Is regular exercise an effective strategy for weight loss maintenance?

Weight regain after weight loss is one of the most significant challenges to successful obesity treatment. Regular exercise has long been touted as a strategy for weight loss maintenance, but the lack of clear evidence in clinical trials has caused some to question its effectiveness. In this review, we present the arguments both questioning and in support of exercise as an obesity therapeutic. Our purpose is to bring clarity to the literature, present a unified perspective, and identify the gaps in knowledge that need to be addressed in future studies. Critical questions remain including sex differences, individual variability and compensatory behaviors in response to exercise, exercise adherence, the role of energy flux and the molecular mechanisms mediating the beneficial effects of exercise after weight loss and during weight regain. Future research should focus on these critical questions to provide a more complete understanding of the potential benefits of exercise on weight loss maintenance.

The role for adipose tissue in weight regain after weight loss.

Weight regain after weight loss is a substantial challenge in obesity therapeutics. Dieting leads to significant adaptations in the homeostatic system that controls body weight, which promotes overeating and the relapse to obesity. In this review, we focus specifically on the adaptations in white adipose tissues that contribute to the biological drive to regain weight after weight loss. Weight loss leads to a reduction in size of adipocytes and this decline in size alters their metabolic and inflammatory characteristics in a manner that facilitates the clearance and storage of ingested energy. We present the hypothesis whereby the long-term signals reflecting stored energy and short-term signals reflecting nutrient availability are derived from the cellularity characteristics of adipose tissues. These signals are received and integrated in the hypothalamus and hindbrain and an energy gap between appetite and metabolic requirements emerges and promotes a positive energy imbalance and weight regain. In this paradigm, the cellularity and metabolic characteristics of adipose tissues after energy-restricted weight loss could explain the persistence of a biological drive to regain weight during both weight maintenance and the dynamic period of weight regain.

Trafficking of dietary oleic, linolenic, and stearic acids in fasted or fed lean rats.

Increasing evidence supports the notion that there are significant differences in the health effects of diets enriched in saturated, as opposed to monounsaturated or polyunsaturated fat. However, the current understanding of how these types of fat differ in their handling by relevant tissues is incomplete. To examine the effects of fat type and nutritional status on the metabolic fate of dietary fat, we administered (14)C-labeled oleic, linolenic, or stearic acid with a small liquid meal to male Sprague-Dawley rats previously fasted for 15 h (fasted) or previously fed ad libitum (fed). (14)CO(2) production was measured for 8 h after tracer administration. The (14)C content of gastrointestinal tract, serum, liver, skeletal muscle (soleus, lateral, and medial gastrocnemius), and adipose tissue (omental, retroperitoneal, and epididymal) was measured at six time points (2, 4, 8, 24, and 48 h and 10 days) after tracer administration. Plasma levels of glucose, insulin, and triglyceride were also measured. Oxidation of stearic acid was significantly less than that of either linolenic or oleic acid in both the fed and fasted states. This reduction was in part explained by a greater retention of stearic acid within skeletal muscle and liver. Oxidation of oleate and stearate were significantly lower in the fed state than in the fasted state. In the fasted state, liver and skeletal muscle were quantitatively more important than adipose tissue in the uptake of dietary fat tracers during the immediate postprandial period. In contrast, adipose tissue was quantitatively more important than skeletal muscle or liver in the fed state. The movement of carbons derived from dietary fat between tissues is a complex time-dependent process, which varies in response to the type of fat ingested and the metabolic state of the organism.

Hyperthermia impairs liver mitochondrial function in vitro.

