Your body is a hybrid fuel organism.
Most people are aware that their body will run efficiently on carbohydrates, but are unaware the it runs even more efficiently running on fatty acids/ketones. In a modern society, where we eat regular meals and we never starve, most of us have never used fatty acids as fuel to the exclusion of glucose. We have a hybrid glucose/fatty-acid/ketone fuel system in our bodies, but in modern societies we run it in glucose mode almost all the time.
Proponents of the Standard American Diet, and the LCHF (Banting) Diet are following two alternative paradigms. They speak the same words, and often use them to mean completely different things. They fail to communicate because their paradigms have entirely different core beliefs. For instance what is a "Low Carb Diet"? Many older studies think that less than 130gm or less than 20% of total calories is "Low Carb." That blocks possibility of using fatty acids/ketones as fuel in the body. A diet where carbohydrates are less than 50gm or 10% of total calories will produce an entirely different result.
Until quite recently most of the public were unaware that there was any alternative to our glucose based metabolism. Although we speak of two systems here, in fact both systems are always present, it's a case of which one is active. People are unaware that a breast fed baby uses it's fatty acid/ketone metabolism exclusively until we begin to feed them carbohydrate foods.
Ketone Creation
In response to low glucose levels, in either starvation, or the absence of dietary carbohydrate, the liver breaks down fatty acids to make the ketone bodies, β-hydroxybutyrate and acetoacetate. But the switch that turns the fatty acid/ketone production "on" is a low level of insulin. This switch operates independently in various parts of the body, but also potentially in the body as a whole.
Ketones transport energy across the body. Your muscles, the heart, the brain, the central nervous system, prefer to use ketones for fuel if ketones are available. That preference for ketones allows for glucose sparing. In other words, the body saves glucose for the important job of sustaining the constant glucose level in the blood, making sure the brain can always have the trickle of glucose it always needs. It's very unusual for the body to make glucose by breaking down muscle.
If any part of the body is over supplied with glucose, a higher level of insulin may be needed to force excess glucose out of the blood stream. (Excess blood glucose creates an emergency.) Various organs indicate that they are "full" by becoming insulin resistant. Or you may call the same condition carbohydrate intolerant. When that's happening the liver, the master chemical factory in the body, begins to turn all this excess glucose into fatty acids, and they are stored as triglycerides in your adipose tissues (around your belly).
General Notes on Metabolism
Oxidation of Food
The food you eat is never oxidized directly. Food is broken down into many chemical parts in a step by step process. This allows the body to control the energy release.
Food mostly becomes either 
Mono-saccharide's, long chain fatty acids, or amino acids.
The energy for your body has TWO primary sources: glucose, and free fatty acids. In extreme situations protein can be also be broken down to produce glucose.
Glycolysis (Breaking down glucose)
Dietary sugars enter the blood stream from the gut quite readily, other carbohydrates likewise are easily digested. Starchy carbohydrates eventually also become glucose. They pass via the liver to body cells.
Glucose is generally considered to be the energy source for human metabolism. Glycolysis (splitting) of glucose creates pyruvic acid, which becomes ATP, which is the main energy source for metabolism.
Insulin, sometimes called the master hormone, helps glucose enter the body cells where it's either turned into ATP for energy or stored as glycogen. Be aware though that this is a small and limited energy store. That process also occurs in the liver. But there is a limit to how much glycogen the liver can store, so excess glucose is converted into free fatty aside. (Free fatty acids can be used directly for energy, but they can't be reverse processed into glucose.) However, since insulin is "on" there is a surplus of free fatty acids too, so they are stored in the adipose tissue as triglycerides. Over time some of these triglycerides also accumulate in the liver, the pancreas and around the kidneys, making these organs less functional.
Lipolysis (Breaking down fats)
Lipolysis is the breakdown of lipids and involves hydrolysis of triglycerides (fats) into glycerol and free fatty acids.
Dietary fat has to be disassembled in the small intestine. Bile salts from the liver coat the fat and give it a surface layer of cells that are both hydrophobic and hydrophilic. (Like the positive and negative ends of a magnet.) The pancreas delivers a strong lipase that breaks the dietary fat into it's essential parts, free fatty acids, cholesterol and monoglycerides. These are assembled in a loose way into micells which enable the passing of these components through the intestine wall into the interstitial space. Free fatty acids, cholesterol and monoglycerides are small enough to pass through the intestine wall, but they are too large to go through capillary walls into the blood stream. The fatty acids from your diet, are now called chyle and are reassembled into chylomicrons in the lymph system and delivered into the blood, via the subclavian and internal jugular veins.
