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Vitamin D Milk

( Originally Published 1939 )


The vitamin D content of milk has been increased by irradiating the cows with ultra-violet light, by feeding them with irradiated yeast or irradiated ergosterol, by directly irradiating the milk itself, or by adding to the milk various vitamin D concentrates such as the un-saponifiable fraction of cod-liver oil, irradiated ergosterol, sun-rayed ergosterol in oil, or ergosterol which is activated by electron bombard-ment, or by the direct addition of irradiated yeast.'

Metabolized vitamin D milk. When irradiated yeast is fed to cows, their milk acquires anti-rachitic properties but is unchanged in all other recognizable respects. Such milk is called "metabolized milk." Dried irradiated yeast, mixed with the grain ration, is fed in the pro-portion of about ½/2 ounce per cow, more or less depending on the milk production. In about 2 weeks, the vitamin D content of the milk reaches the desired level of 430 U.S.P. X units per quart. This procedure appeals to the sanitarian and the certified-milk industry because it imparts the desired vitamin D potency without involving an additional manipulation or treatment of the milk or the addition of substances foreign to milk. About 200 dairies are using this method. Its weakness lies in the fact that dependence on the maintenance of a claimed vitamin D potency rests on the reliability of the herdsman to include the yeast in the feed.

The irradiation of yeast is controlled by patents owned by the Wisconsin Alumni Research Foundation, which licenses the Fleischmann Yeast Company to manufacture irradiated dry yeast and to sub-license dealers to feed it, provided that they use the required amounts to maintain the claimed potency level and that they have bioassays made on their milk at regular intervals.

Irradiation of milk. When milk is exposed to the radiation from some source of ultra-violet light, the milk acquires anti-rachitic properties. The flavor is impaired if the potency exceeds about 135 units per quart of milk. This treatment is covered by the Steenbock patents (U. S. Patents 1,680,818 and 1,817,936), which have been assigned to the Wisconsin Alumni Research Foundation. Any milk dealer who wants to use this process must obtain a license from this organization by contracting to pay certain royalties to the Foundation, to maintain claimed potency levels, to have his milk bioassayed six times a year, and to abide by certain other considerations.

The simplest assemblage of equipment to accomplish this consists of a carbon arc lamp, with reflectors to direct the rays, suspended immediately in front of a horizontal-tube, wash-board type of milk cooler. As the milk flows down the corrugated surface of the cooler, it is flooded with the ultra-violet light. The pump is metered so that it will supply a constant volume of milk.

The type of irradiator in most general use is known as the C-P irradiator, manufactured by the Creamery Package Manufacturing Company, Chicago. This apparatus consists of a slightly tapering metal cylinder of tinned copper or stainless steel, 57 inches long with a diameter at one end of 50 inches and at the other of 40 inches. The cylinder is mounted on brackets which slide on columns and is balanced by counterweights within the columns so that it can be moved up and down for cleaning and inspection. An automatic carbon arc lamp is suspended at the center of the cylinder. A ventilating fan withdraws the ozone from the top of the equipment and discharges it outdoors. A positive fixed-capacity pump meters the milk to the machine.

In operation, milk is fed at a steady and predetermined rate to a distribution trough which encircles the inside of the cylinder at the top, and flows in a thin film down the inner surface of the cylinder, where it is exposed to the irradiation from the lamp.

An irradiator has been developed which uses a quartz mercury lamp as the source of the ultra-violet irradiation. In principle the machine consists of a lamp placed horizontally between two inclined plates so that the ultra-violet rays fall on the thin film of milk which flows down the inner surfaces of the plates. A reasonably constant irradiation output is maintained automatically by means of a light-sensitive cell or target located opposite the burner. An ultra-violet recording meter registers the intensity of the irradiation so that an automatic time record is available for the inspection of the management and the regulatory officials, just as the automatic temperature recorder registers a permanent record of pasteurization performance. This machine is produced by the Hanovia Chemical and Manufacturing Company, Newark, N. J.