The effects of temperature on the relationships among the rates of pyruvate carboxylation, O(2) uptake (J(o)), oxidative phosphorylation (J(p)), and the free energy of ATP hydrolysis (G(p)) were studied in liver mitochondria isolated from 250-g female rats. Pyruvate carboxylation was evaluated at 37, 40, and 43 degrees C. In disrupted mitochondria, pyruvate carboxylase maximal reaction velocity increased from 37 to 43 degrees C with an apparent Q(10) of 2.25. A reduction in ATP/ADP ratio decreased enzyme activity at all three temperatures. In contrast, in intact mitochondria, increasing temperature failed to increase pyruvate carboxylation (malate + citrate accumulation) but did result in increased J(o) and decreased extramitochondrial G(p). J(p) was studied in respiring mitochondria at 37 and 43 degrees C at various fractions of state 3 respiration, elicited with a glucose + hexokinase ADP-regenerating system. The relationship between J(o) and G(p) was similar at both temperatures. However, hyperthermia (43 degrees C) reduced the J(p)/J(o) ratio, resulting in lower G(p) for a given J(p). Fluorescent measurements of membrane phospholipid polarization revealed a transition in membrane order between 40 and 43 degrees C, a finding consistent with increased membrane proton conductance. It is concluded that hyperthermia augments nonspecific proton leaking across the inner mitochondrial membrane, and the resultant degraded energy state offsets temperature stimulation of pyruvate carboxylase. As a consequence, at high temperatures approaching 43 degrees C, the pyruvate carboxylation rate of intact liver mitochondria may fail to exhibit a Q(10) effect.

Disposition of dietary ethanol carbons in rats: effects of gender and nutritional status.

Dietary ethanol is an important contributor to total caloric intake and has been associated with gender-specific alterations in body weight and the risk for coronary heart disease. To understand the metabolic basis of these effects, it is important to first clarify the effects of gender and nutritional state on the metabolic fate of dietary ethanol. Tracer studies were therefore performed using 14C-labeled ethanol in fasted or fed male and female Sprague-Dawley rats (N = 64) previously unexposed to ethanol. 1-(14C)-ethanol (4.5 microCi) was mixed with unlabeled ethanol (for a total ethanol dose equal to 10% of total daily caloric intake) and a 3-kcal liquid meal and administered through gastric feeding tubes. 14CO2 production was measured over the subsequent 8 hours. The 14C content of skeletal muscle, liver, adipose tissue, gastrointestinal (GI) tract, brain, heart, kidney, and serum was determined at 4 time points following tracer administration (20 minutes and 3, 8, and 24 hours; n = 4 at each time point). Tracer content on a whole-body level was significantly greater in skeletal muscle compared with liver in all groups (1.32 +/- 0.02 x 10(6) v 0.27 +/- 0.02 x 10(6) dpm, P < .001). Skeletal muscle tracer content decreased rapidly after 3 hours, whereas liver tracer content remained fairly constant throughout the study period. Fed female rats were the exception, with a significant increase in the tracer content of total liver and liver lipid at 8 hours. The tracer content was higher in the lipid extracts in liver from fed rats compared with fasted rats (1.08 +/- 0.19 x 10(5) v 0.48 +/- 0.08 x 10(5) dpm, P = .002). While male rats exhibited a fairly constant tracer content in adipose tissue throughout the 24-hour period, female rats showed an increase in adipose tissue tracer content at 8 and 24 hours, with levels 3 to 4 times those of the male animals (5.91 +/- 1.42 x 10(4) v 1.55 +/- 0.42 x 10(4) dpm, P = .02). These results demonstrate that (1) skeletal muscle plays an important role in the metabolism of dietary ethanol, (2) the fed state appears to favor the conversion of ethanol-derived carbons to lipid, and (3) female rats have a greater propensity to convert ethanol-derived carbons to lipid and to store these carbons in adipose tissue.

Cell polarization is required for ricin sensitivity in a Caco-2 cell line selected for ricin resistance.

It has been proposed that killing of mammalian cells by ricin requires efficient endocytic delivery to the trans-Golgi network (TGN) prior to retrograde transport to the endoplasmic reticulum and entry to the cytosol. In polarized epithelial cells, an efficient membrane-traffic pathway to the TGN is present from the basolateral but not the apical plasma-membrane domain. Thus one can hypothesize that a ricin-resistant phenotype might be demonstrated by polarized cells that fail to differentiate and thus fail to develop an efficient membrane-traffic pathway from the basolateral plasma membrane to the TGN. We have isolated and studied a ricin-resistant Caco-2 cell clone (Caco-2-RCAr clone 2) which, when grown on plastic, was deficient in differentiation, measured by the development of polarized-cell-surface marker enzymes. The deficiency in differentiation was partially reversed, and ricin sensitivity was restored, when the cells were grown on filter supports. Our data provide the first evidence of a ricin-resistant cell line where resistance is due to the lack of development of polarized cell surfaces. The observed ricin resistance is consistent with the requirement that ricin is delivered to the TGN before its A chain enters the cytosol to mediate cell killing.

