Food intake regulation by diet-induced thermogenesis.

Diet-induced thermogenesis represents energy wasted in the assimilation of food. Since this source of energy loss is variable, it has long been linked to obesity in people who assimilate food at high efficiency, the saved energy converting to adipose tissue. In this report we provide evidence to dispel that notion and, instead, show that the variability in DIT may be linked to the amount of food one consumes during the course of a meal. We call upon the well known temperature sensitive neurons in the hypothalamus acting in concert with the ventromedial nuclei to present a new version of an old 'thermostatic hypothesis' of food intake regulation.

Obesity: thermodynamic principles in perspective.

The energy balance equation applicable to all living organisms was used as a framework on which to construct a critical review of some of the more controversial aspects of the obesity problem. The equation matches energy intake against all the known forms of work that the body does in utilizing that energy, including external and internal work and the work of adipose tissue synthesis (stored energy). Equations representing everyday living conditions, resting, fasting and basal conditions were constructed. The equation applicable to everyday living (working, non-fasting) was used to develop a set of model paradigms to illustrate some of the devices that can be invoked to decrease expenditure and conserve energy. These served as models of how obesity can arise in the absence of calorie overconsumption. The same equation was then used to create a set of opposite paradigms showing how obesity can be prevented by increasing expenditure to waste energy and stabilize body weight when challenged by hyperphagia. In order to see caloric intake and the various work terms in their proper quantitative relationships it was necessary to assign numerical values to the equation. These were selected from published reports of caloric values representative of a non-obese adult of average size engaged in a typical white collar occupation. It was then easy to adjust these assigned values commensurate with the objectives described in the preceding paragraph. Since obesity research is hampered by a confusing array of metabolic interactions it was essential to alter only one of the energy terms at a time, excluding all metabolic interactions except for those unavoidable ones dictated by the laws of thermodynamics. Only in this way could we see the body's multiple energy forms in clear perspective with regard to their real quantitative significance in the energy balance sheet and their potential impact on body weight. Creating these models gave us the added advantage of enabling us better to evaluate the scientific literature because the data we generated, although theoretical, served as excellent standards against which to compare the real data that have emanated from research laboratories.(ABSTRACT TRUNCATED AT 400 WORDS)

Mode of action of lecithin in suppressing cholesterol absorption.

In vitro studies were performed to investigate a possible mechanism by which lecithin suppresses intestinal cholesterol absorption. The hypothesis that lecithin acts by retarding the diffusion of micelles across the unstirred water layer (UWL) was tested by measuring cholesterol influxes (Ji) in segments of rat intestine under conditions with no maximal and minimal effective thickness of the UWL. Ji was sharply reduced by lecithin under all conditions with no apparent relationship to the thickness of the UWL. The hypothesis was further tested by determining if lecithin had any effect on the molecular weight of the micelles. Gel filtration studies indicated no change in molecular weight (range, 21,000-24,000). Finally, actual measurements of the coefficients of free diffusion (D) of the micelles revealed that lecithin in the concentration that caused a 42-90% reduction in Ji caused only a 22% reduction in D. The basis for the decrease in D is unknown, but it was sufficient to account for the decrease in Ji. Therefore, except for the small decrease in micellar diffusion coefficient as a possible contributing factor, the data offered little support for the hypothesis and we concluded that lecithin suppresses cholesterol absorption by some other or additional mechanism, such as by a direct effect on cell membrane or by holding the cholesterol absorption by some other or additional mechanism, such as by direct effect on the cell membrane or by holding the cholesterol in micellar form to reduce its partition coefficient.

Effect of beta-sitostanol (5 alpha-stigmastan-3 beta-ol) on cholesterol absorption from micellar solutions in jejunal loops in situ.

In situ jejunal loops were infused with micellar solutions of cholesterol with or without beta-sitostanol (5 alpha-stigmastan-3 beta-ol), and the uptake of 14C-cholesterol by the loop was followed for 20 minutes. It was found that beta-sitostanol, given as a 'solution-mix' (a solution resulting from the mixture of two separate micellar solutions of cholesterol and beta-sitostanol), at a concentration of 0.30 mM reduced cholesterol uptake. Substituting cholesterol for beta-sitostanol in the 'solution-mix' had no effect on cholesterol uptake by the loop. beta-Sitostanol at a concentration of 0.30 mM in the 'pre-mix' (a solution resulting from pre-mixing of the two sterols prior to preparation of the micellar solution) condition, had no effect on cholesterol absorption. Taken together, these results suggest that the concentration of beta-sitostanol-containing micelles is the important factor in its suppression of cholesterol absorption.

