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Carroll and Richards (7) found that the digestibility of fats in rats was influenced, in part, by the composition of the mineral mix present in the diet. Oleic acid was almost totally digestible when fed as part of a calcium-free diet, but digestibility fell in the presence of various salt mixes and seemed to depend on the calcium:phosphorus (Ca:P) ratio of the salt mix. Thus, digestibility of oleic acid was 78% in diets containing 5% of PhillipsHart salt mix (Ca:P = 1.4) and 42% in diets containing 5% of the HubbellMendelWakeman mix (Ca:P = 4.25).

Mattson et al. (8) examined the effects of calcium- and magnesium-containing diets on the absorption of various triglycerides, specifically OSO, SOO, OSS and SOS (O = oleic acid; S = stearic acid). They found that diets replete in calcium and magnesium affected absorption when stearic acid was in the 1 or 3 position (SOO, SOS) but not when it was in the 2 position (OSO or OSS). In a study of the digestibility of various natural and hydrogenated fats, Calloway et al. (9) concluded that digestibility depended primarily on chain length of the saturated fatty acids and their position in the triglyceride.

Mattson (10) studied the absorption of stearic acid using mixtures of hydrogenated linseed and safflower oils. The oils were mixed or randomized. Absorbability was a function of the total amount of stearic acid and the level of tristearin present in the test fat (Table 5.1).

Metabolism of Stearic Acid-Rich Fats

Bergstedt et al. compared the absorption of tristearin and triolein by the small intestine of rats bearing lymph fistulas (11) as well as the effects of triolein and

Table 5.1 Influence of fat composition on absorbability of stearic acid.

Mix*

Randomized 40

Randomized 70

Randomized

100 95

Source: after Mattson (10).

  • HLO = hydrogenated linseed oil; SFO = safflower oil.
  • 18:0 13

16 16

72 71 97

% tristearin

63 36 91

Coefficient of absorption

96.7

73.2

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tripalmitin on absorption of tristearin (12). Lymph flow was slower in rats given tristearin and its triglyceride content was lower. The distribution of tristearin and triolein in lymph, mucosa, and lumen is given in Table 5.2 and distribution of labeled triglyceride in the intestine is given in Table 5.3. The absorptive index (100% % dose recovered in the lumen) was 94.3 ± 1.0 for triolein and 56.7 ± 7.8 for tristearin. Lymphatic output of triglyceride was higher when tristearin was mixed with triolein than when it was mixed with tripalmitin (12). As in the earlier study (11), more tristearin remained in the lumen when it was undiluted by triolein or tristearin. The absorptive indices for the three fats were tristearin, 63.8 ± 5.1; tristearin + triolein, 94.7 ± 0.9; and tristearin + tripalmitin, 86.8 ± 6.3. Absorbability of tristearin was significantly (P < 0.05) lower than for the other two fats.

Chen et al. (13) compared the absorption of cocoa butter, corn oil and palm kernel oil in lymph fistula rats. Recovery of fatty acids was significantly lower in the rats fed cocoa butter. Cholesterol absorption was also lowest in the cocoa butter-fed rats (Table 5.4).

Table 5.2 Distribution of radioactive lipid in rats fed various triglycerides.

Recovery (% of dose) Triolein* Tristearin*

Lymph

Mucosa

Lumen

Total

  1. 8 ± 4.5
  2. 6 ± 0.4
  3. 7 ± 1.0 69.1 ± 5.0
  4. 4 ± 6.5 3.0 ± 0.9 43.3 ± 7.8 66.7 ± 4.6

Source: after Bergstedt et al. (11).

* Means ± SE. Values obtained 8 hours after infusion of radioactive lipid. Six rats per group.

Table 5.3 Recovery of radioactive lipid (% of dose) remaining in lumen of small intestine after feeding rats triolein or tristearin.

Recovery (% of dose) Triolein* Tristearin*

Small intestine

Segment 1

Segment 2

Segment 3

Segment 4 Cecum

  1. 99 ± 1.02 0.40 ± 0.09 0.26 ± 0.05 0.27 ± 0.02 0.02 ± 0.02
  2. 58 ± 0.42
  3. 40 ± 0.72 4.38 ± 0.78 10.46 ± 3.03 17.42 ± 4.42 7.08 ± 2.31

Source: after Bergstedt et al. (11).

* Means ± SE. Values obtained 8 hours after infusion of radioactive lipid. Six rats per group.

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Table 5.4 Recovery of absorbed fatty acids and cholesterol from rat lymph. Dietary fat

% Recovery

Cholesterol

Corn oil

1001 a2

Palm kernel oil

Cocoa butter

63.0 ab

1 Means ± SE. Corn oil recovery set at 100 other values relative to it.

2 Values bearing same letter are significantly (P < 0.05) different.

Apgar et al. (14) measured the digestibility of corn oil and the stearic acid-rich fat, cocoa butter, in rats. Digestibility coefficients for 5, 10 or 20% corn oil were 92.7 ± 0.9, 96.9 ± 0.1 and 96.3 ± 0.2, respectively. For cocoa butter, the digestibility coefficients (%) were 5%, 58.5 ± 0.5; 10%, 60.3 ± 1.4; and 20%, 71.7 ± 1.3. Body weight gain and food intakes were similar for all groups. Fecal excretion of palmitic acid was roughly similar for all groups (3034%) but rats fed cocoa butter excreted significantly more stearic acid (61% versus 27%).

