Environmental contributions to the obesity epidemic

The current epidemic of obesity is caused largely by an environment that promotes excessive food intake and discourages physical activity. Although humans have evolved excellent physiological mechanisms to defend against body weight loss, they have only weak physiological mechanisms to defend against body weight gain when food is abundant. Control of portion size, consumption of a diet low in fat and energy density, and regular physical activity are behaviors that protect against obesity, but it is becoming difficult to adopt and maintain these behaviors in the current environment. Because obesity is difficult to treat, public health efforts need to be directed toward prevention.

The variability of young children's energy intake

BACKGROUND

Research conducted in the 1930s showed that, given nutritious choices, children can select an adequate diet without adult supervision. Paradoxically, children grew well and were healthy despite patterns of intake at individual meals that were unpredictable and highly variable.

METHODS

To investigate in more detail the energy intake of young children, we measured 24-hour food intake for 15 children, from two to five years of age, on six days. For each of the six days of the study, coefficients of variation were calculated for each child for each of the six meals and snacks (breakfast, lunch, dinner, and morning, afternoon, and evening snacks) and for total daily energy intake.

RESULTS

The children's intake at individual meals was highly variable, but total daily energy intake was relatively constant for each child. The mean coefficient of variation for each child's energy intake at individual meals was 33.6 percent; in contrast, the mean coefficient of variation for each child's total daily energy intake was 10.4 percent. In most cases, high energy intake at one meal was followed by low energy intake at the next meal, or vice versa.

CONCLUSIONS

Although children's food consumption is highly variable from meal to meal, daily energy intake is relatively constant, because children adjust their energy intake at successive meals.

Fat and carbohydrate overfeeding in humans: different effects on energy storage

Both the amount and composition of food eaten influence body-weight regulation. The purpose of this study was to determine whether and by what mechanism excess dietary fat leads to greater fat accumulation than does excess dietary carbohydrate. We overfed isoenergetic amounts (50% above energy requirements) of fat and carbohydrate (for 14 d each) to nine lean and seven obese men. A whole-room calorimeter was used to measure energy expenditure and nutrient oxidation on days 0, 1, 7, and 14 of each overfeeding period. From energy and nutrient balances (intake-expenditure) we estimated the amount and composition of energy stored. Carbohydrate overfeeding produced progressive increases in carbohydrate oxidation and total energy expenditure resulting in 75-85% of excess energy being stored. Alternatively, fat overfeeding had minimal effects on fat oxidation and total energy expenditure, leading to storage of 90-95% of excess energy. Excess dietary fat leads to greater fat accumulation than does excess dietary carbohydrate, and the difference was greatest early in the overfeeding period.

Nutrient balance and energy expenditure during ad libitum feeding of high-fat and high-carbohydrate diets in humans

To study the influence of diet composition on regulation of body weight, we fed 21 weight-stable subjects (11 lean, 10 obese) high-carbohydrate (HC) and high-fat (HF) diets for 1 wk each. Although diet composition was fixed, total energy intake was unrestricted. Subjects had a higher energy intake on the HF (11,039 +/- 2700 kJ/d) than on the HC (10,672 +/- 2617 kJ/d) diet (P less than 0.05), but energy expenditure was not different between diets. On day 7 of the HC diet, carbohydrate (CHO) oxidation was significantly related to CHO intake with the slope of the regression line 0.99, suggesting that overall CHO balance was near zero. However, the slope of the regression line was greater for obese than for lean subjects. On day 7 of the HF diet, fat oxidation was significantly related to fat intake but the slope of the line was 0.50, suggesting that overall fat balance was positive. However, this relationship was due entirely to lean subjects, with obese subjects showing no relationship between fat intake and oxidation.

From instinct to intellect: the challenge of maintaining healthy weight in the modern world

The global obesity epidemic is being driven in large part by a mismatch between our environment and our metabolism. Human physiology developed to function within an environment where high levels of physical activity were needed in daily life and food was inconsistently available. For most of mankind's history, physical activity has 'pulled' appetite so that the primary challenge to the physiological system for body weight control was to obtain sufficient energy intake to prevent negative energy balance and body energy loss. The current environment is characterized by a situation whereby minimal physical activity is required for daily life and food is abundant, inexpensive, high in energy density and widely available. Within this environment, food intake 'pushes' the system, and the challenge to the control system becomes to increase physical activity sufficiently to prevent positive energy balance. There does not appear to be a strong drive to increase physical activity in response to excess energy intake and there appears to be only a weak adaptive increase in resting energy expenditure in response to excess energy intake. In the modern world, the prevailing environment constitutes a constant background pressure that promotes weight gain. We propose that the modern environment has taken body weight control from an instinctual (unconscious) process to one that requires substantial cognitive effort. In the current environment, people who are not devoting substantial conscious effort to managing body weight are probably gaining weight. It is unlikely that we would be able to build the political will to undo our modern lifestyle, to change the environment back to one in which body weight control again becomes instinctual. In order to combat the growing epidemic we should focus our efforts on providing the knowledge, cognitive skills and incentives for controlling body weight and at the same time begin creating a supportive environment to allow better management of body weight.

