The brain depends on many nutrients for function and performance. Importantly, the brain cannot manufacture its own nutrients and therefore depends on delivery of these compounds from the body. The body, of course, gets its source of nutrients from the diet. Therefore, what we choose to eat can either nourish the brain or starve the brain.
The metabolism of the brain requires glucose for its energy and the brain is the only organ besides the heart that can suffer when glucose supply is too low. In fact, glucose, along with oxygen, are easily transported into the brain while the blood-brain-barrier slows the entry of other nutrients such as fatty and amino acids. Fatty acids account for only a small amount of the energy supply in the adult brain. Amino acid breakdown accounts for less than 10% of total brain energy requirements and can only be utilized to a limited extent. This is why a ketogenic diet can cause fatigue, lethargy, and difficulty with concentration and focus. To convert glucose into energy there are several other necessary vitamins including magnesium, riboflavin, pyridoxine, iron, and lipoic acid.
When we eat a healthy whole foods plant-based diet, the supply of glucose is usually sufficient. When we do not, we can live in a state of glucose deficiency. When we are deficient in glucose supply to the brain, not only does the brain have to utilize non-glucose substrates, such as ketones, lactate, and pyruvate, there is an associated increase in GABA levels. GABA is an important inhibitory neurotransmitter that inhibits the activity of neuronal cells and may be responsible for the difficulty in learning and memory when our brain is not receiving enough energy. This can also cause irritability. To minimize the effects of transient states of glucose deficiency, and to try and avoid the need to use other forms of non-glucose energy, the astrocytes of our brain store glycogen which can be converted to glucose during deficient or fasting states. In addition, a glucose depleted state will trigger the adrenal glands and the autonomic nervous system into a state of neoglucogenesis – making new glucose from non-carbohydrate sources.
When we become stressed we have a surge of epinephrine and there is a concomitant increase in glucose because when we are stressed we need to have the ability to focus, concentrate, and make decisions. Optimal functioning of the brain will allow us to be productive, content, and relaxed. Avoiding all sources of carbohydrates can create a low supply of the brain’s preferred source of energy. Complex carbohydrates offer the body – and the brain – the energy it so desires. It is not primarily about the insulin but rather what triggers the insulin release – the type and quantity of the carbohydrates we consume.