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specific needs of the suckling (see previous section). If the nutrition of the mother is normal and adequate (though not necessarily other- wise) the vitamins, like other factors, are supplied in proper ratios; each is adequate in amount but not in excess. When milk of another species is consumed in which the whole balance of the constituents is different, as when cows' milk is substituted for human milk, the adequacy of the vitamin supply must be determined by wide experience or by ad hoc investigations. The former would seem to have proved that raw cows' milk, when at its best, can cover the essential needs of the human infant.
78.
Since it has already received reference, vitamin D may be first discussed. In its absence from the diet, or with a deficiency in its supply sufficiently great, the formation of bone fails to proceed normally, and the infant or young child may display symptoms of rickets. The constitution of this vitamin is approximately known, and it can be produced artificially in a pure form from the substance ergosterol, which in small amounts is somewhat widely distributed in living tissues. Quite inactive itself, this substance is converted into the vitamin when irradiated by ultra-violet light or on exposure to the rays of the sun. This explains the circumstances so abundantly proved that, given an adequate supply of lime and phos- phorus in the food, rickets can be prevented or cured either by an adequate supply of vitamin D or, alternatively, by sufficient exposure to sunlight. It has been said that a geographical map showing the distribution of rickets is practically equivalent to a map of regions deficient in sunshine. The efficacy of the rays is due to the circum- stance that they produce in the body the active vitamin from its inactive precursor.
The climate of this country is such, however, that the aid of the vitamin itself is essential; especially during the winter months when, unfortunately, the supply in cows' milk is at its lowest. It may be said incidentally that it is possible to increase it by irradiating the milk itself, though practical applications of this fact would certainly be difficult.
79. It is important to remember that this vitamin is associated with the fat of the milk. When pure it is resistant to heat treatment, but experiments on animals have suggested that some degree at least of destruction occurs in milk when it is heated. It must not be forgotten, however, that fresh untreated milk is itself liable to be deficient in this vitamin. The highly beneficial influence of cod-liver oil, a rich source of vitamin D, on the assimilation of calcium and phosphorus shown in the experiments on infants previously discussed, suggests that the milk used for those experiments, even when fresh, was unduly deficient in the vitamin. This does not lessen the significance of the results obtained from the direct comparison of the pasteurised with the boiled milk, but it somewhat lessens the practical significance of the experiment as a whole.
80. While, except in a few districts, pronounced rickets has become relatively rare in this country, a minor degree of the
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condition, easily detectable by X-rays, is common among milk-fed infants following on the winter lack of sun. While much more common in bottle-fed children it is frequent even in the breast fed. This circumstance makes it more difficult to decide upon the extent to which pasteurisation by itself increases the liability to rickets among infants fed on cows' milk, and no adequate experimental comparison (which would need to be prolonged) with this special end in view has yet been attempted.
81. In central Europe, towards the end of the war and for some time after it, the milk available, because of difficulties in transport, was apt to be subjected to repeated heating, and severe rickets was rife among children. General malnutrition was of course also prevalent and obscured the issue, but observations made in Vienna in 1921 demonstrated conclusively that a supply of vitamin D cured all the symptoms attributable to rickets (Chick and others, 1923). It is fair to say that, while the unfavourable effect of pasteurisation upon the utilisation of the calcium and phosphates must not be ignored, there is to-day no direct evidence to show, if natural deficiencies in the original milk are first excluded, that it is responsible for the incidence of rickets in a form to be recognised clinically.
82. Reliable evidence has made it sure that a deficiency of vitamin D in the diet of children is a factor of importance in the causation of dental defects (Mellanby, 1934), and other evidence has shown that an adequate addition of raw milk to the food may be a successful prophylactic in this connection (Sprawson, 1982). Once again, however, a conclusive proof that pasteurised milk is less efficient in this respect is yet lacking.
83. In the case of vitamin C the essential facts are clear. When it is absent from the food, or when the supply is greatly deficient, scurvy inevitably follows, alike in children and adults. The long and distressing history of that disease in the past provides ample proof of this, and there are districts in Europe to-day in which it is still being demonstrated. It is equally sure that an adequate supply of the vitamin prevents the disease, and when this is established vitamin therapy works a dramatic cure. Vitamin C has now been isolated in a pure state, and its chemical constitution is probably established. It is heat sensitive, and in milk is undoubtedly destroyed to a greater or less degree during the process of pasteurisa- tion. Oxidation plays, however, a notable part in this destruction, which is therefore less when the milk is heated out of contact with air. As bearing with some significance upon what is desirable in the construction of pasteurisation plants, it is easy to show that the oxidation is accelerated by the catalytic action of traces of metals. The following observations were made for the purpose of the present discussion. Samples from Я fresh milk supply were heated for half an hour in pyrex glass vessels at 145° F.: (a) in open bottles, and (b) sealed from contact with the air. In (a) the milk without additions lost 33 per cent. of its content of vitamin C;
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