Atomic carbon as a terminal ligand: studies of a carbidomolybdenum anion featuring solid-state (13)C NMR data and proton-transfer self-exchange kinetics

Anion [CMo(N[R]Ar)(3)](-) (R = C(CD(3))(2)CH(3) or (t)Bu, Ar = 3,5-C(6)H(3)Me(2)) containing one-coordinate carbon as a terminal substituent and related molecules have been studied by single-crystal X-ray crystallography, solution and solid-state (13)C NMR spectroscopy, and density functional theory (DFT) calculations. Chemical reactivity patterns for [CMo(N[R]Ar)(3)](-) have been investigated, including the kinetics of proton-transfer self-exchange involving HCMo(N[R]Ar)(3), the carbidomolybdenum anion's conjugate acid. While the Mo triple bond C bond lengths in [K(benzo-15-crown-5)(2)][CMo(N[R]Ar)(3)] and the parent methylidyne, HCMo(N[R]Ar)(3), are statistically identical, the carbide chemical shift of delta 501 ppm is much larger than the delta 282 ppm shift for the methylidyne. Solid-state (13)C NMR studies show the carbide to have a much larger chemical shift anisotropy (CSA, 806 ppm) and smaller (95)Mo--(13)C coupling constant (60 Hz) than the methylidyne (CSA = 447 ppm, (1)J(MoC) = 130 Hz). DFT calculations on model compounds indicate also that there is an increasing MoC overlap population on going from the methylidyne to the terminal carbide. The pK(a) of methylidyne HCMo(N[R]Ar)(3) is approximately 30 in THF solution. Methylidyne HCMo(N[R]Ar)(3) and carbide [CMo(N[R]Ar)(3)](-) undergo extremely rapid proton-transfer self-exchange reactions in THF, with k = 7 x 10(6) M(-1) s(-1). Besides being a strong reducing agent, carbide [CMo(N[R]Ar)(3)](-) reacts as a nucleophile with elemental chalcogens to form carbon-chalcogen bonds and likewise reacts with PCl(3) to furnish a carbon-phosphorus bond.

Thermogenesis in humans during overfeeding with medium-chain triglycerides

To test whether excess dietary energy as medium-chain triglycerides (MCT) affects thermogenesis differently from excess dietary energy as long chain triglycerides (LCT), ten male volunteers (ages 22 to 44) were overfed (150% of estimated energy requirement) liquid formula diets containing 40% of fat as either MCT or LCT. Each patient was studied for one week on each diet in a double-blind, crossover design. Resting metabolic rate (RMR) did not change during either week of overfeeding. The thermic response to food (TEF) was greater on day 1 following a meal (1,000 kcal) containing MCT than following an isocaloric meal containing LCT (8 +/- .8% v 5.8 +/- .8% of ingested energy; P less than .05). Moreover, the TEF observed after a 1,000 kcal meal containing MCT increased significantly to 12% (+/- 1.3%) overfeeding. The TEF of the 1,000 kcal meal containing LCT was unchanged by five days of LCT overfeeding (6.6 +/- 1.0% of ingested energy). Energy expenditure during a 20-hour continuous enteral infusion of the diet on day 7 was also significantly higher with the MCT diet than with the LCT diet (15.7 +/- 1.7% v 7.3 +/- .9% of ingested energy; P less than .05). Our results demonstrate that excess dietary energy as MCT stimulates thermogenesis to a greater degree than does excess energy as LCT. This increased energy expenditure, most likely due to lipogenesis in the liver, provides evidence that excess energy derived from MCT is stored with a lesser efficiency than is excess energy derived from dietary LCT.

