Human Physiology, 14th edition (2016)

Regulation of Metabolism 667

acid). This is similar to the way the thyroid hormone receptor

(for triiodothyronine) must dimerize with the RXR receptor to

activate its target genes (chapter 11, section 11.2; see fig. 11.7).

This overlapping of receptors may permit “crosstalk” between

the actions of thyroid hormone, vitamin D, and vitamin A.

In view of this, it is not surprising that thyroxine, vitamin A, and

fig. 11.7) and directly produces the effects of vitamin A. Retinoic
acid is involved, for example, in regulating embryonic develop-
ment; vitamin A deficiency interferes with embryonic development,
while excessive vitamin A during pregnancy can cause birth defects.
Retinoic acid also regulates epithelial membrane structure and func-
tion, and has various effects on the immune system that include the
induction of regulatory T lymphocytes (chapter 15, section 15.3).
These actions may account for the effectiveness of retinoids in treat-
ing acne and other skin conditions.
Vitamin D is produced by the skin under the influence of ultra-
violet light, but usually it is not produced in sufficient amounts for
all of the body’s needs. That is why we must eat food containing
additional amounts of vitamin D, and why it is classified as a vita-
min even though it can be produced by the body. The vitamin D
secreted by the skin or consumed in the diet is inactive in its
original form; it must first be converted into a derivative (1,25-
dihydroxyvitamin D 3 —see fig.  19.21 ) by enzymes in the liver
and kidneys before it can be active in the body. Once the active
derivative is produced, vitamin D helps regulate calcium balance,
primarily by promoting the intestinal absorption of calcium.
The nuclear receptor protein for 1,25-dihydroxyvitamin D 3
cannot activate a gene unless it forms a dimer with the RXR recep-
tor, which binds to the active form of retinoic acid (9- cis retinoic

CLINICAL APPLICATION

Psoriasis is an inherited skin condition characterized by

inflammation and excessive proliferation of keratinocytes (cells

of the epidermis that produce the protein keratin). Immune

responses, particularly those of T cells, are central to the

development of psoriasis. Vitamin D, applied topically on the

skin and/or taken as an oral supplement, and ultraviolet B light,

which stimulates endogenous vitamin D production, are com-

mon psoriasis treatments because of the ability of vitamin D to

reduce inflammation. It may do this by modifying gene expres-

sion and by inducing the formation of regulatory T cells (chap-

ter 15, section 15.3). Hypercalcemia is a possible side effect of

too much oral vitamin D because of its stimulation of intestinal

calcium phosphate absorption.

Vitamin Sources Function Deficiency Symptom(s)

A Yellow vegetables and fruit Constituent of visual pigment;

strengthens epithelial membranes

Night blindness; dry skin

B 1 (Thiamine) Liver, unrefined cereal grains Cofactor for enzymes that catalyze

decarboxylation

Beriberi; neuritis

B 2 (Riboflavin) Liver, milk Part of flavoproteins (such as FAD) Glossitis; cheilosis

B 6 (Pyridoxine) Liver, corn, wheat, and yeast Coenzyme for decarboxylase and

transaminase enzymes

Convulsions

B 12 (Cyanocobalamin) Liver, meat, eggs, milk Coenzyme for amino acid metabolism;

needed for erythropoiesis

Pernicious anemia

Biotin Egg yolk, liver, tomatoes Needed for fatty acid synthesis Dermatitis; enteritis

C Citrus fruits, green leafy

vegetables

Needed for collagen synthesis in

connective tissues

Scurvy

D Fish, liver Needed for intestinal absorption of

calcium and phosphate

Rickets; osteomalacia

E Milk, eggs, meat, leafy

vegetables

Antioxidant Muscular dystrophy (nonhereditary)

Folate Green leafy vegetables Needed for reactions that transfer

one carbon

Sprue; anemia

K Green leafy vegetables Promotes reactions needed for function

of clotting factors

Hemorrhage; inability to form clot

Niacin Liver, meat, yeast Part of NAD and NADP Pellagra

Pantothenic acid Liver, eggs, yeast Part of coenzyme A Dermatitis; enteritis; adrenal insufficiency

Table 19.3 | The Major Vitamins