Plasma leptin concentrations correlate with adiposity, being high in obesity and decreasing after weight loss. Elevation of serum leptin in obesity appears to result from both increased fat mass and increased leptin release from larger adipocytes . However, leptin levels are not constant in blood. They are considered to be pulsatile, with a frequency of about one pulse every 45 min . In healthy subjects, circulating leptin concentrations exhibit a diurnal pattern, with a nadir in the mid-morning and a late-night nocturnal peak [5, 43]. The diurnal leptin pattern is dependent on insulin responses to meals and is therefore influenced by meal timing  and dietary macronutrient composition .
Gender-dependent differences have been found. The distribution of fat in the body differs between the male and female sexes. Females have more body fat and higher plasma leptin levels per gram of fat. Moreover, the brains of male and female rats are differentially sensitive to the cata-bolic action of small doses of leptin (and insulin) .
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