We have more than one nervous system – the central nervous system, which is made up of our brain and spinal cord, and the enteric nervous system, which comprises the nerve tissue in the gut. This is sometimes referred to as the "second brain."

These two nervous systems are connected through the vagus nerve, which travels from the brain to the abdomen, and sends feedback information to the brain, from the gut, and back again in a bi-directional loop. This nerve connects the two brains together and has a profound influence on our mental health. When we have an imbalance in the gut this connection can be disrupted.

We share our life with about 100 trillion organisms that comprise what is called our microbiome. It is made up of good and bad bacteria, fungi and viruses, called microbiota, that help us to perform life-sustaining functions. They inhabit everything from our skin and genitals to our intestines. In the gut these clusters of microbiota are known as the "gut flora".

Our gut flora is fundamental to the breakdown and absorption of nutrients. Without it the majority of our food would not only be indigestible, but we would not be capable of extracting the critical nutritional compounds (vitamins and minerals) needed to function.

Antibiotics, refined sugar, NSAIDs, medications, medical illnesses, processed foods, chronic emotional stress, and pesticides can contribute to a decrease in gut flora.

Environmental factors influencing nutrient supply include the micronutrient content of soil. The more nutrient rich the soil is the higher the nutrition value of foods grown from it. In developed countries there is a substantial depletion of many essential micronutrients in crops, caused by modern agricultural methods. These methods fail to resupply the trace minerals and vitamins that are essential in the human diet. Another factor is the use of herbicides, which often contain glyphosates that bind certain minerals and reduce their bioavailability in the soil.

Sun exposure is critical for Vitamin D synthesis, so indoor work; lack of outdoor recreation, illness, and bedridden status will reduce synthesis and increase the need for dietary supplementation.

Cultural factors affecting nutrient supply include cooking methods, cuisine, and local food patterns. For example vegetarians are advised to take oral micronutrient supplementation. Inadequate food intake whether due to poverty or dieting will also alter the micronutrient status of an individual. Alcohol and recreational drugs, as well as processed foods and sugar with low nutrient density will reduce nutrient absorption.

One of the most important of all factors affecting micronutrient requirements in an individual is genetics (including epigenetics). An individual's genetic polymorphisms affect genetic differences in all aspects of physiologic function. This factor is called biochemical individuality in some nutritional circles and is a critical factor governing the micronutrient requirements of an individual.

A woman's nutrient requirements increase substantially during pregnancy. This is to ensure that the fetus gets an adequate supply. or e.g.: folic acid is important because it can help prevent birth defects. Iron is used to make more blood to supply oxygen to the baby. Choline is a critical nutrient during pregnancy and breastfeeding for neurocognitive development. Iodine is required for a healthy brain and nervous system in the fetus, as well as the production of thyroid hormones for both mother and baby.