NUTRITION IN SPORT

saturated and unsaturated FAs with chain
lengths of 12–20 carbon atoms (Havel et al.
1964).
Small but physiologically important amounts
of FA are also stored as triacylglycerols (TG)
inside the muscle cells: the total muscle mass
may contain up to 300 g of fat of which the major
part is stored within the myocyte as small lipid
droplets (Björkman 1986). FAs liberated from TG
stored in adipocytes are released to blood, where
they are bound to albumin. The albumin concen-
tration of blood is about 6 mm, while the concen-
tration of FA is about 0.2–1.0 mm. As albumin can
bind up to eight FAs, the albumin transport
capacity is far in excess of the amount of FAs
bound under physiological circumstances and
therefore cannot be the limiting factor for FA oxi-
dation by muscle.
FA can also be derived from the triacylglycerol
core of circulating chylomicrons and very low
density lipoproteins (VLDL), which are both
formed from dietary fat in the postabsorp-
tive state. Chylomicrons are formed in the
epithelial wall of the intestine and reach the
blood stream after passage through the lym-
phatic system. VLDLs are synthesized in the
liver after which they are released directly into
the blood stream.

Effects of exercise intensity and

duration of fat metabolism

More than 50 years ago, Christensen and Hansen

(1939) provided evidence from respiratory gas

exchange measurements that fat was a major

fuel for exercise metabolism. Since that time, a

number of investigations have provided evi-

dence that plasma FAs contribute a significant

portion to the energy demands of mild-to-

moderate exercise. However, until recently the

rates of whole-body lipolysis had only been mea-

sured during very low-intensity exercise, and in

untrained or moderately active individuals.

Our understanding of the regulation of

endogenous fat and CHO metabolism in relation

to exercise intensity and duration has been

advanced considerably by modern-day studies

which have used a combination of stable isotope

techniques in association with conventional indi-

rect calorimetry (Romijn et al. 1993, 1995; Sidossis

& Wolfe 1996; Siddossis et al. 1996, 1997). As the

three most abundant FAs are oxidized in propor-

tion to their relative presence in the total plasma

FA pool (Havel et al. 1964), total plasma FA kinet-

ics can be reliably estimated from stable isotope

studies using infusions of either palmitate or

oleate (when the concentrations of total FAs and

fat metabolism during exercise 185

Adipose tissue Blood plasma Muscle

Triacylglycerol

(210 000 kJ)

FFA

Glycerol

FFA

FFA

Albumin FFA Fatty acids

Intramuscular TG

(12 600 kJ)

Glycogen

(c. 8400 kJ)

Mitochondria

Acetyl-CoA

TCA cycle and

electron transport

ATP

Energy

O 2

Glucose

(420 kJ)

Fig. 13.1The storage and
mobilization of peripheral
adipose and intramuscular
triacylglycerol (TG). TG from
peripheral adipose tissue can be
broken down to glycerol and free
fatty acids (FFAs). FFAs can be
mobilized by binding to plasma
albumin for transport into the
systemic circulation to skeletal
muscle. Intramuscular TG can
also be broken down to glycerol
plus fatty acids, which can enter
the mitochondria for oxidation
during exercise. TCA,
tricarboxylic acid. From Coyle
(1997), with permission.