Intravenous feeding involves many problems, some of which remain to be solved. The basic problem is the compounding of utilizable nutrients which can be dispersed in a suitable medium and administered without greatly disturbing the fluid distribution, acid-base balance, or metabolic status of the body. Because a major need in intravenous feeding is the pro-vision of suitable energy-yielding food, carbohydrates are important.
Glucose has been used as the principal source of energy. However it cannot be administered in concentrations appreciably higher in osmotic pressure than the osmotic pressure of the blood. Thus, it cannot furnish more than a small proportion of the total energy requirements without flooding the body with water. Moreover, feeding which needs to be con-tinued more than a day or two must furnish other nutrients. Good prog-ress has been made in compounding mixtures containing fat and carbo-hydrate as the principal sources of energy, with other nutrients such as amino acids, inorganic salts, and vitamins being provided.
It would seem that fructose or invert sugar might have some advantages in intravenous feeding. Glucose utilization is temporarily inhibited fol-lowing a prolonged fast, a condition which may lead to a need for paren-teral nutrients. Fructose utilization shows little or no impairment in fasted rats (91). The evidence seems to be conflicting regarding the renal tolerance to invert sugar as compared with glucose and with fructose (92), but on the basis of the present evidence fructose or mixtures of fructose and glu-cose may prove to be preferable to gluglu-cose as the only carbohydrate in parenteral alimentation.
Owing to the high osmotic pressure of hexose solutions in comparison to that of equivalent amounts of carbohydrate as water-soluble
polysac-90. D. C. Cederquist, W. D. Brewer, R. M. Beegle, A. N. Wagoner, D. Dunsing, and M. A. Ohlson, J. Am. Bietet. Assoc. 28, 113 (1952).
91. G. H. Wyshak and I. L. Chaikoff, / . Biol. Chem. 200, 851 (1953).
92. Anonymous, Nutrition Revs. 11, 299 (1953).
charide, consideration has been given to the effects of phytoglycogen (amylopectin) administered parenterally to rabbits (98, 94). The poly-saccharide causes an elevation of the blood sugar and appears to be con-verted into glucose in the body. More information needs to be obtained concerning the suitability of phytoglycogen for intravenous feeding.
98. D . L. Morris, / . Biol. Chem. 148, 699 (1943).
H. E. Maywald, R. Christensen, and T. J. Schoch, Agr. Food Chem. 3, 521 (1955).
Part II
Dental Aspects of Carbohydrates (95)
Teeth, as a part of a biological structure, are exposed to the same general metabolic influences of carbohydrates as the rest of the animal organism.
These effects will be particularly important during the period of tooth formation. Formation occurs in the human in the prenatal period and be-fore the age of about twelve years, at which time the permanent dentition has been completed, except for third molars. The erupted teeth, as shown by studies using radioactive tracer elements (96), are only slowly affected by general metabolic influences, much more slowly than the bones. The outer covering of the teeth, the enamel, is not capable of self-regeneration, and any changes which occur within the enamel are basically chemical exchanges with substances in the salivary secretions which bathe the teeth constantly. Human saliva, a mixed secretion exhibiting intrinsic variation, is also influenced in its composition by partial solution or emulsification of the foods entering the mouth. Since the saliva is in continuous contact with the teeth over many years, its composition and influence on the tooth sur-faces obviously are of importance. The human mouth in common with the entire gastrointestinal tract as discussed earlier in this chapter also contains many types of microorganisms whose nature and distribution have not been adequately studied. The variable requirements of microorganisms for nu-merous growth factors indicate that the type and growth of microorganisms within the mouth may be extremely dependent upon the composition of the saliva. Although saliva contains many growth substances, the amounts are very small and variations in their amount could greatly influence the growth of microorganisms. Numerous organisms require minimal amounts of carbohydrates, and some will grow in proportion to the amount of carbohydrates present.
Strong evidence is available that the post-eruptive period may be divided into an early period of indefinite length, but perhaps lasting many years, during which the newly exposed surfaces mature. Before complete matura-tion, the teeth may be much more susceptible to attack and to exchange with components of saliva and blood than in the subsequent period.
Carbohydrates, then, are of dental interest through any general meta-bolic effect that they may exert, this effect being primarily operative dur-ing the pre-eruptive period of the tooth. Once the teeth are erupted, the
95. G. Toverud, S. B. Finn, G. J. Cox, C. F. Bodecker, and J. H. Shaw, "A Survey of the Literature of Dental Caries," Publ. 225, National Academy of Sciences, Na-tional Research Council, Washington, D. C , 1952.
96. O. Chievitz and G. Hevesy, Kgl. Danske Vedenskab. Biol. Medd. 13, No. 9 (1937) ; J. F. Volker and R. F. Sognnaes, Am. J. Physiol. 133, 112 (1941) ; R. F. Sogn-naes and J. H. Shaw, J. Am. Dental Assoc. 44, 489 (1952).
carbohydrates apparently exert their main influence through the medium of the saliva on the exposed surfaces of the enamel. This post-eruptive ef-fect may also operate through an influence on the microbiological flora of the mouth.