Final answer:
The statement that polyneuritis is a symptom of an excessive intake of thiamin is false; all other statements are true regarding thiamin's discovery, the deficiency disease beriberi, and its food sources like lean pork.
Step-by-step explanation:
The true statements about thiamin, also known as vitamin B1, are that it was the first B vitamin to be discovered, a deficiency in thiamin is called beriberi, and the most nutrient-dense sources of naturally occurring thiamin are foods such as rice bran, wheat bran, whole grains, nuts, and lean pork. Symptoms of thiamin deficiency can include anorexia, polyneuritis, edema, and cardiovascular changes.
Symptoms of excessive intake, leading to polyneuritis, are generally not a concern as thiamin is excreted in the urine and the body does not store it. Thiamin is involved in vital physiological functions such as the utilization of carbohydrates, maintenance of appetite, and normal brain metabolism. It must be regularly included in the diet as the body does not store thiamin long-term, with any excess being excreted.