Adaptation of rats to diets containing different levels of protein: effects on food intake, plasma and brain amino acid concentrations and brain neurotransmitter metabolism

Food intake, plasma and brain amino acid concentrations, liver amino acid catabolic enzyme activities, and whole-brain neurotransmitter and metabolite concentrations were measured in young rats adapted for 11 d to diets containing from 5 to 75% (in increments of 5%) casein. Food intake was depressed initially in rats fed diets containing 5, 10% or greater than 35% casein. For the duration of the experiment, food intakes of the groups fed the higher protein diets improved on successive days; the length and severity of the depression were proportional to the protein content of the diet fed. Rats fed low levels of protein grew poorly, and their food intake remained depressed. The gradual improvement in growth and food intake of rats fed diets containing more than 35% casein was accompanied by dramatic increases in the activities of serine-threonine dehydratase (SDH, EC 4.2.1.16) and glutamate-pyruvate aminotransferase (GPT, EC 2.6.1.1) in liver. The increase in amino acid catabolic activity was accompanied by decreases in the concentrations of most amino acids in plasma and brain. However, concentrations of branched-chain amino acids, in both plasma and brain, increased in direct proportion to the protein concentration of the diet fed. As a result of these reciprocal responses, the total concentration of indispensable amino acids in brain (IAA) was maintained within a narrow range of values, despite a sixfold range of protein intakes. Whole-brain concentrations of norepinephrine, dopamine and serotonin were not correlated with dietary protein concentration, total food intake or protein intake. Brain concentrations of homovanillic acid and 5-hydroxyindoleacetic acid were correlated inversely with protein intake and that of 3,4-dihydroxyphenylacetic acid was correlated directly with food intake. Protein intake appeared to be related to the animal's ability to maintain brain total IAA content between some upper and lower limits. Our results indicate that this was accomplished initially through downward adjustment of protein intake and subsequently through an increase in catabolic capacity for the amino acids.

Nutrient balance in humans: effects of diet composition

The purpose of this study was to examine the effect of alterations in diet composition on energy expenditure and nutrient balance in humans. Eight adults (three men, five women) ate a high-carbohydrate (60% of calories from carbohydrate) and a high-fat (60% of calories from fat) diet for 7 d each according to a randomized, crossover design. Six subjects were studied for an additional week on a mixed diet (45% of calories from fat). For each subject, total caloric intake was identical on all diets and was intended to provide the subject's maintenance energy requirements. All subjects spent days 3 and 7 of each week in a whole-room indirect calorimeter. Diet composition did not affect total daily energy expenditure but did affect daily nutrient oxidation by rapidly shifting substrate oxidation to more closely reflect the composition of the diet. These results show that diet composition can affect substrate oxidation without producing measurable effects on total energy expenditure.

Dietary carotenoids and certain cancers, heart disease, and age-related macular degeneration: a review of recent research

Key epidemiologic studies show associations between high dietary intakes of certain carotenoid-containing fruits and vegetables and reduced risk of prostate cancer, breast cancer, head and neck cancers, cardiovascular disease, and age-related macular degeneration, although overall the evidence is inconsistent. Little is known about the potential biochemical mechanisms whereby carotenoids might protect against disease, and human intervention trials are limited to high dose beta-carotene, which is not protective against lung cancer or cardiovascular disease. Authoritative scientific organizations continue to emphasize increased consumption of fruits and vegetables but do not make specific recommendations for carotenoids because of a lack of data that directly link them to disease reduction.

Pincer-like amido complexes of platinum, palladium, and nickel

The ligands bis(8-quinolinyl)amine (BQAH, 1), (2-pyridin-2-yl-ethyl)-(8-quinolinyl)amine (2-pyridin-2-yl-ethyl-QAH, 2), o-dimethylaminophenyl(8-quinolinyl)amine (o-(NMe2)Ph-QAH, 3), and 3,5-dimethylphenyl(8-quinolinyl)amine (3,5-Me2Ph-QAH, 4) have been prepared in high yield from aryl halide and amine precursors by palladium-catalyzed coupling reactions. Deprotonation of 1 with nBuLi in toluene affords the lithium amide complex [Li][BQA] (5), whose dimeric solid-state crystal structure is presented. Lithium amide 5 was transmetalated by TlOTf to afford the thallium(I) amido complex [Tl][BQA] (6). An X-ray structural study of 6 shows it to be a 1:1 complex of the BQA ligand and Tl. Entry into the group 10 chemistry of the parent ligand 1 was effected by both protolytic and metathetical strategies. Thus, the divalent chloride complexes (BQA)PtCl (7), (BQA)PdCl (8), and (BQA)NiCl (9) were prepared and fully characterized. An X-ray structural study for each of these three complexes shows them to be well-defined, square-planar complexes in which the auxiliary BQA ligand binds in a planar, eta(3)-fashion. For comparison, the reactivity of ligands 2-4 with (COD)PtCl2 was studied. While reaction with ligand 2 afforded an ill-defined product mixture, ligands 3 and 4 reacted with (COD)PtCl2 to generate the unusual alkyl complexes (o-(NMe2)Ph-QA)Pt(1,2-eta(2)-6-sigma-cycloocta-1,4-dienyl) (10) and (3,5-Me2Ph-QA)Pt(1,2-eta(2)-6-sigma-cycloocta-1,4-dienyl) (11), both of which have been structurally characterized.

