The results of the present study confirm our hypothesis, based on analyses of serum phospholipids conducted in two previous studies of children with SCD in Nigeria [8, 21], that red cell phospholipids of children with SCD contain significantly lower proportions of n-3 polyunsaturated fatty acids and higher proportions of saturated fatty acids relative to non-SCD controls. These results are in agreement with those of Connor and coworkers  who found similar alterations in the fatty acid composition of the red cell phospholipids of U.S. children with SCD. The red cells of the SCD subjects in the present study also contained significantly higher amounts of cholesterol. More importantly, in the present study we found that both the cholesterol content and the percentage of several n-3 fatty acids in the red cells of the subjects were significantly correlated with phase angle. Overall, in the SCD children we studied, cell membrane components that are known to decrease membrane fluidity, such as cholesterol and saturated fatty acids, were associated with a low phase angle while those which increase fluidity, such as long-chain n-3 polyunsaturated fatty acids, were associated with a high phase angle.
The altered fatty acid composition in both the serum and red cell phospholipids of the SCD subjects we studied may be due to several causes. These include: differences in dietary intake between SCD patients and controls, aberrations of the desaturase-elongase pathway that interconverts n-3 and n-6 polyunsaturated fatty acids in SCD, and increased metabolism (e.g., β-oxidation) of the long-chain polyunsaturated fatty acids in SCD. Although we did not obtain dietary information for the subjects in this study, we assumed that the availability of foods containing the essential and long-chain fatty acids did not differ between SCD patients and controls. However, the SCD subjects may have been consuming less food, on average, than the controls.
With regard to the metabolism of fatty acids mentioned above, the activities of the enzymes that are responsible for the elongation and desaturation of the essential fatty acids to produce the long-chain polyunsaturated fatty acids may vary between SCD subjects and controls. In a study of children with protein-energy malnutrition, Decsi and coworkers  used product/substrate ratios of fatty acids in serum phospholipids to speculate that children with protein-energy malnutrition have a reduced level of Δ5-desaturase activity compared to well-nourished children. This particular desaturase is a regulatory enzyme in the pathway responsible for the biosynthesis of long-chain polyunsaturated fatty acids in both the n-6 and n-3 fatty acid families . Since children with SCD exhibit some of the characteristics of malnutrition, such as lower weight-for-age and lower lean body mass, the activity of the)5-desaturase in these children may be decreased, resulting in lower proportions of the long-chain polyunsaturated fatty acids in both serum and membrane phospholipids.
Two important determinants of membrane fluidity are the cholesterol content and the proportions of the fatty acids that comprise the phospholipids of membranes. One way to compare the fluidity of membranes between two or more populations is to determine the mean melting point (MMP) of the fatty acids in the total phospholipid fraction of the membrane. For such a purpose, we used the method of Jensen and Patton  to estimate the MMP of red cell phospholipids. We found that the MMP of the membrane phospholipids of the children with SCD was significantly higher compared to controls, indicating a decrease in membrane fluidity in the SCD subjects. A negative correlation was also observed between MMP and phase angle. This observation suggests that the chemical composition and properties of tissue membranes, specifically the acyl chains of the membrane phospholipids, directly influence the magnitude of the phase angle of an individual.
When we examined the relation between phase angle and red cell cholesterol, a significant negative correlation was obtained between phase angle and red cell cholesterol for the total pool of subjects (SCD patients plus controls) (p = 0.003). However, when regression models were used that included cholesterol, DHA, and FFM, only DHA and FFM were found to be significantly correlated with phase angle.
The implications of the observed alterations in the membrane lipids of children with SCD are not known; however, by affecting the physical properties of tissue membranes, including fluidity and capacitance, these changes could compromise various membrane functions. In an in vitro study using red cells from both healthy subjects and patients with SCD, Kuypers and coworkers  showed that the deformability and stability of SCD red cells could be altered by changing the fatty acid composition of the phosphatidylcholine (PC) molecules of the membrane. For example, when they replaced the native PC with 1-palmitoyl, 2-arachidonoyl-PC in normal cells, the cells exhibited decreased osmotic fragility without a change in hydration. However, replacement of native PC with 1,2-dipalmitoyl-PC resulted in an increase in both osmotic fragility and cellular hydration. In contrast, replacement of the native PC of sickle cells with either 1-palmitoyl,-2arachidonyl PC, 1,2-dipalmitoyl PC, or 1-palmitoyl, 2-oleoyl PC led to increased cellular hydration. The authors concluded that the state of cellular hydration of sickle cells may be modulated by altering the molecular species composition of the membrane's phospholipids. In a study of subjects with type 2 diabetes, Borkman and coworkers  found that decreased insulin sensitivity was associated with a decrease in the proportions of polyunsaturated fatty acids in skeletal-muscle phospholipids. They speculated that changes in the fatty acid composition of muscle membranes could affect insulin sensitivity.
We reported previously significantly lower phase angle values in both African American and Nigerian children with SCD [18, 27]. Because phase angle is related to the amount of lean body tissue and the corresponding amounts of tissue membranes , it could be that the lower phase angle we observed previously in both the Nigerian and African American children were the result of the lower FFM of SCD children compared to their corresponding controls. Patients with SCD, particularly males over the age of 10 years, usually have significantly less FFM than their healthy counterparts . However, in the present study, although there were no significant differences in the mean weight, BMI, or FFM of the controls and the SCD subjects, nevertheless, we still observed a statistically significant difference in the calculated phase angle of the two groups. We suggest, therefore, that factors other than FFM and which are not related to body size, such as the lipid composition of cell membranes, can influence the phase angle of individuals.
It is reasonable to ask whether dietary modification or fatty acid supplementation might change the fatty acid composition of tissue phospholipids in children with SCD. In two separate supplementation studies involving SCD subjects, changes in the fatty acid composition of red cell phospholipids were obtained after supplementation with fish oil containing high amounts of long-chain polyunsaturated n-3 fatty acids. In the first study, Muskeit and coworkers  supplemented 13 SCD patients for 7 months with capsules containing eicosapentaenoic acid, docosahexaenoic acid and vitamin E. This supplementation regimen resulted in increased incorporation of n-3 fatty acids in both plasma cholesterylesters and red blood cells. The double-bond index of the red cells increased, indicating an increase in red cell membrane fluidity and deformability. In the second study, Tomer and coworkers  also demonstrated changes in platelet and red cell activation markers in subjects with SCD following fish oil supplementation for one year, resulting in fewer pain episodes compared to subjects supplemented with n-6 fatty acids.
In summary, we have shown in children with SCD that the lipid composition of their cellular membranes is related to phase angle, a non-invasive whole body measurement of membrane integrity and function. Membrane function is altered in a variety of clinical conditions and the phase angle may provide a rapid, non-invasive means of monitoring the effect of therapies aimed at improving the health of individuals in whom cell membrane function may be deranged. Although it is known that supplementation with n-3 fatty acids produces changes in the membrane composition of cells, it has yet to be determined whether phase angle can be used as an alternative approach for monitoring these changes. Studies of this nature are in progress.