Fortification with vitamin D concentrates. Somewhat prior to the discovery of the ability of ultra-violet light to impart vitamin D to various products, Zucker (U. S. Patent 1,678,454) worked out a process for extracting the unsaponifiable fraction from cod-liver oil, carrying practically all the vitamin D and much of the vitamin A of the oil. The patent was assigned to Columbia University, New York, which licenses the National Oil Products Company, Harrison, N. J., to sell the concentrate to milk dealers for adding to milk. This concentrate, in uniform amounts, is suspended in cream, packed in cans of 5.7 and 14 3/8 ounces respectively, sterilized, and marketed under the name Vitex. The contents of a can are poured into a small amount of milk, a gallon or so, thoroughly mixed, and then this mixture is added directly to the tank of milk to give a potency of 400 U.S.P. X units per quart. This milk is known as "fortified milk." The dealer pays no royalties, but he contracts to maintain the level of vitamin D in his milk at the above unitage, submit the results of three successful bioassays per year, together with record of sales of his fortified milk to check against the amount of Vitex purchased, and maintain his advertising in conformity with the company's standards of ethics and accuracy.

Evaporated milk. Evaporated milk may be irradiated with ultra-violet light in either of the two types of irradiators by flowing it over the exposed surfaces just as ordinary milk except that slightly higher power levels are required to overcome its increased opacity and viscosity. Some brands of evaporated milk are fortified with vitamin D by the addition of cod-liver oil concentrate, and also of irradiated or otherwise activated ergosterol. The milk is sterilized in the cans, just like regular evaporated milk, without detriment to the vitamin D. The potency is maintained at such levels that, when the evaporated milk is diluted to the consistency of regular milk, a quart of the irradiated and of the fortified milk will each contain 135 and 400 U.S.P. X units respectively. Clinical studies have demonstrated that this milk is effectively anti-rachitic.

Public-health relationship. The role of vitamin D in nutrition is to facilitate the assimilation of calcium and phosphorus. The average diet is low in these minerals; vitamin D locks them into the organism very effectively, thereby reducing loss in the excreta. Milk is a unique food in this respect because it contains calcium and phosphorus in abundance in easily assimilable form, it contains some natural vitamin D which can be greatly increased by several processes, and it is a food for infants, children, adolescents, and adults, thereby constituting an admirable vehicle for supplying both the vitamin and the minerals. The investigations of Lewis 2 showed that a smaller number of units of vitamin D is required when vitamin D milk is the source than when other rachitics are administered. This finding has been confirmed by Eliot et al .3

Some persons have feared that such a rich source of vitamin D places this milk in the class of a drug or medicine like viosterol or cod-liver oil. This objection is not warranted, because vitamin D milk carries the vitamin at a level of unitage that is only a rickets prophylactic and a calcifying aid for the mass of the population, and does not usurp the unique role of the concentrates in unusual cases where the services of a physician are needed.

Some people have feared that vitamin D milk may cause hypercalcemia (an excessive retention of minerals in the tissues) . This fear is unfounded. Bills and Wirick showed that 100 times the prophylactic dose of activated ergosterol is harmless for rats in long-time experiments, that 1000 times is just perceptibly harmful, that 4000 times overdosage is definitely injurious, and that 40,000 times overdosage is strongly toxic. The effects of massive doses of vitamin D on 64 dogs and 773 human subjects were studied by Steck and associates, who observed no intoxication when 20,000 units per kilogram were administered for indefinite periods. If milk were enriched with vitamin D to such an extent as to render it toxic, its flavor would be ruined.


Standards. It is customary to express the strength of a solution or preparation of vitamin D in terms of units. In the recent past, several different laboratory procedures were employed to determine these units, but, as no two units expressed the same nutritional strength or metabolic effectiveness of a given vitamin D preparation, there has been much confusion in the medical and biochemical literature in expressing potency and clinical usefulness. However, an international unit has now been adopted.

This unit is defined as the vitamin D activity of 1 milligram of the international standard solution of irradiated ergosterol. Portions were distributed to official control institutions in several nations. In this country, the depository is the U. S. Department of Agriculture in Washington, D. C.

The United States Pharmacopoeia (1934) adopted this international unit, effective as official in this country since January 1, 1935. It is known as the U.S.P. X (1934) vitamin D unit. The U. S. Pharmacopceial Vitamin Advisory Board secured the cooperation of the Bureau of Fisheries in preparing a sample of cod-liver oil to be known as the "reference cod-liver oil." This oil was then accurately standardized by a number of laboratories against the international standard of irradiated ergosterol. It is carefully stored to protect it against deterioration and it is rechecked semi-annually. This oil may be obtained from Dr. E. Fullerton Cook, 43rd Street and Woodland Ave., Philadelphia, Pa., at $2.50 for 30 milliliters.

