Energy consumption, storage, and balance

The majority of ATP synthesized in mitochondria begins with the oxidation of fuel molecules and the release of electrons and protons by the TCA cycles The electrons and protons are delivered by NADH and FADH2 to the inner mitochondrial membrane where the electrons are passed through a series of intermediate compounds and ultimately to molecular oxygen which becomes reduced to H2O in the process The energy provided by the electon flow allows the protons to be translocated from the mitochondrial matrix to the space between the inner and outer membranes which creates an energy gradient that powers the phsophorylation of ADP to form ATP Fatty acids are a rich source of energy on an equal weight basis they surpass carbohydrates in this property Many tissues are capable of oxidizing fatty acids by way of a mechanism called beta-oxidation When the fatty acid enters the cell it is first activated by coenzyme A to acyl-CoA The oxidation of fatty acids occurs primarily within the mitochondrion and produces energy through oxidative phosphorylation

Short-chain fatty acids can pass directly into the mitochondrial matrix and form acyl-CoA derivatives in the matrix Long-chain fatty acids and their CoA derivatives are incapable of crossing the inner mitochondrial membrane so a membrane transport system is necessary The carrier molecule for this system is carnitine Normally the concentration of the ketone bodies in the blood is very low but will increase in situations of accelerated fatty acid oxidation that occurs during body fat reduction consuming low-energy low-carbohydrate diets or in uncontrolled type 1 diabetes Under such conditons an abundance of free fatty acids is released by adipocytes into the circulation which exceeds the ability of tissues to oxidize them Glucose-requiring tissues including the brain and red blood cells cannot use fatty acids for energy and their need for alternative fuels increases The liver is able to handle excess free fatty acids by converting them to the so-called ketone bodies in a process called ketogenesis Following beta-oxidation the liver converts the excess acetyl-CoA to acetoacetate beta-hydroxybutyrate and acetone

Fat loss strategies often include consumption of low-CHO and or low-energy diets Mild increased in ketone bodies called ketosis is to be expected during fat loss and usually poses no harm However in uncontrolled diabetesm mellitus in starvation or with prolonged consumption of a very-low-CHO diet ketone bodies can rise to dangerous levels that lower the pH of the blood resulting in ketoacidosis Untreated ketoacidosis can lead to low blood pressure dehydration coma and death Energy is constantly being used by every cell in the body Humans must consume food on a regular basis to meet energy demands When the amount of food energy matches energy expenditure over time a person is in energy balance A person who habitually consumes energy in excess of energy needs is said to be in positive energy balance and will convert the unused energy into triacylglycerols for storage as body fat