The combination of supplements does not usually interfere with their functioning and, in some cases, may even be beneficial. For instance, vitamin C helps the absorption of iron, while vitamin D and vitamin K (especially K) complement each other. According to Dr. David Jenkins, taking these vitamins and minerals in moderation usually causes no harm.
However, the National Institutes of Health warns that taking high doses of vitamin C can lead to stomach cramps and diarrhea.Research has suggested that possible reasons for deficiency include a lower dietary intake of vitamin D among obese men and women compared to non-obese individuals, the possibility that obese people will expose their skin less to the sun (not only due to worse absorption), and a decrease in the absorption rate of vitamin D among people who have undergone bariatric or gastric bypass procedures. Some providers suggest combining iron with vitamin C to maximize absorption, but the jury is still out on this. If tests show that your vitamin D level is normal, you'll need to repeat the tests every two to three years, unless you have major changes in your overall health. However, very few foods have enough vitamin D to meet the recommended daily intake, and sunlight can be unreliable in certain climates. For patients with low bone mass, treatment with an antiresorptive agent is needed or, if bone mass is very low and there are fractures, an anabolic agent is needed in addition to vitamin D.
In addition, some vitamins may interfere with the effectiveness of the medication and even exacerbate side effects. For years, health care providers have recommended that postmenopausal women take calcium and vitamin D supplements to prevent osteoporosis, a disease that weakens bones and is a major cause of devastating fractures in old age. Compared to people with lighter skin, black people have more melanin in their skin, which affects the body's ability to synthesize vitamin D.That said, Kitchin said that a multivitamin can help compensate for some deficiencies in a person's diet, especially if they avoid certain food groups such as meat or dairy products. Men and women who rely on vitamin D solely through their diet do not usually exceed 288 IU a day on average. If you choose the supplement route, Clifford recommends taking between 400 and 800 IU of vitamin D3 (cholecalciferol) a day.
Whether you're taking a daily multivitamin to improve your overall health or depending on herbal supplements to fill nutritional gaps, it's important to be aware of potential interactions. Even if none of the supplements separately exceeds the maximum limit for a given nutrient, combining several pills - such as a multivitamin and an additional vitamin D capsule - can result in higher doses than recommended. So if you're already taking dietary supplements for other reasons, it's important to consider whether you need additional vitamin D as well. It's best to consult with your doctor or nutritionist before adding any new supplement to your routine.