“The turnover of proteins in normal cells and body fluids provides a good marker of ageing. Proteins are broken down into their constituent amino acids ready for recycling. When this process occurs inefficiently, granules of a yellow fatty substance called lipofuscin begin to accumulate in the cells. Protein recycling becomes increasingly inefficient as cells age and so the level of lipofuscin granules offers a measure of cell age. Lipofuscin has the effect of slowing cell metabolism, so as it builds up the cell metabolism becomes slower and less efficient in a vicious circle of ageing.”