Ideally, one should purchase vitamins with a reliable hechsher. For example, the Star-K currently certifies various Shaklee vitamins and nutritional supplements rich in vitamins and minerals. Much time is spent reviewing ingredients, processes and equipment. A mashgiach inspects the factory on a regular basis to ensure all is in order. Unfortunately, kosher certified vitamins are not always readily available to the consumer.
Natural products are often non-kosher.
In today's society, all natural has become a buzzword meaning pure or healthy. This does not mean the product is necessarily kosher. A product may be used as kosher only if the store or the package bears a reliable hechsher, or a Rav knowledgeable in kashrus laws and manufacturing procedures, has carefully researched and approved the product. The same ingredient may be kosher or non-Kosher, depending on it's source. Ingredient panels should be reviewed by someone familiar with Kashrus.