Effects of a nonenergy fat substitute on children's energy and macronutrient intake

To determine whether children adjusted their energy intake in response to covert manipulations in the proportion of energy from dietary fat, 24-h food intake of 29 2- to 5-y-old children was measured over four 2-d blocks. In this within-subject crossover design, in the first three meals of the first day of each block, children consumed foods containing dietary fat or a nonenergy fat substitute, which provided 10% of total daily energy intake. Children compensated for the missing energy; cumulative energy intake differed by only 100 kJ over 2 d. Substitution for dietary fat reduced the percent of energy from fat from 38.7% to 36.4%. Children's intake at individual meals was highly variable (mean CV 24.7%) relative to the variability of total daily energy intake (CV = 8.6%). To produce this pattern, children adjusted energy intake across successive meals. Use of a fat substitute at 10% of energy from dietary fat did not significantly reduce 24-h energy intake.

Evaluation of the potential for olestra to affect the availability of dietary phytochemicals

It has been hypothesized that phytochemicals found in fruits and vegetables are responsible for the inverse association observed between diets high in fruits and vegetables and risk of certain chronic diseases and cancer. This paper assesses the potential for olestra to affect the absorption of dietary phytochemicals and estimates the effect of olestra on the availability of carotenoids when olestra-containing snacks and foods containing carotenoids are eaten in free-living diets. Experimental data compiled on the effects of olestra on the availability of 29 compounds, mainly nutrients and oral medications, showed that olestra affects the availability of only molecules having octanol-water partition coefficients greater than approximately 7.5. Partition coefficients compiled for 382 dietary phytochemicals showed that only two classes of phytochemicals, phytosterols and carotenoids, contain molecules with octanol-water partition coefficients in the range in which olestra could potentially affect bioavailability. The potential effect on the bioavailability of phytosterols would be <10% and would not be expected to be of concern inasmuch as the hypothesized benefit of consuming pharmacological amounts of phytosterols is to reduce cholesterol availability, a function also of olestra. A 5.9% reduction in the average effective beta-carotene intake was calculated for individuals eating olestra-containing snack foods in free-living diets. The calculation was made by assuming that carotenoid bioavailability would be reduced to the extent measured in human clinical studies each time olestra-containing snacks and carotenoid-containing foods are eaten together and that all snacks eaten are made with olestra. Among individuals with low carotenoid intakes (the lowest 10%) the calculated reduction was 6.0%; for heavy snack eaters (the top 10%) it was 9.5%. These effects on carotenoid bioavailability are similar to those that can occur with other dietary factors.

Protein intake, brain amino acid and serotonin concentrations and protein self-selection

Analysis of evidence of associations among dietary protein content, brain amino acid and serotonin concentrations, and protein self-selection by rats suggests that 1) protein intake is not regulated precisely, although rats will select between low and high protein diets to obtain an adequate, but not excessive, amount of protein; 2) associations between brain serotonin concentration and protein intake are weak, although consumption of single meals of protein-deficient diets will elevate brain serotonin concentration; 3) the nature of signals that drive rats to avoid diets containing inadequate or excessive amounts of protein remains obscure; (4) whole brain amino acid and serotonin concentrations are quite stable over the usual range of protein intakes, owing to competition among amino acids for uptake across the blood-brain barrier and effective metabolic regulation of blood amino acid concentrations; 5) protein intake and preference are not in themselves regulated, but what appears to be regulation of intake and preference is a reflection of the responses of systems for control of plasma amino acid concentrations; and (6) the relative stability of the average protein intake of groups of self-selecting rats (which gives the appearance of regulation) results from averaging the variable behavioral responses--learned aversions and preferences--of rats to the variety of sensory cues arising from diets that differ in protein content.