Elovson (15) injected carboxyl-labeled stearic acid into the jugular vein of rats and 5 min later recovered 8% of the radioactivity in liver oleic acid. When [3,4-3H]-stearic acid was injected intravenously into rats, [3H]-oleic acid was recovered from liver lipids. However, before postulating a major conversion of stearic to oleic acid, one must take into account the mode of administration (intravenous injection), the rapid reutilization of 14CO2 obtained as a result of the metabolic decarboxylation of stearic acid. The lability of 3H must also be taken into account. Emken (16) found that the conversion of deuterated stearic to oleic acid in five male subjects averaged 9.2%.

Leyton et al. (17) fed rats emulsions of carboxyl-labeled lauric, myristic, palmitic, stearic, oleic, linoleic, a-linolenic, dihomo a-linolenic or arachidonic acids. Recovery of 14CO2 from rats given labeled stearic acid was 56% lower than from those given oleic acid and 22% lower than from rats given palmitic acid. The saturated fatty acids were incorporated uniformly into liver triglycerides but were incorporated more rapidly into phospholipids as their chain length increased. Incorporation of stearic acid into phosphatidyl choline compared to that of palmitic acid and was higher than for lauric or myristic acid.

Incorporation of stearic acid into phosphatidyl inositol or phosphatidyl ethanolamine was higher than for the other fatty acids. Wang and Koo obtained lymph chylomicrons containing carbon-labeled stearic, myristic or linoleic acids (18) or stearic, palmitic or oleic acids (19) from rats and injected them intravenously into recipient rats. Stearic acid was removed from the plasma more slowly than the other fatty acids and was incorporated more slowly into hepatic

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triglycerides. The percentage of [14C]-stearate appearing in liver phospholipids was higher than for the other fatty acids.

Several investigators have studied the effects of stearic acid-rich fats vis-à-vis cholesterol and lipoprotein metabolism. Feldman et al. (20) studied cholesterol absorption and turnover in rats fed tristearin, triolein or safflower oil. Diets containing 0.025% cholesterol and 10% of the test fat were fed for 6 weeks. Tristearin-fed rats gained significantly less weight than did the control or other test groups. Rats fed tristearin absorbed significantly less cholesterol and removal of cholesterol from the plasma was most rapid in this group. They also exhibited the lowest cholesterol levels, 58 ± 2 mg/dl compared to 77 ± 1 mg/dl in triolein-fed rats and 80 ± 1 mg/dl in those fed safflower oil. Sterol turnover and synthesis were higher in rats fed tristearin. There were no differences in neutral or acidic steroid excretion.

In a second study (21), trilaurin, trimyristin, tripalmitin and tristearin were compared. Cholesterol was administered by gavage rather than by diet, but again absorption was lowest and turnover highest in the tristearin-fed group. Waterman et al. (22) compared the effects of tallow and safflower oil on growth, plasma lipids and lipogenesis in rats, pigs and chicks. Growth was similar in pigs fed the two fats; plasma cholesterol levels were also similar (116 ± 6 mg/dl in the safflower oil group and 125 ± 4 mg/dl in the tallow group) and plasma triglyceride levels were significantly lower in the tallow-fed group (37 ± 6 mg/dl) than in the safflower oil group (56 ± 6 mg/dl). Weight gains in rats or chicks fed either fat were similar. There were no significant differences in plasma cholesterol levels but rats fed tallow had significantly higher triglyceride levels than those fed safflower oil. In all three species, adipose tissue fatty acid synthesis was considerably lower in tallow-fed animals. Kritchevsky et al. (23) studied cholesterol metabolism in rats fed 14% cocoa butter, palm kernel oil, coconut oil or corn oil. Cholesterol absorption was lowest in rats fed cocoa butter and cholesterogenesis from either acetate or mevalonate was highest.

Monsma and Ney (24) fed rats diets containing 0.2% cholesterol and 16% fats containing increasing amounts of stearic acids. The fats were lard (15% stearic acid), beef tallow (19% stearic acid) and cocoa butter (35% stearic acid). The control fat was corn oil (2% stearic acid). They found reduced absorption of stearic acid as its level in the diet increased. In a subsequent study (25), rats were fed 0.035% cholesterol and 16% corn oil (2% stearic acid), butterfat (18% stearic acid), beef tallow (16% stearic acid), palm oil (4% stearic acid) and coconut oil (3% stearic acid) for 6 weeks. Data are presented in Table 5.5

Rats fed butterfat exhibited significantly lower cholesterol levels than those fed tallow or palm oil. Rats fed beef tallow had the highest triglyceride levels. Of interest is the observation that the highest amounts of plasma cholesteryl ester were found in rats fed the fats containing the highest levels of stearic acid. It would be interesting to know if the fatty acid composition of the plasma cholesteryl esters varied with dietary fat. High-density lipoproteins (HDL) from

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