High Intensity Interval Training or HIIT is training where you alternate between intervals of high-intensity and low-intensity exercise. An example would be sprinting all-out for 100 meters then walking for 100 meters then repeating. One cannot sprint at full intensity for a prolonged time because sprinting and any other form of high-intensity cardio utilizes the anaerobic energy systems, specifically the Phosphocreatine system, which power short-term activity. After going all out, one must rest and give the Phosphocreatine system time to “recharge”. Additionally after sprinting all-out a couple times you are going to start feeling fatigued. After 10 sprints you are pretty much worn out. While you might not be able to sprint at full speed anymore you can still walk and jog as the intensity is not as high as sprinting and other uses the aerobic energy systems.
The rate of turnover in a metabolic pathway, otherwise known as the metabolic flux , is regulated based on the stoichiometric reaction model, the utilization rate of metabolites, and the translocation pace of molecules across the lipid bilayer .  The regulation methods are based on experiments involving 13C-labeling , which is then analyzed by Nuclear Magnetic Resonance (NMR) or gas chromatography-mass spectrometry (GC-MS) -derived mass compositions. The aforementioned techniques synthesize a statistical interpretation of mass distribution in proteinogenic amino acids to the catalytic activities of enzymes in a cell. 
A vitamin is an organic compound needed in small quantities that cannot be made in cells. In human nutrition , most vitamins function as coenzymes after modification; for example, all water-soluble vitamins are phosphorylated or are coupled to nucleotides when they are used in cells.  Nicotinamide adenine dinucleotide (NAD + ), a derivative of vitamin B 3 ( niacin ), is an important coenzyme that acts as a hydrogen acceptor. Hundreds of separate types of dehydrogenases remove electrons from their substrates and reduce NAD + into NADH. This reduced form of the coenzyme is then a substrate for any of the reductases in the cell that need to reduce their substrates.  Nicotinamide adenine dinucleotide exists in two related forms in the cell, NADH and NADPH. The NAD + /NADH form is more important in catabolic reactions, while NADP + /NADPH is used in anabolic reactions.