Intestinal absorption and lymphatic transport of cholesterol and beta-sitostanol in the rat.

The intestinal absorption of cholesterol and beta-sitostanol (the saturated analogue of beta-sitosterol) were measured and their absorptions compared in the presence and absence of cholestyramine. After test meals containing [(3)H]cholesterol and [(14)C]beta-sitostanol without added cholestyramine, 4-day fecal collections yielded an average of 51% of the fed cholesterol and 83% of the fed beta-sitostanol. In separate lymph transport studies without cholestyramine, 36% of the fed cholesterol was recovered in lymph in 24 hours compared to only 2% of the fed beta-sitostanol. Thus, while total recoveries of the two labeled compounds in feces plus lymph were nearly identical (51% + 36% = 87% for cholesterol and 83% + 2% = 85% for beta-sitostanol) their distribution in the two compartments was markedly different, reflecting the relative nonabsorbability of beta-sitostanol. Adding cholestyramine to the test meal caused fecal excretion of cholesterol to increase to 73%, independent of the dose of cholestyramine used. Cholestyramine had no effect on the fecal excretion of beta-sitostanol (average excretion after cholestyramine, 85%). The relative non-absorbability of beta-sitostanol compared to cholesterol is clearly evident in this study and leads us to suggest its possible use as a lipid-soluble, nonabsorbable reference compound for measurement of the absorption of cholesterol and other lipids. Further data are presented to justify its use for this purpose.-Hassan, A. S., and A. J. Rampone. Intestinal absorption and lymphatic transport of cholesterol and beta-sitostanol in the rat.

The effect of phosphatidylcholine and lysophosphatidylcholine on the absorption and mucosal metabolism of oleic acid and cholesterol in vitro.

The absorption and mucosal metabolism of [14C]oleic acid and [3H]cholesterol were studied using everted sacs of rat jejunum in an in vitro incubation system. The labeled compounds were present in the incubation mixture either singly or together as mixed micelles with bile salt and monoacylglycerol and in the presence or absence of phosphatidylcholine or lysophosphatidylcholine. The presence of cholesterol or phosphatidylcholine markedly suppressed oleic acid absorption. We suggest that both compounds interacted with the micelles causing changes in micellar mass, charge or configuration leading to possible interference with access of the fatty acid to the cell membrane. Lysophosphatidylcholine enhanced oleic acid absorption and stimulated incorporation of the fatty into mucosal triacylglycerol. When the incubation temperature was lowered to suppress metabolism lysophosphatidylcholine had no effect. The results suggest that the increased absorption occurring at the higher temperature was secondary to enhanced glycerol acylation. Lysophosphatidylcholine had only a minimal effect on cholesterol absorption and no effect on cholesterol acylation. Evidence is presented showing that lysophosphatidylcholine is itself well absorbed and variously metabolized. We conclude that phosphatidylcholine and lysophosphatidylcholine have quite divergent effects on lipid absorption but the full elucidation of their mechanisms of action must await further study.

Studies on micellar fatty acid uptake by rat intestine in vitro with reference to the role of bile.

1. The uptake and esterification to trigylceride of oleic acid in micellar form was studied in rat intestine in vitro. Sacs of the upper half of the everted intestine taken from bile fistula rats were incubated in a buffered solution containing mono-olein, (14)C-labelled oleic acid and bile salt (sodium taurocholate (NaTch) in concentrations exceeding the critical micellar concentration).2. At 37 degrees C incubation temperature increasing the NaTch concentration enhanced both uptake and esterification. Adding whole rat bile caused uptake to decrease at all NaTch concentrations but had only a slight and variable effect on esterification.3. Lowering the incubation temperature to 0 degrees C suppressed esterification but had no effect on uptake.4. At 0 degrees incubation temperature adding whole bile still decreased fatty acid uptake but had no effect on esterification.5. It is concluded that intestinal fatty acid uptake from micelles is a non-energy requiring process and that a non-bile salt component in bile exists which can suppress this process.6. It is suggested that lecithin may be the non-bile salt component and that it suppressed uptake by interacting either with the micelles or with the epithelial membrane to reduce fatty acid permeability.

The effect of lecithin on intestinal cholesterol uptake by rat intestine in vitro.

