Final answer:
Excess niacin consumption often leads to diarrhea, among other symptoms like dermatitis and peptic ulcers. It's crucial to consume vitamins within the recommended limits to prevent toxicity.
Step-by-step explanation:
The main sign of consuming excess amounts of niacin is symptom B. Diarrhea. While niacin is essential for good health, its overconsumption can lead to a range of negative side effects, known as niacin toxicity. In particular, symptoms of too much niacin intake can include diarrhea, skin conditions such as dermatitis, and in more severe cases, issues like peptic ulcers and hyperglycemia. It is important to adhere to recommended dietary allowances and tolerable upper intake levels to avoid such adverse effects.