Iron, magnesium and fish oil supplements are the most common culprits of digestive discomfort when taken on an empty stomach, so it is important to be mindful when consuming them with a meal or snack. Water-soluble vitamins, such as vitamins C and B, including folate, are often recommended to be taken on an empty stomach. This is because the body takes what it needs from these vitamins and excretes the rest through the urine, meaning that it doesn't need food to be absorbed. It is beneficial to take these types of vitamins as soon as you wake up, before a meal, or two hours after a meal.
Since these vitamins require water to break down, it is important to take them with water so that the body can use them to their full capacity. Iron is another important supplement that is better absorbed when taken on an empty stomach, one to two hours before a meal. Some doctors suggest taking iron with orange or apple juice to aid absorption. Vitamins A, D, E and K are fat-soluble vitamins and should not be taken on an empty stomach.
The best time to take vitamin D and other fat-soluble vitamins is after eating foods that contain fat, according to Cleveland Clinic. It is beneficial to take supplements at the same time every day, to develop a healthy habit and ensure that the body receives the necessary nutrients on a daily basis. If you still experience nausea even while eating food with your multivitamin, you may need to change your supplement strategy. If taking any of your supplements on an empty stomach causes any type of irritation, try eating a very light snack (e.g., if a supplement makes you nauseous, such as prenatal multivitamins), take it before bed so you can sleep through the side effects.
If you're concerned, consult a health professional before taking dietary supplements or making major changes to your diet.