NAD and life-span 2006 2006 2003 2003 2003 2007 2007 2005 Ageing, dietary restriction and NAD 2006 2005 2005 2005 2006a 2007 Ageing and accumulation of altered proteins 2006b 2006 Protein glycation and ageing 2007 2007 2007 2007a b NAD and accumulation of methylglyoxal, an endogenous glycating agent NAD is essential for the metabolism of the glycolytic intermediate glyceraldehyde-3-phosphate (G3P) via the action of glyceraldehyde-3-phosphate dehydrogenase (GAPDH), generating 1,3-diphosphoglycerate (1,3DPG) and NADH as products. It is argued above that in the AL condition, glycolysis would be continuous, which would tend to lower NAD levels and raise NADH levels. This would occur especially should mitochondrial-mediated NADH re-oxidation to NAD be correspondingly lowered to compensate for the extra ATP synthesised via glycolysis, assuming cellular ATP demand remains unchanged. Limitation of NAD availability would lower GAPDH activity and promote an accumulation of G3P. The immediate precursor of G3P is dihydroxyacetone phosphate (DHAP); both of these trioses can glycate proteins. More importantly, however, is the fact that both G3P and DHAP can spontaneously decompose into methylglyoxal (MG), a highly toxic and very reactive glycating agent. It is likely therefore that changes in NAD availability could strongly influence MG production. 2006a 2007 2007 2007 2006a 2006 2007 2005 2006 2003 2006 2006 1990 2006 2006 2000 2003 1965 1966 2007 2007 2006 2007 1 Table 1 Predicted effects of aerobic exercise and fasting induced by caloric restriction or intermittent feeding, on NAD and NADH levels, methylglyoxal (MG) levels, mitochondrial (mito) activity and sirtuin activity Conditions NAD NADH  MG Mito activity Sirtuin activity Ad libitum fed Low High High  Lowered Lowered Fasting High Low Low Increased Increased Aerobic exercise  High Low Low Increased Increased Increased MG levels are partly responsible for the increased generation of altered proteins that accompanies ageing Any situation such as fasting which maintains NAD levels, either via regeneration from NADH, or by synthesis de novo or via a scavenging pathway, would facilitate metabolism of the MG precursors G3P and DHAP, and so decrease the incidence of MG-induced macromolecular damage. The increase in free-radical-mediated damage which occurs during AL feeding, compared to the CR and IF conditions, might occur as a result of not only MG-induced generation of ROS following its reaction with proteins etc., but also via plasma membrane-mediated NAD(P)H-oxidase activity. Furthermore, because less ATP is required from mitochondrial function due to continuous ATP synthesis via glycolysis in the AL-fed state, the decreased supply of electrons (as acetyl-CoA or from NADH) to the electron transport chain would tend to produce more incompletely reduced oxygen moieties i.e. oxygen free-radicals. Any increased intra-mitochondrial ROS production could also increase the probability of mitochondrial dysfunction. 1998 2001 2006 2002 Tissue differences in ageing susceptibility 2005 2006 2005 2007 2003 The beneficial effects of functional mitochondria on NAD regeneration 2007 2006 2007 2007 Other functions induced by DR 2003 2007 2006 2007 2006 Conclusion It is proposed that dietary-induced changes in NAD and NADH levels, as revealed by their regulation of sirtuin activity, may also control the concentration of deleterious glycolytic intermediates G3P and DHAP, and thereby also control formation of MG and generation of protein AGEs. The accumulation of MG and protein AGEs may compromise tissue function including mitochondrial activity and thereby contribute to organism ageing. Conversely, conditions that stimulate mitochondrial function will help regenerate NAD, maintain sirtuin activity and decrease formation of protein AGEs, intra- and extra-mitochondrial ROS can thereby delay ageing onset.