1 How Dietary Deficiency Studies Have Illuminated the Many Roles …
5
When offspring of the carotene-deficient chickens were also fed the carotene-free
diet from the time of hatching, all died within three months. This series of experiments
suggested to Palmer that there was no correlation between yellow pigment and the
growth, fecundity, or reproduction of chickens, at least for one generation (Palmer
1919).
Jack Drummond and Katharine Coward used a different approach to determine
if vitamin A was associated with any yellow pigments. They generated vitamin A
deficient rats with failing health and then fed them the same deficient diet supplemented with butter, animal fat, lard, or one of nine plant oils. A rough approximation
of vitamin A content was determined based on the growth promoting capacity of the
supplement relative to butter (arbitrarily considered 10). Carotene and xanthophyll
content were estimated by evaluating the color of the unsaponifiable fraction obtained
from 10 g of each fat or oil. The results were interesting. Lard, cottonseed, peanut,
sesame, and olive oils were devoid of lipochrome pigments and showed essentially
no growth promoting activity. Cod liver oil was equivalent to butter in its growth
promoting activity, but had no detectable lipochrome pigments. Linseed oil, palm
oil, and maize oil had both growth promoting activity and relative high levels of
carotene and xanthophyll. Although still puzzling, these observations pointed to an
interpretation that lipochrome pigments seemed connected to vitamin A, but were
not themselves, vitamin A (Drummond and Coward 1920).
This new interpretation was born out by an experiment carried out by Moore, who
reared young albino rats on a vitamin A deficient diet until their condition indicated
all reserves of vitamin A were depleted. Some rats were killed at this point, while
others were fed a diet supplemented with purified carotene and killed only after
complete cures were observed. Using colorimetric and spectrophotometric analyses
that discriminated between carotene and vitamin A, Moore’s results showed that only
the livers of rats receiving excessive doses of purified carotene had measureable levels
of vitamin A. From these experiments, Moore concluded that carotene produced
similar biological outcomes as vitamin A because it served as a precursor to vitamin
A. Moore ended his report in Biochemical Journal with this visionary comment,
The fact that a widely-distributed plant pigment such as carotin has an important biological
function in the maintenance of animal life is in itself a most noteworthy finding. It can be
safely prophesied that further elucidation of its role in the pathology and physiology of the
cell and higher organisms will occupy the attention of a host of investigators for years to
come (Moore 1930).
Subsequent experiments demonstrated that in contrast with carotene, which is
strongly pigmented and which exhibits multiple absorption maxima at 495, 463, 436,
348 and 280 nm, vitamin A is a colorless, unsaturated alcohol with a broad spectral absorption band extending from 280 to 370 nm, with a maximum near 328 nm
(Morton and Heilbron 1928; Morton and Spring 1930). Paul Karrer and coworkers
established the chemical structure of β-carotene in 1930 and proposed a structure
for vitamin A in 1931 that was experimentally confirmed by Heilbron, Morton and
Webster in 1932 (Karrer et al. 1931; Heilbron et al. 1932).
The outcome of studies seeking the identity of the unknown factor, vitamin A,
that began in the late 1800s and that continues today, has been the development of
5
When offspring of the carotene-deficient chickens were also fed the carotene-free
diet from the time of hatching, all died within three months. This series of experiments
suggested to Palmer that there was no correlation between yellow pigment and the
growth, fecundity, or reproduction of chickens, at least for one generation (Palmer
1919).
Jack Drummond and Katharine Coward used a different approach to determine
if vitamin A was associated with any yellow pigments. They generated vitamin A
deficient rats with failing health and then fed them the same deficient diet supplemented with butter, animal fat, lard, or one of nine plant oils. A rough approximation
of vitamin A content was determined based on the growth promoting capacity of the
supplement relative to butter (arbitrarily considered 10). Carotene and xanthophyll
content were estimated by evaluating the color of the unsaponifiable fraction obtained
from 10 g of each fat or oil. The results were interesting. Lard, cottonseed, peanut,
sesame, and olive oils were devoid of lipochrome pigments and showed essentially
no growth promoting activity. Cod liver oil was equivalent to butter in its growth
promoting activity, but had no detectable lipochrome pigments. Linseed oil, palm
oil, and maize oil had both growth promoting activity and relative high levels of
carotene and xanthophyll. Although still puzzling, these observations pointed to an
interpretation that lipochrome pigments seemed connected to vitamin A, but were
not themselves, vitamin A (Drummond and Coward 1920).
This new interpretation was born out by an experiment carried out by Moore, who
reared young albino rats on a vitamin A deficient diet until their condition indicated
all reserves of vitamin A were depleted. Some rats were killed at this point, while
others were fed a diet supplemented with purified carotene and killed only after
complete cures were observed. Using colorimetric and spectrophotometric analyses
that discriminated between carotene and vitamin A, Moore’s results showed that only
the livers of rats receiving excessive doses of purified carotene had measureable levels
of vitamin A. From these experiments, Moore concluded that carotene produced
similar biological outcomes as vitamin A because it served as a precursor to vitamin
A. Moore ended his report in Biochemical Journal with this visionary comment,
The fact that a widely-distributed plant pigment such as carotin has an important biological
function in the maintenance of animal life is in itself a most noteworthy finding. It can be
safely prophesied that further elucidation of its role in the pathology and physiology of the
cell and higher organisms will occupy the attention of a host of investigators for years to
come (Moore 1930).
Subsequent experiments demonstrated that in contrast with carotene, which is
strongly pigmented and which exhibits multiple absorption maxima at 495, 463, 436,
348 and 280 nm, vitamin A is a colorless, unsaturated alcohol with a broad spectral absorption band extending from 280 to 370 nm, with a maximum near 328 nm
(Morton and Heilbron 1928; Morton and Spring 1930). Paul Karrer and coworkers
established the chemical structure of β-carotene in 1930 and proposed a structure
for vitamin A in 1931 that was experimentally confirmed by Heilbron, Morton and
Webster in 1932 (Karrer et al. 1931; Heilbron et al. 1932).
The outcome of studies seeking the identity of the unknown factor, vitamin A,
that began in the late 1800s and that continues today, has been the development of