Two of the most widely used unofficial units are the Steenbock and the A.D.M.A. units. The U.S.P. X unit being taken as equal to 1, the Steenbock unit equals 0.37 and the A.D.M.A. unit equals 3.25.

The error inherent in the technic of bioassay limits the accuracy of determinations to variations of about 10 percent on either side; accordingly, duplicate determinations may differ over a range of about 20 percent.

There is evidence that the rat unit is not a correct measure of the therapeutic effectiveness of vitamin D for all animals, but it seems clear that the rat unit does constitute a reasonably satisfactory measure of the relative effectiveness of different sources of vitamin D milks for human use.

Potency. Natural milk contains about 25 U.S.P. X units of vitamin D per quart—a little more in summer than in winter. Commercial vitamin D milk has been studied clinically by feeding it to more than 1000 infants during a score or more investigations. All these vitamin D milks were found to be helpful in preventing rickets, in improving calcification, and in promoting growth. However, the degree of their effectiveness was not equal. The confusion attendant on this situation led the Council on Foods of the American Medical Association to re-view the situation in an endeavor to ascertain whether the units by the respective processes possessed different degrees of therapeutic effectiveness and what constituted a reasonably effective level of unitage in a quart of milk.

Jeans carefully reviewed the clinical findings of all the available published investigations .6 He showed that 135 U.S.P. X vitamin D units to the quart of milk were effective to prevent rickets, but this amount of vitamin D approached closely the minimum effective level. He pointed out that prevention of rickets was not to be considered the criterion of vitamin D intake because this amount did not permit the best growth of infants, nor did it cause retention of calcium and phosphorus as great as was considered desirable. Levels of 400 U.S.P. X units of vitamin D were thought to be adequate for satisfactory retentions of calcium and phosphorus. The Council on Foods approved these presentations and announced that it would not accept any other vitamin D enriched common food than vitamin D milk. The Council allows no claim that 135 units to the quart of milk is adequate for infants, but it does allow claims that 400 units of vitamin D in milk is greater than usually required for the prevention of rickets in normal infants and provides a margin of safety when customary amounts of milk are taken.

The more recent work of Eliot and her associates has confirmed Jeans's conclusions that a dosage of 400 U.S.P. units is preferable to lower ones by providing a level nearer to the usual dose of cod-liver oil (namely, 2 or 3 teaspoonfuls), which gives a reasonable margin of safety for the rapidly growing infant. When milk was the carrier of the vitamin D, the potency was greater than that of cod-liver oil, even when the oil was at a higher dosage level, and the potency of one type of vitamin D milk at the 400-unit level was equal to that of another in the prevention of rickets.

Labeling. On account of the newness of the problem, the con-fusion attendant on the relative valuation of the units, the earlier uncertainty as to the relative therapeutic effectiveness of the different sources, and the difficulty of officially controlling the level of vitamin D units introduced into milk, the regulatory officials have been slow to adopt regulations for the production and labeling of vitamin D milk.

The New York City Department of Health 8 requires that the outer caps and labels shall, in addition to the regular information, bear the words, "Vitamin D Milk," and shall carry statements as to the method by which the vitamin D content was increased and the minimum number of vitamin D units in a quart of the milk. Moreover, the caps shall be covered with a hood which completely covers the pouring lip of the bottle. The Council on Foods requires that the labels state the source of the vitamin D and the number of units.

Although vitamin D milk is sold by several hundred dairies in most (if not all) of the states and many communities, the regulatory officers of several cities prohibit its sale. Generally, their objection is based on the difficulty in measuring the number of vitamin D units by the limited laboratory means at the disposal of the health officer, the lack of unanimity as to what should be the level of unitage, the legal uncertainty as to just what a "vitamin D milk" is, and conservatism in permitting a product to be sold in milk on which so much more clinical research remains to be done. The merchandising abuses which have been practiced by some vendors of vitamin D products have fostered reluctance by some regulatory officials and refusal by others to permit the sale of vitamin D milk in their territories.

Laboratory bioassay methods. Several methods have been devised to determine the number of vitamin D units in milk. They all depend on the estimation of the amount of calcium phosphate in the bones of the rat.

