Fw: NATAP: Low-Carb Diet Increased Weight Loss

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Low-Carbohydrate Diet Works in the Liver to Stimulate Weight Loss

MedPage Today

January 22, 2009

DALLAS, Jan. 22 -- Low-carbohydrate diets may lead to increased fat burning and more efficient use of glucose precursors, investigators here concluded.

Action Points  

    * Explain to patients that a low-carbohydrate diet alters energy production and metabolism in the liver, with increased rates of utilization of substrates like lactate and amino acids.

    * Point out that the findings are based on a small study.

Obese and overweight patients lost twice as much weight during two weeks of carbohydrate restriction compared with calorie counting, Browning, M.D., of the University of Texas Southwestern Medical Center, and colleagues reported online in Hepatology.

The observation came from a small clinical study that focused on diet-induced changes in hepatic glucose and energy metabolism in the liver and its implications for nonalcoholic fatty liver disease.

"Energy production is expensive for the liver," Dr. Browning said in a statement. "It appears that for people on a low-carbohydrate diet, in order to meet that expense, their livers have to burn excess fat."

Carbohydrate restriction appeared to stimulate increased fat burning throughout the body, he added.

U.S. dietary recommendations have generally emphasized consumption of a low-fat, low-calorie diet. Despite successful efforts to reduce fat consumption, the proportion of overweight and obese individuals in the population has continued to increase, the authors noted.

The increased prevalence of obesity has been accompanied by a similar increase in the occurrence of metabolic liver disease, characterized by excess triglyceride accumulation that leads to inflammation, fibrosis, and cirrhosis.

In a subset of patients, relatively benign nonalcoholic fatty liver disease evolves in the more morbid condition of nonalcoholic steatohepatitis. How this transition occurs has been unclear, although some studies showed a strong association with dietary carbohydrate intake, the authors said.

Low-carbohydrate diets alter hepatic glucose production by changing the rates of glycogenolysis and gluconeogenesis, they continued. Moreover, an empiric relationship between gluconeogenesis and trichloroacetic acid cycle flux effects an "energetic rheostat" that allows the liver to match energy production with the requirements of gluconeogenesis.

Dr. Browning and colleagues conducted a clinical study aimed at gaining a better understanding of hepatic energy production and its relationship to gluconeogenesis under varied conditions of macronutrient intake.

Using nuclear magnetic resonance spectroscopy, the authors measured sources of hepatic glucose and trichloroacetic acid cycle flux in seven normal-weight participants, seven overweight participants assigned to carbohydrate restriction, and seven overweight participants assigned to a low-calorie diet.

Carbohydrate restriction was associated with a predominance of hepatic gluconeogenesis from lactate/amino acids (GNGPEP). The contribution of glycerol to gluconeogenesis was similar in all three groups, despite evidence of increased fat oxidation in the carbohydrate-restricted participants.

In the overweight participants assigned to a low-calorie diet, 40% of their glucose came from carbohydrate-derived glycogen stored in the liver.

In contrast, participants on the low-carbohydrate diet got 20% of their glucose from glycogen and derived more of their energy requirements from metabolism of liver fat.

The alteration in hepatic glucose metabolism was associated with an average two-week weight loss of 9.5 lb. in participants assigned to carbohydrate restriction, whereas calorie restriction led to a mean weight loss of 5 lb.

"Together, these data imply that the trichloroacetic acid cycle is the energy patron of gluconeogenesis," the authors said. "However, the relationship between these two pathways is modified by carbohydrate restriction, suggesting an increased reliance of the hepatocyte on energy generated outside of the trichloroacetic acid cycle when GNGPEP is maximal."

Dr. Browning said the findings have implications for treatment of obesity, diabetes, insulin resistance, and nonalcoholic fatty liver disease. EXCERPTS from main articleAlterations in hepatic glucose and energy metabolism as a result of calorie and carbohydrate restriction

Hepatology Nov 2008Volume 48, Issue 5

Pages: 1487-1496 D. Browning 1 2 *, Weis 4, Jeannie 2, Santhosh Satapati 2, Merritt 2, Craig R. Malloy 1 2 5, C. Burgess 2 3

1Department of Internal Medicine, The University of Texas Southwestern Medical Center at Dallas, Dallas, TX

2The Advanced Imaging Research Center, The University of Texas Southwestern Medical Center at Dallas, Dallas, TX

3Department of Pharmacology, The University of Texas Southwestern Medical Center at Dallas, Dallas, TX

4Department of Medicine, Texas Tech University Health Science Center School of Medicine at Amarillo, Amarillo, TX

