After ingestion, dietary carbohydrates have four
metabolic fates (1,2):
1. Used as fuel substrate and oxidized in
several tissues of the body, mainly as glucose;
2. Storage as
glycogen in skeletal muscle (80%
stored) and liver;
3. Storage as triacylglycerol (TAG);
4. Conversion of glucose molecules into C3
precursors in the splanchnic area which pass on to the liver where they are
used as substrates for gluconeogenesis.
Glycogen is a homopolysaccharide composed of subunits of glucose. The synthesis of glycogen consists of adding individual glucose units to an existing glycogen chain, forming extensively linked branches every 8-12 residues on the glycogen molecule. The more extensively branched the molecule is, the greater the ability it has to rapidly supply energy.
The body's capacity for storing glycogen is
limited. Most of dietary glucose is
stored as glycogen in liver and skeletal muscles during the postprandial period
and is subsequently released and oxidized (3). The capacity for storing large
amounts of glycogen is relatively large (4,5,6,7,8).
Looking at the actual storage of glycogen in the
body it was reported back in 1975 that a nonobese man weighing 70 kg stores
about 350 g of muscle glycogen and 40-50 g of liver glycogen (9), equivalent
to about 1600kcal of energy stored in the body in the form of glycogen.
Liver glycogen varies in relation to the
patterns of eating and fasting (10). Liver glycogen concentrations vary in the
range of 8-81g (50-500 mmol glycosyl residues/kg tissue in the post-absorptive
state) with a mean of 44g (270 mmol glycosyl residues/kg liver) (11).
Skeletal muscle glycogen concentrations can also
vary, concentrations depend on upon the muscle being measured (12). In biopsy
samples from the quadriceps femoris muscle glycogen concentrations were found
to be in the range of 60-l20 mmol glycosyl residues/kg with a mean of 85 mmol
(14 g) glycosyl residues/kg tissue (12).
The body’s glycogen reserves are usually
maintained at between 250 and 500 g in a 70 kg adult man. More precisely, others
reported that glycogen storage capacity in man is 15 g/kg body weight and can
accommodate a gain of ≈500g before net lipid synthesis contributes to
increasing body fat mass (13). For a 70-kg man with ≈40% of his weight as
skeletal muscle and a liver weighing 1.8 kg, one can estimate that ≈3 mol
glycosyl residues or almost 500 g of glycogen are stored in the body.
However, higher values have been reported. For
subjects with a mean body weight of 72kg, Hedman (14) calculated maximal
values of 700 g glycogen. Others came to the same conclusion but suggested
that a further 100g could be stored with 2 weeks of carbohydrate overfeeding or
by using the glycogen-loading technique (15).
Back in 1967, the highest values ever reported
were above 4/100g of muscle in three subjects (16), and liver glycogen content
ranged from 14.3 g to 80.1 g per kg wet liver tissue, with a mean of 43.7 with
2.4 g water per g glycogen (17). If those highest values are extrapolated to
the whole body, then up to 4.3 mol glycosyl residues or some 700 g of
glycogen could be stored in the body.
Bergstrom et al (16) reported values in the range
500-800 g in some of their subjects who followed a glycogen-loading
technique. The maximum increase in stored glycogen ever observed was 1146g
in one subject (13). Data derived from this overfeeding study (16) suggest
that the glycogen stores can maximally accommodate 800-900 g of carbohydrate
and perhaps as much as 1-1.1 kg in trained athletes, which are among the
highest glycogen storage values reported in the literature (13).
This glycogen serves as the primary supplier of
energy during most forms of exercise (18). Glycogen is stored with two
to four times its weight of water (19).
In addition, the extracellular fluid can
accommodate less than an extra 10 g glucose in order to avoid glucosuria.
Bonus:
Why do we store carbohydrate as polymers and not
monomers?
The reason why carbohydrate is stored in polymeric form (glycogen) is
dictated by osmotic pressure considerations
(20). Osmotic pressure is the hydrostatic pressure required to stop the net
flow of water across a membrane separating solutions of different compositions
(21).
(21)
A 70 kg man stores about 70 g of glycogen in
the in the cytosol of a 1.5 kg liver,
and the total cytosolic volume of this liver is about 1 l. The osmolarity of
body fluids is maintained at around 300mOsM.
If carbohydrate was stored as glucose monomers a 0.43 Mol of glucose
in 1 l would exert an osmotic pressure of 430mOsM, above the threshold of 300mOsM (20). The solution is to store it as
polymers with a very low osmotic pressure.
