The obvious answers are food and water, and then next we may answer with the suns energy, and oxygen in the air we breathe through our lungs and skin.
But it goes much much deeper than this. We are also nourished by resources of energy inside and outside of our perceived physical bodies. Firstly with the energy of emotion, and mental processes, and by electromagnetic forces, sound and light, and by exchanges occuring with other dimensional planes of existence.
The term "emotion" could be referred to as e-motion, "energy in motion". There are vast opportunities for using what we percieve as negative emotions such as shame, guilt, anger, resentment, and fear as energy resources. It is possible to turn our upsets into energy that can actually serve us in positive ways, rather than letting it drain us. The mental processes that often fuel emotion are also resources.
Electromagnetic forces exist as a consequence of minerals and water above and beneath the ground for example. There are also energetic exchanges in our bodies in response to man-made structures and energetic devices, and forces created by the gravitational pull of other planets and stars. The gravitational pull of the moon not only governs our ocean tides on earth, it also pulls on the water in our bodies, and changes our biochemistry for example.
There are also influences on our energy that relate to the nature of the density in which our percieved reality is centred. Quantum Physics has proven objects can exist in more than one place at once, but this is only skimming the surface of explaining the multidimensional nature of energy that forms our physical and energetic bodies here on Earth. Its is a well known saying "We are spiritual beings having a human experience".
Why is lack of energy the root cause of disease?
The words "lack of energy" attempt to explain an imbalance of energy. It's not that the energy does not exist, it is existing in a form that is inaccessible or unavailable.
From the beginning of our existence, the design of our bodies ensures our best chance of survival. This means that our bodies have an incredible wisdom that is largely subconscious to us, which makes decisions on how to function in any situation. This of course includes how we breathe, digest food, how we move our bodies, and how we use our senses, and how we process what happens to us.
Most of the decision-making for the way we function and behave on a daily basis comes from our subconscious. Essentially, our subconscious mind protects us and ensures we survive. A person trying to drown on purpose will experience the power of their subconscious mind as it ensures their survival, and makes that kind of ending to life very difficult.
To keep us safe, it is mainly our subconscious mind that manages any stressors, toxins and traumas that we experience. In ideal circumstances, and on the most part for many of us, the toxins created during cellular processes or taken up by the body are dealt with, causing minimal problems for the body. Stressors such as dehydration cause the person to seek out the water it needs, and trauma such as injuries or arguements are generally felt, healed and forgiven.
However, in the case of disease, the body for some reason has not been able to access enough energy to naturally release, detox or transform. The subconscious mind protects the body from damage by those stressors, toxins and trauma, and in doing so, uses energy resources that would otherwise keep the person feeling vibrant. Essentially, energy is being used to protect the body, rather than heal it. When the body knows it's safe to let its protection down, and energy is accessible to make that shift safely and relatively painlessly, then long term healing can occur.
Here are some examples of how the body protects itself from stressors, toxins and trauma. For example, when the organs swell to protect the tissues from the inflammatory effects of candida fungus and gluten; when high cholesterol is in the bloodstream in an attempt to prevent bleeding of the walls of veins and arteries being scratched by undissolvable table salt; when the fat in the brain, and fatty deposits around the rest of the body accumulate toxins, such as aluminium, arsenic and mercury to keep them out of the bloodstream; when there is poor uptake/digestion of water because it contains corrosive chloride and fluoride; when the brain "forgets" extremely traumatic experiences and the conscious mind won't access this memory because it causes stress. In all of these examples, it is body processes shutting down or over-reacting as a means of protection from harm.
Of course the subconscious has a wisdom that understands shutting down or over-reacting is harmful, and sends help signals that suggest to the body that it needs to rest, eat more, or get more energy in other ways to help it correct the problem. But many of us do not hear our body call for help, unless a health situation gets really noticable.