The most suitable method for general control work is the bioassay technic known as the "line test." It was developed in McCollum's laboratory in connection with his discovery that rickets can be experimentally produced and controlled by dietary manipulation. It depends on the appearance of the epiphyseal cartilage in the neck of the long bone. This is the area where, in normal healthy growth, calcium phosphate is deposited for the formation of bone cells, whereas in a rachitic condition there is little or no such deposition but only a wide cartilaginous band. The measurement of the degree of calcification of this cartilage band constitutes the line test. This name is given because the examination is made on a section of the bone, and the degree of calcification appears as a more or less continuous line. See details of the official method of bioassay in the Journal of Milk Technology, 1, 49 (March, 1938).

Only a relatively few laboratories are equipped to make these bioassays. It is customary for the licensing agencies to designate reputable biochemical laboratories where dealers of vitamin D milk may send their milk to be bioassayed. Usually, arrangements are made for surprise samples to be collected by an impartial agency. This is the local health department when it is interested (as it usually is) or the messenger service of one of the large telegraph companies. The bottle of vitamin D milk is collected from a delivery wagon, sealed, and packed in an iced shipping container for forwarding to the bioassay laboratory. Usually, a 1-quart bottle is an adequate sample. This may be preserved with 2 drops of a 10 percent formalin solution and suitably refrigerated."

The amount of vitamin D in milk is determined preferably on the cream or butterfat which is removed from this milk. This is done in order to obviate any disturbing effect which the calcium and phosphorus of the milk may exert on the rachitogenic ration. Such a procedure is justified because the vitamin is fat soluble and does not reside in the non-fat portion of milk.

Detailed instructions for making the bioassay of vitamin D milk will appear in the forthcoming Standard Methods for the Examination of Dairy Products (seventh edition of Standard Methods of Milk Analysis).

Control procedure. Although several firms sell products or a process for the fortification of milk with vitamin D, only three overwhelmingly dominate their respective fields in metabolized milk, irradiated milk, and fortified milk. In each case, the contract of sale between the supplying firms and the purchasing dairy stipulates that license to use the given process is issued on the dairy's assurance that it will produce full potency as claimed, that high quality of product will be maintained, and that surprise samples of the treated milk will be bioassayed in reputable laboratories at stated and regular intervals. The Wisconsin Alumni Research Foundation controls the metabolized- and irradiated-milk processes, and Columbia University owns the process of making the cod-liver oil concentrate. Both these organizations require that the firms which operate under their respective licenses enforce the above quality standards on pain of revocation of license or refusal to sell. By these means, the merchandising of vita-min D milk has not been attended with some of the abuses perpetrated by the sale of other vitaminized foods.

The bioassay reports themselves are made in multiplicate, and copies are sent to the licensing institution, to the licensor company, to the health officer, to the Council on Foods of the American Medical Association if the given vitamin D milk has been accepted by them, and to the dairy which sold the milk. One state and one city have provided their own laboratory facilities for bioassaying this milk. Else-where this work is usually done by several state agricultural experiment stations or by the biochemical departments of approved colleges.

All copy for advertising by the dairies is reviewed by the licensing company, which also sends representatives to instruct the wagon drivers (milk salesmen) as to the proper claims that they are to make in retail selling.

In addition to this control by the commercial licensing organizations, the dealers in vitamin D milk are under the same regulatory control by the local health officials as they are for selling any other milk product.


1. J. H. SHRADER, Arch. Phys. Therapy, X-ray, Radium, 15, 709 (1934).

2. J. M. LEwis, New York State J. Med., 34, 685 (1934).

3. M. M. ELIOT and associates, J. Pediat., 9, 355 (1936).

4. C. E. BILLS and A. M. WIRICK, J. Biol. Chem., 86, 117 (1930).

5. I. E. STECK et al., Ann. Internatl. Med., 10, 9M (1937).

6. P. C. JEANS, J. Am. Med. Assoc., 106, 2066, 2150 (1936).

7. Council on Foods, ibid., 108, 206 (1937).

8. Department of Health, New York City, Miscl. 17, 1935, p. 62.

9. W. B. PALMER et al., Am. Pub. Health Assoc. Year Book, 1934-5, p. 62.

10. J. Assoc. Offic. Agr. Chemists, 20, 78 (1937).

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