Effects of olestra, a noncaloric fat substitute, on daily energy and fat intakes in lean men

Nutrient and energy intakes, hunger, and fullness were examined after the replacement of 36, 20, or 0 g fat in breakfast with olestra, a noncaloric fat substitute. Twenty-four lean, nondieting men (aged 21-30 y) participated in a placebo-controlled, three-condition crossover design. Self-selected, ad libitum intakes at lunch and dinner were monitored in the laboratory. Evening snacks and breakfast the next day were assessed through food diaries. Visual-analog-scale ratings including hunger and fullness were collected throughout the test days. Single-meal olestra substitution produced a significant dose-related reduction in the amount and percentage of energy from fat consumed daily with a reciprocal increase in carbohydrate intake. Daily energy intakes were not significantly different nor did ratings of hunger and fullness vary systematically between conditions. Consumption of olestra can reduce fat intake and increase carbohydrate intake without affecting total daily energy intake or usual patterns of hunger and fullness.

Effect of fat-free potato chips with and without nutrition labels on fat and energy intakes

This study investigated the effect on fat and energy intakes of fat-free potato chips made with olestra compared with regular potato chips. Ninety-five participants (unrestrained and restrained males and females) were tested in 2 conditions. In the information condition, participants were given nutrition information about the chips and were aware that the chips differed in fat and energy contents. In the no-information condition, participants were not aware of the differences. In both conditions, participants ate either regular or fat-free potato chips ad libitum for an afternoon snack in a crossover design in two 10-d periods. To assess 24-h intake, participants completed food diaries twice in each 10-d period. The results showed that all groups significantly reduced their fat and energy intakes in the snack when eating the fat-free chips compared with the regular chips (P< 0.0001). Also, potato chip intake did not differ across time for either type of chip. Over 24 h all participants had lower fat intakes (P< 0.05) when eating the fat-free potato chips compared with the regular chips, but 24-h energy intake was not significantly different between groups. When information was provided, restrained participants ate more of the fat-free chips than the regular chips; however, this increase did not negate the reductions in fat and energy associated with eating the fat-free chips. This study showed that substituting fat-free (olestra-containing) potato chips for regular-fat chips can help reduce fat and energy intakes in short-term (within meal) situations and reduce fat intake over 24 h.

Long-term effects of dietary organic and inorganic selenium sources and levels on reproducing sows and their progeny1,2,3,4

An experiment evaluated the effects of feeding either a basal non-Se-fortified diet, two Se sources (organic or inorganic) each providing 0.15 and 0.30 ppm Se, or their combination (each providing 0.15 ppm Se) on gilt growth and sow reproductive performance. The experiment was a 2 x 2 + 2 factorial conducted in a randomized complete block design in three replicates. One hundred twenty-six crossbred gilts were started on one of the six treatment diets at 27.6 kg BW. During the grower phase, animals were bled at 30-d intervals with three gilts killed per treatment at 115 kg BW for tissue Se analysis. Fifteen gilts per treatment were bred at 8 mo of age and were continued on their treatment diets for four parities. Sow serum collected within parity was analyzed for Se and glutathione peroxidase (GSH-Px) activity. Tissue Se was determined from five 0-d-old pigs per treatment from fourth-parity sows. Three sows per treatment were killed after the fourth parity for tissue Se analysis. Similar treatment performance responses occurred from 27 to 115 kg BW. Serum Se (P < 0.01) and GSH-Px activity (P < 0.05) increased for both Se sources to 0.30 ppm Se during the grower and reproductive periods. Serum Se and GSH-Px activity decreased from 70 to 110 d postcoitum in all treatment groups, but increased at weaning (P < 0.01) in the Se-fortified groups. The number of pigs born (total, live) increased (P < 0.05) with the 0.15 ppm Se level for both Se sources. Tissue and total body Se content of 0-d-old pigs increased with Se level (P < 0.01) and also when the organic Se source (P < 0.01) was fed to the sow. When sows were fed either Se source, pig serum Se (P < 0.01) and GSH-Px activity (P < 0.05) increased at weaning. Colostrum and milk Se concentrations increased (P < 0.01) with Se level for both Se sources, but were substantially greater (P < 0.01) when sows were fed organic Se. The combination of Se sources had sow milk and tissue Se values that were similar to those of sows milk and fed 0.15 ppm organic Se. The fourth-parity sows had greater tissue Se concentrations when organic Se level was increased (P < 0.01), more so than when sows were fed inorganic Se. These results suggest that both Se sources resulted in similar sow reproductive performances at 0.15 ppm Se, but sows fed the organic Se source had a greater transfer of Se to the neonate, colostrum, milk, weaned pig, and sow tissues than sows fed inorganic Se.