Cyclins and the G2/M transition.

The entry of a cell into mitosis is regulated by an elaborate network of kinases and phosphatases that control both for the timing of cell division and the complete reorganization of the cellular architecture. The mitotic cyclin/Cdks form part of large multiprotein complexes whose other components are only now beginning to be identified. The continuing identification of proteins that contribute to these complexes and changes in the composition of these complexes are likely to give a more integrated view of how mitotic cyclin/Cdk complexes are regulated and how they function-not only to induce mitosis, but also to aid further mitotic progression. Furthermore, assigning specific G2/M functions to distinct mitotic cyclin/Cdk complexes will require the identification of differences in substrate specificities between the mitotic cyclin/Cdk complexes, perhaps in parallel with specific cyclin knockouts in mice. Such investigations will be complicated by potential functional overlap between mitotic cyclin/Cdk complexes in vitro and in vivo. Although cyclin/Cdk1 is thought to be the major kinase that initiates the onset of mitosis, a more complete understanding of how cells move from G2 to a mitotic state will require further identification of kinases operating upstream, downstream and in parallel with Cdk1, their substrates and their relationship with one another during the G2/M transition.

Characteristics of mitochondria isolated from type I and type IIb skeletal muscle.

Mitochondria isolated from rabbit soleus (98% type I) and gracilis (99% type IIb) skeletal muscle were compared for compositional differences. Whole muscle mitochondrial contents were 14.5 +/- 1.2 mg/g of wet weight in soleus and 5.3 +/- 0.6 mg/g in the gracilis muscle, a 2.7-fold difference. Maximal pyruvate plus malate oxidase activity in gracilis mitochondria was roughly 75% of that in soleus mitochondria. In contrast, glycerol 3-phosphate (G-3-P) oxidation was 10-fold greater in gracilis mitochondria. Both soleus and gracilis mitochondria exhibited additive pyruvate and G-3-P oxidase activities. In general, citric acid cycle enzyme activities were higher in soleus mitochondria. A notable exception was isocitrate dehydrogenase, which was twofold higher in gracilis mitochondria. Substrate cytochrome c reductase activities indicated that the electron transport chain (ETC) of soleus mitochondria possess roughly twice the capacity for both NADH and succinate oxidation. Similarly, the maximal activities of NADH dehydrogenase and succinate dehydrogenase were roughly twofold higher in soleus mitochondria. The findings demonstrate that mitochondria isolated from types I and IIb skeletal muscle differ substantially in composition. Furthermore, the relatively similar pyruvate plus malate oxidase activities in the face of markedly different ETC capacities suggest that the interaction between matrix dehydrogenases and the ETC may differ in mitochondria isolated from types I and IIb skeletal muscle.

Effect of choline supplementation on fatigue in trained cyclists.

The availability of choline, the precurser of the neurotransmitter, acetylcholine, in the diet is sufficient to provide the body's requirements under normal conditions. However, preliminary evidence indicates that depletion of choline may limit performance, while oral supplementation may delay fatigue during prolonged efforts. A double-blind cross-over design was used to determine the relationship between plasma choline and fatigue during supramaximal brief and submaximal prolonged activities. Twenty male cyclists (ages 23-29) with maximal aerobic power (VO2max) between 58 and 81 ml.min-1.kg-1 were randomly divided into BRIEF (N = 10) and PROLONGED (N = 10) groups. One hour after drinking a beverage with or without choline bitartrate (2.43 g), cyclists began riding at a power output equivalent to approximately 150% (BRIEF) and 70% (PROLONGED) of VO2max at a cadence of 80-90 rpm. Time to exhaustion, indirect calorimetry and serum choline, lactate, and glucose were measured. Increases in choline levels of 37 and 52% were seen within one hour of ingestion for BRIEF and PROLONGED groups, respectively. Neither group depleted choline during exercise under the choline or placebo conditions. Fatigue times and work performed under either test condition for the BRIEF or PROLONGED groups were similar. Consequently, trained cyclists do not deplete choline during supramaximal brief or prolonged submaximal exercise, nor do they benefit from choline supplementation to delay fatigue under these conditions.

