Fatigue, even after getting enough sleep, is a general sign that can indicate poor absorption of several nutrients. Low iron, low magnesium, and low intake of calories, in general, can all cause persistent fatigue.

The muscles of the small intestine mix food with digestive juices from the pancreas, liver, and intestine, and push the mixture forward for further digestion. The walls of the small intestine absorb water and the digested nutrients into your bloodstream.

The body uses the nutrients for energy, growth, and cell repair.

Digestion is a process where the body breaks down food into smaller particles to absorb them into the bloodstream. Complete digestion of food takes anywhere between 24 to 72 hours and depends on several factors, including the type of food eaten and the presence of digestive issues.

Having a weak gut lining, food allergies, microbiome imbalances such as bacterial overgrowth, damage to the intestines from infection, surgery, pancreatic insufficiency, autoimmune disease–all of these are possible causes that lead to poor nutrient absorption.

Providing you don’t have underlying medical conditions as outlined above, there are a few ways to improve your nutritional absorption.

1. Eat a variety of foods in one meal.
2. Pair vitamin C-rich foods with iron.
3. Include healthy fats with each meal.
4. Take a probiotic.
5. Avoid drinking tea at mealtimes.

Foods that naturally are nutrient-rich include fruits and vegetables. Lean meats, fish, whole grains, dairy, legumes, nuts, and seeds also are high in nutrients.

“Digestion slows down during sleep, so taking your nutrient supplement late at night would not be associated with an efficient absorption.” Neil Levin, a clinical nutritionist at NOW Foods, agrees that morning is best for multivitamins and any B vitamins.

Any beverage or food containing caffeine such as coffee, tea, chocolate and some sodas can inhibit the absorption of vitamins and minerals and increase their excretion from the body.

For maximum absorption, the best time to take vitamin D and other fat-soluble vitamins is after you’ve eaten foods that contain fat. And you don’t need much: Dr. Perez-Gallardo says even small amounts of low- or whole-fat milk or yogurt will do the trick. So will eating food cooked with oil.

Most people do not commonly experience side effects with vitamin D, unless too much is taken. Some side effects of taking too much vitamin D include weakness, fatigue, sleepiness, headache, loss of appetite, dry mouth, metallic taste, nausea, vomiting, and others.

Vitamin D deficiency can lead to a loss of bone density, which can contribute to osteoporosis and fractures (broken bones). Severe vitamin D deficiency can also lead to other diseases. In children, it can cause rickets. Rickets is a rare disease that causes the bones to become soft and bend.