To determine true digestibility of nutrients, endogenous losses from the digestive tract must be measured. Because consistent values for endogenous losses of fat (ELF) in pigs have yet to be determined, the calculation of fat digestibility has mostly been limited to apparent digestibility.

Endogenous losses of fat may be influenced by the form of fat – extracted or intact – and by the concentration of fiber in the diet. The objective of this experiment was to determine the effect of the form of dietary fat on ileal and total tract digestibility of fat in growing pigs. To do so, it was necessary to measure endogenous losses of fat and to correct for the difference in amounts of dietary fiber in diets containing extracted fat and diets containing intact fat.

Estimation of endogenous losses of fat: ileal and total tract

To measure the digestibility of extracted fat, diets containing 2.0%, 4.0%, and 6.0% corn oil (dry matter basis) were formulated from a basal diet of maize-flaking grits, casein, sucrose, and cornstarch. To measure the digestibility of intact fat, an additional four diets were formulated by adding 3.0, 6.0, or 9.0% intact fat from whole corn germ meal to a basal diet based on casein, maize-flaking grits, cornstarch, and defatted corn germ meal.  Ileal and fecal samples from pigs on the different diets were analyzed for fat content, and apparently digested fat was calculated by subtracting the fat content of the samples from the fat content of the diets.

Endogenous losses of fat were estimated by regressing the total amount of apparently digested fat against the concentration of fat in the diet. The y-intercept of the regression was considered the ELF, and the slope was the true digestibility of fat. ELF and total digestibility were calculated at the end of the ileum and over the entire intestinal tract. The results are summarized in Tables 1 and 2.

Table 1. Apparent ileal digestibility (AID), apparent total tract digestibility (ATTD), true ileal digestibility (TID), and true total tract digestibility (TTTD) of fat in diets containing extracted fat

Table 2. Apparent ileal digestibility (AID), apparent total tract digestibility (ATTD), true ileal digestibility (TID), and true total tract digestibility (TTTD) of fat in diets containing intact fat

There was no difference between estimates of ELF at the end of the ileum and over the total digestive tract for pigs fed extracted fat (i.e., corn oil). For pigs fed intact fat (i.e., from corn germ meal), ELF was greater (P < 0.05) over the entire intestinal tract than at the end of the ileum, indicating that endogenous fat loss was induced by the diets containing intact fat. This is likely due to a greater concentration of fiber in these diets, which may facilitate microbial growth and fat synthesis in the hindgut. The estimates of ELF at the end of the ileum and over the entire intestinal tract were greater (P < 0.05) for intact fat than for extracted fat.

Testing the influence of fiber

When formulating diets to test the effect of extracted vs. intact fat on digestibility, a possible confounding factor is that the diets containing intact fat were higher in fiber due to the inclusion of corn germ. To test whether the concentration of fiber affects fat digestibility, four diets were formulated.  Increasing concentrations of neutral detergent fiber (NDF) in the form of Solka-Floc powdered cellulose were added to a basal diet consisting of maize-flaking grits, casein, sucrose, cornstarch, and 4.0% corn oil.

There was no change in values for apparent ileal digestibility (AID), true ileal digestibility (TID), or true total tract digestibility (TTTD) of fat as increased amounts of purified NDF were added to the diet (Table 3). A small quadratic response in the apparent total tract digestibility (ATTD) of fat was observed with increased NDF.  These results are not consistent with previous studies that have shown that an increase in dietary NDF decreases the apparent digestibility of fat. It is possible that purified NDF stimulates less microbial growth in the hindgut, leading to less microbial fat synthesis -- and therefore less fat loss -- than naturally occurring NDF.

Table 3. Effect of concentration of dietary NDF on apparent ileal digestibility (AID), apparent total tract digestibility (ATTD), true ileal digestibility (TID), and true total tract digestibility (TTTD) of fat

Comparing extracted and intact fat

Values for true ileal and total tract digestibility of extracted fat were greater (P < 0.05) than for intact fat (Tables 1 and 2). There was no difference between the TID and TTTD values of either extracted or intact fat, indicating that no net absorption of fat occurs in the hindgut of the pig.

True digestibility of extracted fat was greater than that of intact fat not only over the total tract, but also at the end of the ileum. This means that the lower ELF of extracted fat compared with intact fat cannot be the sole factor determining its greater digestibility. Another factor may be that intact fat is encased in cell membranes and is therefore more resistant to the formation of emulsions and enzymatic digestion than extracted fat.

Differences in digestibility values for extracted and intact fat imply that the metabolizable and net energy that pigs derive from these sources of fat will differ as well. True ileal and total tract digestibility values may thus be more accurate than values for total fat in determining the energy content of the diet.

Key points

• In growing pigs, extracted fat has a greater apparent and true digestibility both at the end of the ileum and over the entire intestinal tract than intact fat.
• The concentration of dietary fiber had only a small effect on apparent and true digestibility of fat in this experiment. It is possible that naturally occuring fiber would encourage more microbial growth and lead to more synthesis of fat (and thus, ELF) than the purified fiber used in the study.
• Apparent and true ileal digestibility and total tract digestibility of fat were similar for both extracted and intact fat. This indicates that there is no net absorption of fat in the hindgut of pigs.

This research report is based on the following peer-reviewed publication:

Kil, D. Y., T. E. Sauber, D. B. Jones, and H. H. Stein. 2010. Effect of the form of dietary fat and the concentration of dietary neutral detergent fiber on ileal and total tract endogenous losses and apparent and true digestibility of fat by growing pigs. J. Anim Sci. 88:2959-2967.