Metabolic differences between obesity-prone and obesity-resistant rats

We compared, across several physiological variables, rats most and least susceptible to develop obesity when given a high-fat diet. After 4 wk of eating a high-fat diet (60% of calories from fat), rats in the upper (obesity prone, OP) and lower (obesity resistant, OR) quartiles for weight gain were further studied. OP rats ate significantly more than OR rats, but this did not completely explain differences in their susceptibility to dietary obesity. No differences in 24-h energy expenditure were found between groups. OR rats had a significantly lower 24-h respiratory quotient, indicative of a greater relative proportion of fat oxidation and lower plasma levels of free fatty acids (FFA) than OP rats. Thus the ability to avoid dietary obesity produced by a high-fat diet may depend on an ability to increase fat oxidation in response to increased fat intake. Insulin sensitivity, measured by a euglycemic insulin clamp, was significantly higher in OR than OP rats. We cannot determine from these data whether insulin resistance developed as a consequence of elevated FFA levels or whether the ability to oxidize FFA declined as a result of development of insulin resistance. In summary, we propose that rats able to resist becoming obese on a high-fat diet have the ability to adjust the composition of fuel oxidized to the fuel composition of the diet with a minimum increase in body fat. The specific mechanisms by which this occurs are unknown but may be related to effects of diet on insulin sensitivity.

Dietary carotenoids and lung cancer: a review of recent research

Several hundred carotenoid research studies have been published since 1996, when two major intervention trials showed a lack of protective effect of beta-carotene supplements against lung cancer. Recent epidemiologic studies continue to show an association between high dietary intake of beta-carotene and lower risk of lung cancer. New research is attempting to clarify the apparently contradictory results of intervention and epidemiologic studies. Promising areas of investigation include characterizing biologic activities of carotenoids and gaining further insight into whether they may serve primarily as markers for a healthy lifestyle or diet.

Medium-chain fatty acids: evidence for incorporation into chylomicron triglycerides in humans

The purpose of this study was to evaluate the fatty acid composition of chylomicron triglycerides isolated from subjects fed liquid-formula diets containing 40% of total energy as medium- (C8:0 and C10:0) or long-chain (C16-C18) triglycerides (MCT, LCT) for 6 d. Medium-chain fatty acids (MCFA) comprised 8% of total chylomicron triglyceride fatty acids after the first MCT meal. After 6 d of continued MCT feeding, chylomicron triglyceride MCFA content increased to 13%. When subjects were fed the LCT (soybean oil) diet, C16:0, C18:1, and C18:2 comprised nearly 90% of the chylomicron triglyceride fatty acids. The mass of triglyceride transported in chylomicrons isolated from subjects fed the MCT diet was approximately 20% of that found when subjects consumed the LCT diet. We conclude that although total triglyceride production during MCT ingestion is low, the chylomicron triglycerides that are synthesized contain significant amounts of MCFA.

Plasma lipids and lipoproteins during 6 d of maintenance feeding with long-chain, medium-chain, and mixed-chain triglycerides

This study assessed fasting plasma lipids and lipoproteins and postprandial plasma lipids in healthy male subjects fed liquid-formula diets containing 40% of total energy as long-chain (LCT, primarily C18:1 and C18:2), medium-chain (MCT, C8:0-C10:0), or mixed-chain (structured lipid, STL, mostly C8:0, C10:0, and C22:0) triglycerides for 6 d. None of the diets altered plasma cholesterol concentrations. HDL cholesterol was decreased 14% by the STL diet (P < 0.044) and 15% by the MCT diet (P < 0.004) but was unchanged by the LCT diet. Plasma triglycerides were elevated 42% by the MCT diet (P < 0.006), but were unaltered by either the STL or LCT diets. Neither the STL nor the MCT diets produced changes in fasting lipoprotein lipid composition; however, during the LCT diet, VLDLs became enriched in triglyceride and LDLs became enriched in cholesterol. Postprandial triglyceridemia was significantly greater after subjects consumed the LCT diet than it was after they consumed either the STL or MCT diets, which were similar. Short-term feeding of MCT and STL diets produces significant changes in lipid metabolism. An understanding of the long-term effects of these diets awaits further study.