Mitochondrial function during heavy exercise.

Maximal rates, coupling, and control of oxidative phosphorylation were studied in isolated skeletal muscle mitochondria from rat and rabbit. Mitochondria were incubated under various conditions of temperature, pH, and substrate availability. A 20% decrease in coupling (ADP/O) was observed at 43 degrees C as compared to 37 degrees C in rat mixed skeletal muscle mitochondria. Changes in pH from 7.00 to 6.20 affected neither coupling nor maximal (state 3) respiration rates. Changing the substrate supply from pyruvate to palmitoyl-carnitine (+ malate) did not alter ADP/O, but markedly degraded the energy state sustained at submaximal ATP turnover. Thus, carbohydrate depletion may be associated with inhibition of contractile function and the recruitment of less economical higher threshold motor units. State 3 respiration of mitochondria from rabbit Type IIb fibers oxidizing pyruvate+malate+alpha-glycerophosphate was 27% higher than that of mitochondria from Type I rabbit skeletal muscle. However, the ADP/O ratio in the Type IIb preparation was 18% lower. The experimental findings suggest that temperature, substrate supply, and energetic differences between slow twitch and fast twitch motor units may impact the economy of mitochondrial oxygen utilization during heavy aerobic exercise, and thus contribute to the slow component of oxygen uptake.

Inhibition of apical but not basolateral endocytosis of ricin and folate in Caco-2 cells by cytochalasin D.

Apical and basolateral endocytic pathways in polarised Caco-2 cells were investigated by following the uptake, recycling and transcytosis of the galactose-binding protein toxin ricin, as a membrane marker. Differences in the extent and kinetics of lectin uptake, recycling and transcytosis were observed at the apical and basolateral domains and altered with the age of the cell monolayer. Treatment of polarised Caco-2 cells with cytochalasin D showed a domain-specific, concentration-dependent inhibition of apical endocytosis of ricin. Inhibition of apical endocytosis by cytochalasin D was not due to a gross change in brush border morphology, although actin stress fibres within the cell body were disrupted. It is not clear whether inhibition of apical endocytosis in polarized epithelial cells by cytochalasin D is caused simply by disruption of a mechanochemical motor involving microvillar actin filaments. The cytochalasin D effect was also observed when measuring uptake of folate, suggesting apical domain-specific inhibition of caveolar, as well as clathrin-mediated, endocytic routes.

The post-synthetic sorting of endogenous membrane proteins examined by the simultaneous purification of apical and basolateral plasma membrane fractions from Caco-2 cells.

A subcellular fractionation method to isolate simultaneously apical and basolateral plasma membrane fractions from the human adenocarcinoma cell line Caco-2, grown on filter supports, is described. The method employs sucrose-density-gradient centrifugation and differential precipitation. The apical membrane fraction was enriched 14-fold in sucrase-isomaltase and 21-fold in 5'-nucleotidase compared with the homogenate. The basolateral membrane fraction was enriched 20-fold relative to the homogenate in K(+)-stimulated p-nitrophenylphosphatase. Alkaline phosphatase was enriched 15-fold in the apical membrane fraction and 3-fold in the basolateral membrane fraction. Analytical density-gradient centrifugation showed that this enzyme was a true constituent of both fractions, and experiments measuring alkaline phosphatase release following treatment with phosphatidylinositol-specific phospholipase C showed that in both membrane fractions the enzyme was glycosyl-phosphatidylinositol-linked. There was very little contamination of either membrane fraction by marker enzymes of the Golgi complex, mitochondria or lysosomes. Both membrane fractions were greater than 10-fold purified with respect to the endoplasmic reticulum marker enzyme alpha-glucosidase. Protein composition analysis of purified plasma membrane fractions together with domain-specific cell surface biotinylation experiments revealed the presence of both common and unique integral membrane proteins in each plasma membrane domain. The post-synthetic transport of endogenous integral plasma membrane proteins was examined using the devised subcellular fractionation procedure in conjunction with pulse-chase labelling experiments and immunoprecipitation. Five common integral membrane proteins immunoprecipitated by an antiserum raised against a detergent extract of the apical plasma membrane fraction were delivered with the same time course to each cell-surface domain.