This turned out to be a long post. Therefore, I have broken it into three parts. Part one will discuss the relationship of carbohydrates, lipids (fats) and protein. Part two will discuss energy production and usage under normal and exercise conditions. Part three will discuss energy usage during short and long term fasting programs. A bibliograpy is included in ‘Part Three’. This may be more information than most are interested in. However, if you understand how your body stores, creates & uses energy from the food you eat, you will be less likely to be taken in by all of the over hyped weight loss and cleanse programs out there. No doubt about it, there is a lot of hype about cleanses & weight loss programs. Some of it is true, some of it is partially true and some of it is so patently misleading that it should be criminal! Some of the beliefs about nutrition have been repeated for so long or repeated by so many that they are taken at face value in spite of no scientific evidence or even in spite of scientific evidence to the contrary.
Let me make it clear that I am not a scientist, nor an expert. I have studied human physiology at the undergraduate level fairly extensively. I have worked in the alternative health field for 15 years. I read a lot and I try to apply some logic to understanding the natural healing process.
Here in lies a paradox. The living human body as a whole is not easily studied by standard laboratory techniques. Chemical reactions must be isolated and random variables eliminated in an attempt to apply the scientific method. Organic chemists and physiologists have learned and continue to learn a lot of valuable information using these techniques. Unfortunately, science and scientific medicine have a hard time arriving a ‘facts’, ‘proofs’ & ‘cures’ when confronted with too many variables, some of which they have no way of sensing, quantifying or reproducing in a consistent way.
However, each individual human body is a complex entity influenced by DNA, health history, diet, lifestyle and beliefs both spiritual and secular. And yes, I do believe that our strongly held beliefs can change physical reality in profound ways. The human body has a tremendous ability to heal itself, especially if we can support it through healthy diet, lifestyle and positive thoughts. This is natural healing. This dichotomy between what science can prove and what experience tells us sometimes happens, leaves patients and practitioners of natural healing free to believe that almost anything is possible. Knowing that many of us want to believe, some have capitalized on this to feed us what we want to hear so that we will buy their products. So, now that I have that lengthy preamble out of the way, I would like to present a brief scientific explanation of the physiology of energy storage and retrieval in the human body as it applies to daily activities, physical exercise, fasting and/or weight loss.
Our food is generally classified as carbohydrate, fat or protein. Actually, most of our food is, in it’s natural state, before it is processed, is a combination of all three, plus enzymes, micro nutrients, vitamins and minerals. To discuss the topic of energy pathways in any coherent way we must talk about how the body processes these things individually.
Carbohydrates may be simple sugars such as glucose, sucrose & fructose or more complex sugars and starches such as many root vegetables and plant fiber or the animal starch glycogen. Glycogen is how energy is stored in our liver and muscle tissue. 
Lipids (fat) may be fatty acids (saturated or unsaturated), glycerides (mono, di, & triglyceride), prostaglandins, steroids, phospholipids and glycolipids. It is triglycerides that absorb and store lipid soluble vitamins, drugs or toxins in our bodies. Fatty acids are also stored in the liver for use in bile salt, steroid, and energy production. 
Proteins are made from smaller compounds called amino acids, which are linked together by peptide bonds. Proteins are classified as dipeptide, tripeptide or polypeptide. 
All three of these are made up of carbon (C), Nitrogen (N), oxygen (O) and Hydrogen (H) molecules, which form compounds that can be broken down and in many cases rebuilt by the body as needed. Breaking down carbohydrates, fats and proteins releases more energy than is required to break the molecular bonds. This energy can then be used by the body to build other compounds it needs. Adding a phosphate group to a substrate such as adenosine diphosphate (ADP) creates adenosine triphosphate (ATP), which is the energy exchange molecule used by chemical processes throughout the body.
Click here to go to Part Two
Don Gillmore, Licensed Massage Therapist(15 yrs), B.S.- Life Sciences