Glycolysis or Lipolysis: one or the other.
At any one time in a cell only one of those processes can function. The switch is the hormone insulin. If glucose is high, insulin is "on," and the priority is to reduce blood sugar and burn glucose to make ATP. If insulin is low, blood sugars are normal, so insulin is "off" and fatty acid sources of ATP can be used. If insulin is very low, the liver converts some free fatty acids into ketones, which are small and water soluble, and easily transported in the blood. In particular they can easily fuel the brain.
It is possible via gluconeogenesis for amino acids from dietary protein to be converted into glucose. Muscle tissue can be broken down into amino acids can become a fuel, but that's irregular, as in the case of illness or starvation. If your body really needs glucose, it can always make some, even if there is none in the diet.
What is Metabolism?
John Perry (8 minutes)
Published by: Stated Clearly - May 2019
Video URL: https://youtu.be/nRq6N5NGD1U
Normally when we think of metabolism, we think of weight loss fads like the Keto diet, the Paleo diet or intermittent fasting. Metabolism, however, is way more interesting than weight loss trends. It is the process used by your body to transform Cap'n Crunch into brain cells, and some scientists believe it his has huge implication for the origin of life!
Energy Use in the Body
The energy for your body has TWO prime sources glucose, and free fatty acids. It's possible in starvation for body protein to be broken down to provide energy, but that's not normally the case.
There are two metabolic states: where glucose is abundant, and where glucose is scarce. This is better described as insulin "on" and insulin "off", but because we seldom measure insulin, we use glucose as a bad, approximation.
Glucose Abundant State
Glucose can readily be transformed into ATP, in body cells, but the reverse is not true. That's a very important fact. Once glucose moves into body cells and becomes ATP, it can't reverse itself. Excess glucose (a monosaccharide) becomes the fat around your waist can only come from carbohydrate sources.
When you eat fat, it's turned into free fatty acids. Using the Standard American Diet model, in the presence of insulin, free fatty acids compete with glucose to fuel the body. Glucose is supplied to the brain because free fatty acids cannot cross the blood brain barrier. The heart and muscles prefer to burn free fatty acids if they are abundant.
Fatty acids in the blood are readily accepted by all the bodies cells except the red blood cells, the central nervous system and the brain. Inside your cells, free fatty acids are accepted by the mitochondria, where β-oxidation creates Acetyl-CoA (pronounced a-see-tell-co-eh) which becomes ATP for energy output in the Krebs Cycle.
When glucose is high, insulin is "on" because the body is trying to restore the glucose level in the blood to normal. Insulin helps glucose enter the muscle cells of the body, and promotes the creation of glycogen. Insulin also turns on fat storage.
Fat storage involves collecting 3 x free fatty acids and connecting them with a bond of glycerol making triglycerides. That removes both some glucose and some free fatty acids into storage, as fat in your adipose tissues.
Glucose Scarce State
When glucose is low, insulin is "off" because the body is trying to keep the glucose level in the blood to normal. Free fatty acids in the blood are not converted into triglycerides, they are directly used for energy production, being converted into ATP both in muscle cells and in the liver. Excess ATP, in the liver is converted into ketone bodies, β-hydroxybutyrate and acetoacetate.
The fat you eat can't (there are some tiny exceptions) become glucose. These are one way bio-chemical processes. Excess glucose becomes fat. In reverse body fat must be used as energy, by burning free fatty acids and ketones. The switch in both cases is the amount of insulin in the blood.
Athletes and Ketosis
If you are an athlete involved in some long-distance high-energy activity, and adapted to nutritional ketosis, your body prefers to burn ketones.
Under exercise stress, ketone oxidation conserves glucose; it also conserves glycogen stored in your body cells, and it conserves protein, protecting your muscles from breaking down.
Hence athletes tend to be protected from "hitting the wall" after about two hours of strenuous exercise, because their glycogen stores are never fully depleted.
Can You Improve Your Metabolism?
Both weight training and aerobic exercise help to improve your metabolism. Exercise creates a demand for extra energy in your cells, and the body responds by creating more mitochondria. That's one reason why high intensity interval training works so well.
If you also choose to restrict carbohydrate to less than 50g a day you will soon be in ketosis. Then your metabolism while mostly running on fatty-acids or β-hydroxybutyrate, will be conserving glucose, and be more efficient than it can be, just running on glucose.
Do the things you love to do. That will make you more enthusiastic and more energetic.
Get plenty of sleep, the body needs recovery time.
Avoid sports drinks and fruit juice. We want to improve the quality of the body engine, to make it work better, not damage it with excess glucose, sucrose and fructose.
