NUTRITION IN SPORT

and inhibition of PFK, thus slowing the rate of
glycolysis and glycogenolysis. This forms the
basis of the ‘glucose–fatty acid cycle’ proposed
by Randle et al. (1963), which has for many years
been accepted to be the key regulatory mecha-
nism in the control of CHO and fat utilization by
skeletal muscle. However, recent work has chal-
lenged this hypothesis and it seems likely that

the regulation of the integration of fat and CHO

catabolism in exercising skeletal muscle must

reside elsewhere, e.g. at the level of glucose

uptake into muscle, glycogen breakdown by

phosphorylase or the entry of fatty acids into the

mitochondria. A detailed discussion is beyond

the scope of this review; for further details, see

Hargreaves (1995) and Maughan et al. (1997).

biochemistry of exercise 31

Muscle glycogen

1

+

+

• 
  


• 
  


• 
  


• 
  


• 
  

AMP, Pi, Ca2+, adrenaline (cAMP)

ATP

ADP

Glucose-1-P

Glucose-6-P

Fructose-6-P

Fructose-1,6-bp

PEP

Pyruvate

5

Acetyl-CoA

Oxaloacetate

TCA cycle

Succinyl-CoA

α-ketoglutarate

Citrate

ATP

NADH

ATP, NADH

Ca2+, ADP, CoA, NAD+

4

3

2

10

9

8

6

7

Glucagon, adrenaline,

noradrenaline

Insulin

Liver glycogen

Plasma glucose

Adipose tissue and

muscle

triacylglycerol

Adrenaline, glucagon,

cortisol

Insulin

Fatty acids

Fatty acyl-CoA

β-oxidation

NAD+

Muscle protein

Cortisol

Insulin

ADP, AMP, Pi, NH+ 4

ATP, H+

Activators

Inhibitors

Amino acids

+

• 
  

+

• 
  

+

+

• 
  

+

+

• 
  

Fig. 2.6Metabolic pathways of importance to energy provision during exercise showing the main sites of
regulation and the principal hormonal and allosteric activators and inhibitors. Enzymes: 1 , glycogen
phosphorylase (muscle); 2 , glycogen phosphorylase (liver); 3 , hexokinase; 4 , phosphofructokinase; 5 , pyruvate
dehydrogenase; 6 , hormone-sensitive lipase; 7 , carnitine acyl-transferase; 8 , 3-hydroxyacyl dehydrogenase; 9 ,
citrate synthase; 10 , proteases. AMP, adenosine monophosphate; cAMP, cyclic AMP; PEP, phosphoenolpyruvate;
Pi, inorganic phosphate; TCA, tricarboxylic acid.