SUMMARY1. The effect of exercise on blood ketone body concentrations was studied in trained athletes and in sedentary subjects pedalling a bicycle ergometer.2. Although the untrained subjects had higher heart rates and blood lactate concentrations at the same work load as the athletes, neither group developed ketonaemia even after intense or prolonged exercise.3. Older subjects developed post-exercise ketonaemia, reaching maximum about 3 hr after exercise.4. A high-carbohydrate diet before the exercise could prevent the onset of postexercise ketonaemia and a low-carbohydrate diet enhanced it. The highest postexercise blood ketone levels were recorded in marathon runners after a 'glycogenstripping' regimen.5. Concentrations of free fatty acids, glucose, growth hormone and insulin in blood after exercise followed different patterns from that of ketones.6. Post-exercise ketosis, when it occurs in untrained subjects, may be due to a lower carbohydrate intake than that of athletes.

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The body density of 50 healthy young women was determined by underwater weighing, and five skin-fold thicknesses and five girths were measured on each subject. The mean body density was 1.0467 g/ml (sd ± 0.0122), corresponding to 20.06 ± 4.63% by weight of fat in the body by the formula of Keys and Brozcaronek or 22.91 ± 5.58% by the formula of Siri. The mean specific gravity at water temperature 37.5 C was 1.0537 (sd ± 0.0122), corresponding to 22.13 ± 7.08% fat by the formula of Rathbun and Pace. The skin-fold measurement that showed the highest correlation with a composite criterion of skin folds was that over the iliac crest (r = +.92). The best prediction of body density was from skin-fold measurements over the iliac crest and back of arm, which gave a multiple correlation of –.74. Submitted on October 9, 1961

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