Every cell is surrounded by a membrane made of fat molecules. Which fatty acids sit there affects how flexible the membrane is. Polyunsaturated fatty acids such as EPA and DHA have a different shape from saturated fatty acids, which makes the membrane more mobile.
The fatty acids in the membranes come partly from the diet. That is why a blood sample of red blood cells can say something about diet over time.
Membrane fluidity is a biological concept. It is not measured directly in an at-home test.
Sources
- [source pending] Source needed: textbook or review on how dietary fatty acids are incorporated into cell membrane phospholipids