“Energy” is defined in science as the ability to do work. For example, “Person A is able to lift 50 ft./lbs. over a one-minute period.”
In the field of nutrition “energy” refers to calories or kilocalories. For example, “Fat provides 9 calories/gram, while carbohydrate provides 4 calories/gram.” A person burns energy/calories as a part of sustaining life, maintaining body temperature, and during activities.
These two definitions do not seem to totally describe mental or emotional energy as experienced by individuals.
During workshops held by the International Life Sciences Institute in 2004, scientists discussed the concept of “mental energy.” The group defined mental energy as the ability to perform mental tasks, the intensity of feelings of energy/fatigue, and the motivation to accomplish mental and physical tasks. Core aspects of mental energy include 1) cognition (knowledge that is gained through perception, reasoning or intuition), 2) mood (a state of mind or feeling) of energy, and 3) motivation (an incentive for action). The group noted that factors that can influence mental energy include, among others, genetics, nutrition, pain, and sleep.
Hypoglycemia is an example of nutrition affecting mental energy. When a person does not eat carbohydrates at the right time in the right amount, blood sugar (glucose), and then brain glucose, goes down. This may result in foggy thinking, feeling weak, shaky, or emotional.
Vitamins and minerals are essential for converting the energy (calories) in food to the form of energy used by the cells (ATP). If any of these vitamins or minerals are in short supply, this energy conversion isn’t sufficient and people feel tired, unable to motivate themselves to act and do what they want to do. The energy that isn’t converted to ATP is channeled into fat storage. Examples of vitamins and minerals that are essential for converting food energy to ATP include thiamin (vitamin B-1), riboflavin (vitamin B-2), ascorbic acid (vitamin C), iron, and magnesium.
If a person feels low in mental energy or “tired all the time,” they need to assess their nutritional intake as well as sleep, health, medication side effects, and overall mental health.