Angiotensin-converting enzyme activity decreases during fasting

Serum angiotensin-converting enzyme (ACE) activity varies directly with thyroid hormone levels in states of altered thyroid function. Because T₃ levels decrease during fasting, ACE activity was examined to ascertain if it was reduced in this low T₃ condition. Eighteen obese euthyroid subjects were hospitalized and placed on a weight-maintaining diet for 4 days. Nine subjects (Group 1) underwent a fast (50 kcal/day) for 8 days. Nine (Group 2) subjects received T₃ (5 μg q 3 h) during an identical fast. Weight loss was identical in both groups (-4.4 kg). Serum T₃ fell in Group 1 from 104 ± 8 to 50 ± 4 ng/d/(p<.05) but was unchanged in Group 2 (114 ± 11 ng/dl fed vs. 120 ± 14 ng/dl fasted). Blood pressures fell significantly in Group 1 (mean systolic: 112 → 104 mmHg; diastolic: 71 → 65 mmHg, p<0.05), but not in Group 2 subjects. ACE activity fell progressively in Group 1 subjects during fasting (14.4 ± 1.6 U/ml fed vs. 12.8 ± 1.4 U/ml fasted p<0.05). ACE activity was not decreased significantly early in the fast in patients given T₃, but by late fast (days 6-8) was reduced to the same degree as in Group 1 subjects. Glucose and insulin levels fell similarly in both groups. Conclusions: (1) ACE activity is reduced during starvation. This effect is not mediated by T₃. (2) Blood pressure reduction during fasting may result from the low T₃ levels, but not from decreased ACE activity. Interpretation of serum ACE activity must be viewed in the context of a patient's diet.