1. Sacs 20 cm long were obtained from the upper half of the small intestine of bile fistula rats (bile duct cannulated 48 hours previously). The sacs were everted, filled with oxygenated phosphate buffer and incubated 1 hr at 37 degrees C in 25 ml. of a buffered micellar solution of oleic acid (0.6 mM), mono-olein (0.3 mM), sodium taurocholate (4.8 mM) and (3)H-labelled cholesterol (0.15 mM) plus glucose (28 mM).2. After incubation the amount of [(3)H]cholesterol taken up by the mucosal tissue was measured. It averaged 200 n-mole/hr.g tissue wet wt. +/- 6 (S.E.).3. Adding 3 ml. whole rate bile with other factors unchanged caused cholesterol uptake to decrease by 50% in confirmation of previous studies.4. Adding purified lecithin obtained from rat liver tissue, and from egg yolks, similarly decreased cholesterol uptake. A significant response was obtained with 2.5 mg liver lecithin (concentration 0.13 mM) and a near maximum response with 15 mg (concentration 0.80 mM). 10 mg lecithin decreased uptake by an amount equivalent to that obtained with 3 ml. whole bile.5. Lecithin is an active component of whole bile causing reduced intestinal cholesterol uptake from micelles.6. The decreased uptake of cholesterol in the presence of lecithin may have been the result of expansion of the cholesterol-containing micelles with consequent reduction in cholesterol permeability.

Bile salt and non-bile salt components in bile affecting micellar cholesterol uptake by rat intestine in vitro.

1. The uptake of micellar cholesterol was measured in sacs of the upper half of everted rat intestine. Sacs of 20 cm length were incubated 1 hr in 25 ml. phosphate buffer containing fatty acid, monoglyceride and (3)H-labelled cholesterol in micellar form with the bile salt, sodium taurocholate, as the dispersing agent.2. Sacs obtained from bile fistula rats (bile duct cannulated 48 hr previously) took up more than twice as much cholesterol as did sacs obtained from untreated control rats.3. In experiments utilizing bile-deficient sacs increasing the sodium taurocholate concentration caused an increase in cholesterol uptake. Conversely, adding a small amount of whole bile caused a decrease in cholesterol uptake.4. The inhibitory effects of the bile were greatly enhanced if the bile was pre-treated with cholestyramine to remove the bile salts.5. It is concluded that bile has a variable effect on intestinal cholesterol absorption depending upon its relative concentration of bile salt and a non-bile salt component having opposite actions.6. It is suggested that the variable effect may be related to the physico-chemical dispersion of cholesterol and that the non-bile salt component may be lecithin.

The effects of bile salt and raw bile on the intestinal absorption of micellar fatty acid in the rat in vitro.

1. The uptake, esterification and transport of [(14)C]oleic acid were studied using sacs of rat everted small intestine incubated in 25 ml. of a buffered mixture of sodium taurocholate, glyceryl mono-oleate and (14)C-labelled oleic acid in micellar form.2. Intestine obtained from bile fistula rats (bile duct cannulated 48 hr previously) showed elevated rates of (14)C uptake into the tissue total lipid compared with sham-operated controls.3. Nearly all of the excess (14)C uptake in the bile fistula group was in the form of free fatty acid. Both groups showed similar rates of [(14)C]oleic acid incorporation into tissue triglyceride and also similar, though small, amounts transported into the serosal fluid.4. In further experiments using intestine from bile fistula rats the addition of 1 ml. of fresh rat bile to the incubation mixture reduced the (14)C uptake to approximately control levels. The addition of 2-3 ml. of fresh bile similarly reduced the uptake and increased (14)C incorporation into the triglycerides of mucosal tissue and serosal fluid.5. These responses were not entirely the result of the bile salts contained in fresh bile since increasing the taurocholate concentration per se caused uptake, esterification and transport all to increase. In the presence of the higher taurocholate concentration the addition of fresh bile still caused a decrease in (14)C uptake.6. There was no significant effect of either fresh bile or taurocholate on the transport of the 3-O-methyl analogue of D-glucose under comparable conditions.7. It is concluded that raw bile contains one or more components other than bile salts which may be important in determining fatty acid absorption.

The relation between L-methionine uptake and sodium in rat small intestine in vitro.

1. Uptakes of L-methionine and mannitol by rat jejunum in vitro were measured over test periods from 5 to 120 sec after 30 min pre-test periods in the presence or absence of Na.2. The initial stage in methionine uptake was dependent on the presence of Na(+) and to a lesser extent on the K(+) concentration. In contrast mannitol uptake was independent of Na and K.3. The initial stage in methionine uptake can be reactivated 30-60% within 5 sec by replacing an Na-deficient intestine into an Na-containing medium.4. Initial methionine uptake was greater with a normal intracellular and low medium Na concentration than with a high medium and low intracellular Na concentration. It is suggested that the intracellular Na concentration is a critical factor, more important than the Na gradient, in determining the rate of amino acid transfer across the luminal membrane.