercise whereas, in the CHO conditions, rates of lipolysis appeared to equal lipid oxidation rates, thus implying that lipolysis limits fat oxidation (see Figure 9.24).
However, when intralipid and heparin was infused during an additional glucose trial, lipid oxidation rates were enhanced by 30%
(4.0µmol.kg−1. min−1) compared with the glucose-only trial (3.1µmol.kg−1. min−1), but they were still not restored to levels occurring during fasted exercise (6.1µmol.kg−1. min−1).
Taken together, while these data suggest that only small elevations in insulin can attenuate lipolysis (i.e. 10−30µU/ml), they also demonstrate a limitation within the muscle cell itself
70 ∗†
FAST FRUCTOSE GLUCOSE GLUCOSE+LIPID
REST EXERCISE
60 50 40 30
Plasma Insulin (µU/ml)
20 10 0
−60 −50 −40 −30 −20 −10
Time (mins) Meal
0 10 20 30 40 50 60
Figure 9.22 Plasma insulin concentration after feeding and during exercise (adapted from Horowitzet al., 1997)
FAST 0.9
0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0
FRUCTOSE GLUCOSE GLUCOSE+LIPID
REST EXERCISE
Plasma FFA (mM)
Time (mins) Meal
∗
∗
∗
−60 −50 −40 −30 −20 −10 0 10 20 30 40 50 60
Figure 9.23 Plasma FFA concentration after feeding and during exercise (adapted from Horowitzet al., 1997)
∗
∗ Lipolysis
Fat Oxidation 9
8 7 6 5 4 3 2
9 8 7 6 5 4 3 2
FAST
µmol/kg/minµmol/kg/min
GLUCOSE Trial
(a)
(b)
FRUCTOSE
FAST GLUCOSE
Trial
FRUCTOSE
Figure 9.24 Rates of lipolysis and fat oxidation during exercise from (a) 20–30 minutes and (b) 50–60 minutes (adapted from Horowitzet al., 1997)
during CHO-fed conditions. In accordance with reduced lipid oxidation following CHO-feeding, CHO oxidation was increased due to increased glucose uptake (and oxidation) as well as muscle glycogenolysis. The enhanced rates of glycogenol- ysis was suggested to be due to increased allosteric activation of phosphorylase, given that AMP and Pi production is greater during conditions of reduced plasma FFA availability, as is the case with CHO feeding.
In an effort to ascertain the source of limitation to lipid oxidation within the muscle follow- ing CHO feeding, Coyle et al. (1997) infused octanoate (an MCFA) or palmitate (an LCFA) during 40 min of exercise at 50% VO2max after an overnight fast or 60 minutes after ingesting
1.4 g/kg of glucose. As expected (based on the previously discussed study), plasma FFA and lipid oxidation were higher in the fasted trials, while CHO oxidation was lower in this condition, compared with the glucose trials (see Figure 9.25).
However, the major finding of this study was that the percentage of palmitate oxidized during the glucose trial was reduced compared with fast- ing (70 vs. 86%, respectively), whereas octanoate was unaffected (99 vs. 98%, respectively). These data therefore suggest that LCFA uptake into the mitochondria is reduced with CHO feeding.
When taken in the context of previous sections in this chapter, it becomes increasingly apparent that any condition which accelerates glycolytic flux (e.g. increased intensity, muscle glycogen, glucose feeding) can regulate intramuscular lipid metabolism, which again points to a carnitine- mediated limitation. Furthermore, more recent data has demonstrated that the increased insulin and decreased adrenaline levels which accompany glucose ingestion during exercise appear to result in an attenuation of intra-muscular HSL activity (Wattet al., 2004), thus highlighting an additional point of control.
Given the effects of acutely manipulating CHO availability prior to and during exercise on lipid oxidation, these data are of obvious implications for individuals wishing to lose body fat. In situations where this is the main goal of the exercise session (e.g. in training sessions, as opposed to competition, where performance is the aim), it would be advisable to perform exercise in fasted conditions (e.g. first thing in the morning) or many hours after CHO ingestion.
Indeed, the negative effects of CHO ingestion on exercise-induced lipolysis can persist for up to six hours after feeding (Montain et al., 1991).
Furthermore, data also demonstrate that fat oxi- dation was reduced by 30% during an eight-hour recovery period when CHO was ingested after exercise, as opposed to ingested before exercise (Schneiteret al., 1995).
Over the last few years, many researchers have also observed that training in condi- tions of reduced endogenous and exogenous carbohydrate availability enhances oxidative
Carbohydrate Oxidation (µmol/kg/min) 210 200 190 180 170 160 140 150 130 120
(b) Time (min)
∗
0 10 20 30 40
40 35 30 25 20 15
Fat Oxidation (µmol/kg/min)
10
0 10 20
Time (min) (a)
30
∗
40 FAST-Palmitate
GLUCOSE-Palmitate FAST-Octanoate GLUCOSE-Octanoate
Figure 9.25 Rates of fat and CHO oxidation during exercise in fasted and glucose trials (adapted from Coyle et al., 1997)
adaptations of skeletal muscle and augments the training response. Such data are particularly interesting as they appear to challenge traditional sports nutrition guidelines advising that athletes should consume carbohydrate before, during and after daily training sessions. Interested readers are directed to recent reviews summarizing these exciting new findings (Hawley & Burke, 2010).
Given that insulin is a potent inhibitor of lipolysis, one nutritional strategy to minimize this suppression is to consume low glycemic carbohydrate (LGI)-based foods in the hours prior to exercise, as opposed to those that are high glycemic (HGI). LGI foods do not produce as big an insulin response, so, as a result, plasma FFA availability and glycerol are higher during exercise undertaken in the hours after LGI meals, as opposed to HGI meals (see Figure 9.26).
Consequently, lipid oxidation during exercise is higher with LGI CHO ingestion, compared with HGI ingestion, and muscle glycogen utilization is reduced (Weeet al., 2005).
For this reason, athletes are typically advised to consume LGI foods as the pre-competition meal (such feeding strategies are also associated with better maintenance of plasma glucose levels during exercise), although it should be noted that effects of GI on substrate utilization during exercise appear to be minimal when large
amounts of CHO are ingested during exercise, as is typically the case in prolonged endurance events (Burke et al., 1998). Where athletes are concerned with maximizing body fat loss (such as those in weight-making sports), it is advised that LGI carbohydrates should form the majority of the CHO input to the daily diet (Mortonet al., 2010).