The answer is simple: cell membrane flexibility. Every cell has a cell membrane. When this cell membrane is rigid, it does not work well. When it is flexible, the chemicals that run our bodies - hormones, proteins, enzymes, vitamins, minerals, fats, etc - can move in and out of cells efficiently as needed, thus making the cells healthier, since the materials they need to function well are available. When the cells work well, the tissues that are made of cells work well. When tissues work well, the whole system works well and ultimately leads to overall improved health of the entire person.
For example, when there are enough omega-3 fatty acids available through either diet or supplementation, they will be absorbed into cells in the heart, making their cell membranes flexible, but strong. Consequently, the heart and arteries are stronger and therefore the entire cardiovascular system benefits. In fact, in the same way that omega-3 fats make cell membranes more flexible, the dangerous trans fats do the exact opposite - they are absorbed into the cell membranes making them stiff and unable to do their job. Just as stiff joints or stiff arteries are unhealthy, so are inflexible cell membranes. And since cell membranes are an integral part of every tissue in the body, the level of omega-3 fatty acids a person has can affect just about everything (see below).
SpectraCell's HS-Omega-3 Index® measures the amount of two very important omega-3 fatty acids - EPA and DHA - in a person's red blood cells.