Fatness as a Determinant of Minimal Weights for Menstrual Cycles

As shown in Table 1 and Figure 1, total body water as a percentage of body weight is an index of fatness. In a study in the United States, this index in each of the 181 girls followed from menarche to the completion of growth at ages 16-18 years provided a method of determining a minimal weight for height necessary for menarche in primary amenor-rhea and for the resumption of normal ovulatory cycles in cases of secondary amenorrhea, when the amenorrhea was due to undernutrition or intensive exercise. These weights have been found useful in the evaluation and treatment of patients with primary or secondary amenorrhea due to weight loss.

Percentiles of total body water/body weight, which are percentiles of fatness, were made at menarche and for the 181 girls at age 18 years, the age at which body composition stabilized. Patients with amenorrhea due to weight loss, other possible causes having been excluded, were studied in relation to the weights indicated by the diagonal percen-tile lines of total water/body weight percent (Figure 3). It was found that 56.1% of total water/ body weight, the 10th percentile at age 18 years (equivalent to approximately 22% fat of body weight), indicated a minimal weight for height necessary for the restoration and maintenance of menstrual cycles. For example, a 20-year-old woman whose height is 165 cm (65 in.) should weigh at least 49 kg (108 lb) before menstrual cycles would be expected to resume (Figure 3).

The weights at which menstrual cycles ceased or resumed in postmenarcheal patients age 16 years and older were approximately 10% greater than the minimal weights for the same height observed at menarche (Figure 4). The explanation was that both early and late-maturing girls gain an average of 4.5 kg of fat from menarche to age 18 years. Almost all of this gain is achieved by age 16 years, when mean fat is 15.7 ± 0.3 kg, 27% of body weight. At age 18 years, mean fat is 16.0 ± 0.3 kg, 28% of the mean body weight of 57.1 ± 0.6 kg. Reflecting this increase in fatness, the total water/body weight percent decreases from 55.1 ± 0.2% at menarche (12.9 ± 0.1 years in this sample) to 52.1 ± 0.2% (standard deviation 3.0) at age 18 years.

Because girls are less fat at menarche than when they achieve stable reproductive ability, the minimal weight for onset of menstrual cycles in cases of primary amenorrhea due to undernutrition or exercise is indicated by the 10th percentile of fractional body water at menarche, 59.8%, which is equivalent to approximately 17% of body weight as fat. For example, a 15-year-old girl whose completed height is 165 cm (65 in.) should weigh at least 43.6 kg (96 lb) before menstrual cycles can be expected to begin (Figure 4).

The minimum weights indicated in Figure 4 would be used also for girls who become amenor-rheic as a result of weight loss soon after menarche, as often occurs in cases of anorexia nervosa in adolescent girls.

The absolute and relative increase in fatness from menarche to ages 16-18 years coincides with the period of adolescent subfecundity. During this time, there is still rapid growth of the uterus, ovaries, and oviducts.

Other factors such as emotional stress affect the maintenance or onset of menstrual cycles. Therefore, menstrual cycles may cease without weight loss and may not resume in some subjects even though the minimum weight for height has been achieved. Also, these standards apply only to Caucasian US females and European females since different races have different critical weights at menarche and it is not known whether the different critical weights represent the same critical body composition of fatness.

Since the prediction of the minimum weights for height is based on total water/body weight percent (not fat to body weight percent), successful prediction may be related to the ratio of lean mass to fat, which is normally approximately 3:1 at menarche and 2.5:1 at the completion of growth at age 18 years. No prediction can be made above the threshold weight for a particular height.

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