Would you like to know more? Subscribe for daily updates!
References:
1. Abumrad, N. N., Cherrington, A. D., Williams,
P. E., Lacy, W. W. & Rabin, D. (1982). Absorption and disposition of a
glucose load in the conscious dog. American Journal of Physiology 242,
E398-E406.
2. Bjorkman, O., Eriksson, L. S., Nyberg, B.
& Wahren, J. (1990). Gut exchange of glucose and lactate in basal state and
after oral glucose ingestion in postoperative patients. Diabetes 39,747-751.
3. Ebiner, J. R., Acheson, K. J., Doerner, D.,
Maeder, E., Arnaud, M. J., JCquier, E. & Felber, J. P. (1979). Comparison
of carbohydrate utilization in man using indirect calorimetry and mass
spectrometry after an oral load of 100 g naturally-labelled 13C-glucose.
British Journal of Nutrition 41,419-429.
4. Passmore R, Swindells YE. Observations on the
respiratory quotients and weight gain of man after eating large quantities of
carbohydrate. BrJ Nutr 1963; 17:33 1-9.
5. Acheson, K. J., Flatt, J. P. & Jdquier,
E. (1982). Glycogen synthesis versus lipogenesis after a 500 gram carbohydrate
meal in man. Metabolism 31, 1234-1240. Journal of Clinical Nutrition
48,24@-247.
6. Acheson, K. J., Schutz, Y., Bessard, T.,
Ravussin, E., JCquier, E. & Flatt, J. P. (1984). Nutritional influences in
lipogenesis and thermogenesis after a carbohydrate meal. American Journal of
Physiology 246, E62-E70.
7. Acheson, K. J., ThClin, A., Ravussin, E.,
Arnaud, M. J. & JCquier, E. (1985). Contribution of 500 g naturally
labelled I3C dextrin maltose to total carbohydrate utilization and the effect
of antecedent diet in man. American Journal of Clinical Nutrition 41,881-890.
8. Acheson KJ, Schutz Y, Bessard T, Anantharaman
K, Flatt JP, Jéquier E. Glycogen storage capacity and de novo lipogenesis
during massive carbohydrate overfeeding in man. Am J Clin Nutr 1988;48:240-7.
9. Felig P, Wahren T. Fuel homeostasis in
exercise. N Engl T Med 1975;293(21): 1078-84.
10. Nilsson LH. Liver glycogen content in man in
the post absorptive state. Scand J Clin Lab Invest l973;32:3 17-23.
11. Hultman E, Nilsson LH. Liver glycogen in
man. Effect of different Diets and muscular exercise. Adv Exp Med Biol 1971;
11:143-51.
12. Hultman E. Muscle glycogen in man determined
in needle biopsy specimens method and normal values. Scand J Gin Lab Invest
1967; 19:209-17.
13. Acheson KJ, Schutz Y, Bessard T,
Anantharaman K, Flatt JP, Jéquier E. Glycogen storage capacity and de novo
lipogenesis during massive carbohydrate overfeeding in man. Am J Clin Nutr
1988;48:240-7.
14. Hedman R. The available glycogen in man and
the connection between rate of oxygen intake and carbohydrate usage. Acta
Physiol Scand l957;40:305-2l.
15. Bjorntorp P. Sjostrom L. Carbohydrate
storage in man: speculations and some quantitative considerations. Metabolism
l998;27(suppl 2): 1853-65.
16. Bergstrom J, Hermansen L, Hultman E, Saltin
B. Diet, muscle glycogen and physical performance. Acts Physiol Scand l967;7 1:
140-50.
17. Nilsson LH. Liver glycogen content in man in
the post-absorptive state. Scand J Clin Lab Invest l973;32:3 17-23.
18. Costill DL. Carbohydrate for athletic
training and performance. Bol Asoc Med P R 1991;83(8):350-3.
19. Olsson KE, Saltin B. Variations in total
body water with muscle glycogen changes in man. Acts Physiol Scand l970;80:1
1-8.
20. JT Brosnan. Comments on metabolic needs for glucose and
the role of Gluconeogenesis. European Journal of
Clinical Nutrition (1999) 53, Suppl 1, S107±S111
21. Lodish H, Berk A, Zipursky SL, et al. Molecular Cell Biology. 4th edition. New York: W. H. Freeman; 2000.
21. Lodish H, Berk A, Zipursky SL, et al. Molecular Cell Biology. 4th edition. New York: W. H. Freeman; 2000.
