While the jury is still out when it comes to the health benefits of vitamins, it still stands that people are more likely to be deficient in some vitamins more than others. Take vitamin A, the name for a fat-soluble group of retinoids, for example.
Although vitamin A is essential for healthy vision, immune function, clear skin, and a well-regulated thyroid, most Americans don't get their recommended daily intake of it, according to the Office of Dietary Supplements (700 micrograms recommended for women, compared to an average of 580 consumed). The problem is even more widespread in underdeveloped countries.
"This fat-soluble vitamin plays an important role in the regulation of thyroid hormones, and not getting enough vitamin A in your diet is associated with reduced levels, according to a 2012 study published in the Journal of the American College of Nutrition," said Jill Corleone, a registered dietitian and health coach, on Livestrong. "Thyroid hormones regulate how your body's cells use energy, which affects the rate of your metabolism. When thyroid hormone levels are too low, parts of your body slow down, and this may cause a decrease in your metabolism."
Considering we can get our necessary vitamins through food alone, Corleone recommends consuming 700 to 900 micrograms of vitamin A a day to "keep your metabolism going at a good clip." Vitamin A–rich foods include sweet potatoes, broccoli, carrots, spinach, beef liver, squash, dried apricots, bell peppers, and pumpkin pie.
If your doctor has confirmed that you have a vitamin A deficiency, they could recommend a vitamin A supplement to balance you out—though your daily intake should never exceed 1500 to 3000 micrograms. "Because vitamin A is fat soluble, the body stores excess amounts, primarily in the liver, and these levels can accumulate," explains the Office of Dietary Supplements. "When people consume too much vitamin A, their tissue levels take a long time to fall after they discontinue their intake, and the resulting liver damage is not always reversible."
For more, read up on the "natural" health supplements that actually do more harm than good.