Vitamin B12 is an essential nutrient that helps the body in many ways. It is essential for Red Blood cell formation, DNA production, Nerve functionality and more. Low levels of vitamin b12 can cause symptoms of muscle weakness, tingling sensation in hands and feet, nausea, decreased appetite, weight loss, tiredness and psychological problems. Testing your b12 levels determines whether you are getting enough nutrients from your diet or whether you need to supplement it with medication.