What are the warning signs of vitamin B12 deficiency? Symptoms of Vitamin B12 Deficiency Weakness, tiredness, or lightheadedness. Heart palpitations and shortness of breath. Pale skin. A smooth tongue. Constipation, diarrhea, loss of appetite, or gas. Nerve problems like numbness or tingling, muscle weakness, and problems walking. Vision loss. How can I raise my B12 levels fast? To increase the amount of vitamin B12 in your diet, eat more of foods that contain it, such as: Beef, liver, and chicken. Fish and shellfish such as trout, salmon, tuna fish, and clams. Fortified breakfast cereal. Low-fat milk, yogurt, and cheese. Eggs. How serious is a B12 deficiency? If left untreated, the symptoms of vitamin B12 deficiency tend to worsen and irreversible problems involving the nerves and brain may develop. The risk of experiencing a number of serious complications, including heart failure, may also increase. What is the main cause of vitamin B12 deficiency? However, the most common ...