Vitamin D is one of the most essential vitamins for our health, and is also one of the most common vitamins people are deficient in. Anyone with a chronic disease should have their vitamin D levels checked by their doctor and take supplements if they are found to be deficient. According to prevention.com, low levels of vitamin D can contribute to many different health conditions and slow down the healing process for others. MORE: Eating a balanced diet is sensible advice for anyone, but it is even more important if you’ve been diagnosed with cancer. Breast Cancer  Breast cancer patients who have normal levels of vitamin D are twice as likely as those with low levels to survive the disease. There is also a correlation between other cancer patients' survival rate and their vitamin D levels. Aggressive Prostate Cancer A study found that men with low levels of vitamin D were four to five times more likely to have aggressive prostate cancer than those with healthy levels. Alzheimer's Disease Vitamin D deficiency has also been linked to an elevated risk of developing dementia or Alzheimer's disease. The risk of Alzheimer's increases the lower your vitamin D levels are. As you age, skin becomes less adept at turning the sun's rays into vitamin D so you're more likely to have a deficiency, increasing your risk of developing dementia and Alzheimer's later in life. Depression People with low levels of vitamin D are twice as likely to develop depression. Researchers believe that areas in the brain responsible for controlling mood have receptors for vitamin D, so not receiving enough vitamin D may lead to these areas under-performing. MORE: Here are five foods that could help fight breast cancer.  Heart Disease People with low levels of the vitam