Reply to HS Kahn and R Valdez

¹ Department of Physiology University of Murcia 30100 Murcia Spain E-mail: garaulet{at}um.es
² Department of Biochemistry University of Granada Granada Spain
³ Department of Biochemistry Arrixaca Hospital Murcia Spain
⁴ Department of Edocrinology Arrixaca Hospital Murcia Spain

Dear Sir:

We read with interest the comments made by Khan and Valdez in reference to our article "Site-specific differences in the fatty acid composition of abdominal adipose tissue in an obese population from a Mediterranean area: relation with dietary fatty acids, plasma lipid profile, serum insulin, and central obesity" (1). They make 2 main points: 1) the differences we found in the fatty acid composition of perivisceral adipose tissue and other intraabdominal depots might be due to the medical condition of the patients from whom the samples were taken and 2) the potential association between insulin resistance and the fatty acid composition of adipose tissue might be obscured by undiagnosed diabetes among our subjects.

As regards the first point, the statistical analysis to establish possible differences in the fatty acid composition of the different types of adipose tissue was only carried out in those patients for whom we had obtained adipose tissue samples from the 3 regions studied (subcutaneous, perivisceral, and omental). In these subjects, a two-way (subject and fatty acid) analysis of variance was applied. For this group, then, it is safe to say that the perivisceral fat was significantly more saturated than were the other fats. We are sorry that this aspect was not clear in our article.

However, when we reanalyzed our patients with insulin resistance as a variable, as suggested by Khan and Valdez, we found some interesting results. The obese population studied was divided into 2 groups according to the degree of insulin resistance, as determined by the HOMA index (homeostasis model assessment; 2). Those with a HOMA index 3.8 were designated as being insulin resistant (n = 29), and those with a HOMA index <3.8 were designated as non–insulin resistant (n = 55) (3). We found no significant association between the fatty acid composition of subcutaneous fat and plasma insulin concentrations in either group. The correlation in perivisceral fat was only significant in the non-insulin-resistant group: 22:6n-3 (r = 0.37, P = 0.032) and 20:4n-6 (r = 0.39, P = 0.019). The metabolic significance of these findings is unclear, and perhaps other studies are necessary. However, there were strongly significant associations between the composition of omental fat and insulin values in both groups: 14:0 (r = 0.75, P = 0.021), 18:0 (r = 0.83, P = 0.006), and total saturated fatty acids (r = 0.76, P = 0.018) were positively correlated and 18:1n-9 (r = –0.75, P = 0.019) and total monounsaturated fatty acids (r = -0.82, P = 0.008) were negatively correlated with insulin concentrations in the insulin-resistant group. A negative and significant correlation was also found between 18:1n-9 (r = -0.59, P = 0.02) and insulin concentrations in the non-insulin-resistant group. In reference to dietary composition, we found a significant correlation between 18:3n-3 (r = -0.43, P = 0.028), total polyunsaturated n-3 fatty acids (r = -0.41, P = 0.036), and insulin only in the insulin-resistant group. These findings confirm that the correlation between insulin and the fatty acid composition both in the diet and in adipose tissue differ depending on whether patients are studied as a whole or divided into groups on the basis of their insulin resistance, as Khan and Valdez suggested.

In conclusion, as mentioned in our article, the fatty acid composition of perivisceral, omental, and subcutaneous abdominal tissue is different. Our results after the use of insulin resistance as a variable lend weight to our original finding that the higher 18:1n-9 and polyunsaturated fatty acid contents and the lower saturated fatty acid content of the diet and adipose tissue of the Mediterranean population than of the non-Mediterranean population may mitigate the harmful effects of obesity.

REFERENCES

1. Garaulet M, Pérez-Llamas F, Pérez-Ayala M, et al. Site-specific differences in the fatty acid composition of abdominal adipose tissue in an obese population from a Mediterranean area: relation with dietary fatty acids, plasma lipid profile, serum insulin, and central obesity. Am J Clin Nutr 2001;74:585–91.
2. Matthews DR, Hosker JP, Rudenski AS, Naylor BA, Teacher DF, Turner RC. Homeostasis model assessment: insulin resistance and B cell function from fasting plasma glucose and insulin concentrations in man. Diabetologia 1985;28:412–9.
3. Ascaso JF, Romero P, Real JT, Priego A, Valdecabres C, Carmena R. Insulin resistance quantification by fasting insulin plasma values and HOMA index in a non-diabetic population. Med Clin (Barc) 2001;117:530–3.