The heat increments of mixtures of steam-volatile fatty acids in fasting sheep

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Abstract
Adult castrated male sheep (4) were each equipped with a Perspex cannula passing through a permanent rumen fistula. The animals were starved 4 days, then given mixtures of steam-volatile fatty acids to study whether the proportion of acetic acid was related to the size of the heat increment whea the mixture of acids provided the sole source of energy. Previous work showed that when acetic acid was given alone to fasting sheep its heat increment was 41% of the energy metabolized; similar values for propionic and n-butyric acids were 13% and 16% respectively. A mixture of propionic and n-butyric acids in molar proportions of 32 reduced the heat increment to 9% of energy metabolized. Subsequent addition of acetic acid to the mixture resulted in a small rise in heat increment until the molar proportion of acetic acid was 90% and heat increment 15%. Metabolism of each of the acids is apparently modified by presence of the others. Each of the mixtures caused a reduction in exretion of urinary N, with the reduction being much the same for each mixture. Mean N-sparing effect was 1.6 [plus or minus] 0.3 mg N/cal metabolized. Blood sugar increased with administration of all steam-volatile acid mixtures except the one containing 90% acetic, 6% propionic and 4% n-butyric on a molar . basis, where an initial fall was followed by a rise and slight ketosis was apparent. Acetic acid alone caused marked reduction in blood sugar for the first 2 days.