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We previously reported in populations exhibiting all the spectrum of insulin sensitivity (SI) values correlations between SI and blood viscosity eta suggesting that high eta is an additional symptom of the insulin resistance syndrome. However, due to the elevation of insulinemia (I) which is usually associated with insulin resistance it remained to determine whether this relationship was explained by SI or I. We analyzed SI with the minimal model procedure in 108 nondiabetic subjects and analyzed correlations of SI with blood rheology (eta, RBC aggregation and rigidity). Across quartiles of SI (defined after log transformation since distribution of SI was not normal), hematocrit and red cell rigidity remained stable, while aggregability and plasma viscosity (etap) increased in the lowest quartile. SI was correlated to only two rheological parameters: etap (r= -0.280, p=0.005) and Myrenne index M1 (r= -0.219, p=0.044). Among SI, I, age and BMI multivariate analysis selected only BMI as a determinant of either whole blood viscosity (etawb: r= -0.301, p=0.004) and RBC disaggregation threshold (gammaD: r= -0.331, p=0.013), only I as determinant of M1 (r=0.254, p=0.03), and a combination of BMI (p=0.009) and SI (p=0.007) for etap. Although age and obesity are factors of hyperviscosity, the hemorheological disturbances found in insulin resistance are not fully statistically "explained" by those two factors. While hyperaggregability (measured with M1) is rather related to hyperinsulinism, etap is influenced by SI and should be further investigated as a simple marker for the follow up of insulin-resistant states.

We investigated relationships among body composition, blood rheology, and exercise performance in 14 rugbymen (19-31 yr, weight 65.8-109.2 kg, height 1.7-1.96 m, body mass index 21.7-33.1 kg/m2) who underwent a standardized submaximal exercise session on cycloergometer corresponding to 225 kJ over 30 min. The rheologic response to exercise was measured with the MT90 viscometer and the Myrenne aggregometer. Dehydration, evaluated by precision weighing, resulted in a loss of 360 to 973 g water, i.e., 1.69 to 4.32 g/kJ. This loss of water is not correlated to plasma volume contraction as assessed by the equation of Greenleaf. Hemorheologic changes are observed, but they are correlated neither to water loss, nor to plasma volume contraction. A 36% increase in blood viscosity (p < 0.01) is mainly explained by a red blood cell rigidification (p < 0.02), although hematocrit and plasma viscosity also increase (p < 0.01). Isometric adductor strength (specific ergometer) is correlated to erythrocyte flexibility (r = 0.680, p < 0.01). Red cell aggregability (Myrenne aggregometer) is correlated to fat mass measured by bioelectrical impedance (r = 0.634, p < 0.02). Aerobic working capacity index W170 is negatively correlated to the increase in plasma viscosity during exercise (r = -0.546, p < 0.05), suggesting that this event is less important in stronger individuals. This study shows that fat mass, even within a physiological range, is a determinant of erythrocyte aggregability, suggesting that training-induced alterations in body composition play a role in the specific hemorheologic profile of athletes. In addition, both erythrocyte flexibility and the magnitude of fluid shifts during exercise appear to be related to fitness in these sportsmen.

While it is well established that blood viscosity is decreased in sportsmen and related to fitness, the involvement of fibrinogen in this relationship is less well defined. Relationships among fitness, rheology and fibrinogen were investigated in 32 football players (age 17-33 years: 19 professionals and 13 leisure players). A submaximal 25 min exercise-test was performed and allowed the calculation of aerobic working capacity. Aerobic working capacity (W170 and VO2 max) was negatively correlated to fibrinogen (r = -0.531, p < 0.01 and r = -0.623, p < 0.01), while on the whole sample the correlation to viscosity and erythrocyte aggregation was not significant. When subjects were divided into two subgroups according to their plasma fibrinogen concentration, the aerobic working capacity (either expressed as W170 or VO2 max) is higher when plasma fibrinogen level is lower than 2.7 g/l. Thus, there is a highly significant negative correlation between fibrinogen and fitness in these sportsmen, independent of blood rheology. These data suggest that rheology and fibrinogen are to some extent separate determinants of an individual's fitness.

The life-extending effects of regular exercise are related to a decrease in both coronary and peripheral vascular morbidity, associated with some improvements in cardiovascular risk factors. A possible link between the beneficial metabolic and hemodynamic effects of exercise could be blood rheology, which is markedly affected by exercise. We propose here a description of the hemorheological effects of exercise as a triphasic phenomenon. Short-term effects of exercise are an increase in blood viscosity resulting from both fluid shifts and alterations of erythrocyte rheologic properties (rigidity and aggregability). Increased blood lactate, stress, and acute phase play a role in this process. Middle-term effects of regular exercise are a reversal of these acute effects with an increase in blood fluidity, explained by plasma volume expansion (autohemodilution) that lowers both plasma viscosity and hematocrit. Long-term effects further improve blood fluidity, parallel with the classical training-induced hormonal and metabolic alterations. While body composition, blood lipid pattern, and fibrinogen improve (thus decreasing plasma viscosity), erythrocyte metabolic and rheologic properties are modified, with a reduction in aggregability and rigidity. On the whole, these improvements reflect a reversal of the so-called "insulin-resistance syndrome" induced by a sedentary lifestyle. Since impaired blood rheology has been demonstrated to be at risk for vascular diseases, the hemorheologic effects of exercise can be hypothesized to be a mechanism (or at least a marker) of risk reversal. This latter point requires further investigation. The physiological meaning of the triphasic pattern of exercise-induced alterations of blood rheology is uncompletely understood, but increased blood fluidity may improve several steps of oxygen transfer to muscle, as clearly demonstrated in hypoxic conditions. Increasing evidence emerges from the literature, that blood fluidity is a physiological determinant of fitness.