5Veterans Affairs Medical Center, Dallas, TXSubjects in the low-carbohydrate arm experienced greater total weight loss over the 2-week period than those in the low-calorie arm.... There was no significant change in weight in the weight-stable group....At enrollment, subjects were assigned to either a low-carbohydrate diet or a calorie restricted diet.....studied....... a group of overweight/obese subjects undergoing weight-loss via dietary restriction......The study was restricted to subjects whose BMI was >25 kg/m2 and <35 kg/m2.......An additional group of seven subjects who were lean (BMI < 25 kg/m2) was also recruited to act as a weight stable comparison groupThese findings may explain, in part, the correlation between carbohydrate intake and severity of liver disease in individuals with NAFLD.[7] Understanding the alterations to cellular energetics that occur with simple macronutrient manipulation may be important for understanding and treating NAFLD and other metabolic disorders associated with obesity.[34]Since the seminal observation of Keys[1] in 1980, the recommended diet in the United States has been the low-calorie, low-fat diet. This diet originated primarily as an inexpensive approach to prevent cardiovascular disease but has now become the recommended treatment for overweight and obesity in clinical practice.[2] Despite the success of clinicians and the U.S. Public Health Service in reducing the U.S. population's fat intake and increasing its carbohydrate intake over the past 30 years, the prevalence of obesity has continued to rise.[3][4] During this same period, metabolic liver disease has become increasingly prevalent, taking the form of excess triglyceride accumulation in the liver that can result in inflammation, fibrosis, and cirrhosis.[5][6] The transition of this type of liver disease, known as nonalcoholic fatty liver disease (NAFLD), from relatively bland, inactive steatosis to a more morbid inflammatory condition, termed nonalcoholic steatohepatitis, occurs in a subset of individuals. The reason this transition takes place is unclear; however, some investigators have found a strong association between dietary carbohydrate intake and severity of both steatosis and steatohepatitis.[7][8] Current evidence suggests that a high carbohydrate diet leads to increased hepatic de novo lipogenesis,[9] likely as the result of the molecular mediators carbohydrate and sterol response element binding protein.[5] Such an increase in hepatic fat synthesis would be anticipated to be associated with hepatic steatosis; however, the connection between carbohydrate intake and inflammatory activity remains elusive. The changes in hepatic metabolism and energy production that occur as a consequence of changes in dietary carbohydrate intake may be important in the pathogenesis and progression of NAFLD.

Several studies have used stable isotopes to investigate the impact of carbohydrate intake on hepatic glucose metabolism.[10-12] These studies clearly show that low-carbohydrate diets result in a reorganization of hepatic glucose production by changing the rate of glycogenolysis and, to a lesser degree, the rate of gluconeogenesis (GNG). However, little is known about the effect of carbohydrate restriction on the origin of gluconeogenic precursors (GNG from lactate/amino acids or glycerol) or its consequence on hepatic energy homeostasis. There is an empirical relationship between GNG and tricarboxylic acid (TCA) cycle flux[13]: suppressed rates of GNG result in impaired hepatic TCA cycle flux,[14][15] while increased GNG is accompanied by elevated TCA cycle flux.[16] This relationship appears to be an 'energetic rheostat', allowing the liver to match energy production with the requirements of GNG. If the increased rates of GNG observed during carbohydrate restriction are the result of increased conversion of lactate/amino acids to glucose, energy production at the level of the TCA cycle may be altered in a coordinated manner.

To gain a better understanding of hepatic energy production and its relationship to GNG under conditions of varied macronutrient intake, we used 2H and 13C tracers combined with nuclear magnetic resonance (NMR) spectroscopy[17] to simultaneously assess endogenous glucose production (EGP), glycogenolysis, GNG from lactate/amino acids (GNGphosphoenolpyruvate [PEP]), GNG from glycerol (GNGglycerol) pyruvate cycling, and TCA cycle flux in human subjects following a carbohydrate restricted or calorie restricted diet.Patients and Methods

Materials.

Deuterium oxide (2H2O) (70%) and [u-13C]propionate (99%) were obtained from Cambridge Isotopes (Andover, MA). Sterility-tested and pyrogen-tested [3,4-13C]glucose was obtained from Omicron Biochemicals Inc. (South Bend, IN). Other common reagents were purchased from Sigma (St. Louis, MO).

Participants.

The study was comprised of two groups of seven subjects aged 18 to 65 years that were matched for age, body mass index (BMI), gender, and ethnicity. To eliminate confounding variables, participants chosen were of stable health, using no medications known to alter hepatic glucose metabolism, participating in no weight loss diet or using no diet pills at enrollment or within 6 months prior to enrollment, and without baseline ketonuria. The study was restricted to subjects whose BMI was >25 kg/m2 and <35 kg/m2. All subjects had a normal nutritional status, no ethanol intake, and did not participate in regular exercise above the activity required for daily living.

An additional group of seven subjects who were lean (BMI < 25 kg/m2) was also recruited to act as a weight stable comparison group. These subjects were all of normal health, had a normal nutritional status, no ethanol intake, and did not participate in regular exercise above the activity required for daily living.

The protocol and consent form were approved by the Institutional Review Board of the University of Texas Southwestern Medical Center and all participants provided written informed consent prior to enrollment.

Design.

At enrollment, subjects were assigned to either a low-carbohydrate diet or a calorie restricted diet. Prior to initiating the diet, all subjects underwent a teaching session with the General Clinical Research Center (GCRC) dietician to explain the appropriate use of the diet history questionnaire, determine food preferences, and provide dietary instruction. At 2 weeks after enrollment, subjects were admitted to the inpatient GCRC for overnight metabolic studies. On admission (16:00; 24-hour clock), subjects were provided a dinner consistent with their assigned diet, after which they were fasted until completion of the study. Between 22:00 and 09:00, subjects received two stable isotope tracers orally: [u-13C]propionate (about 1,200 mg) at 08:30 and divided doses of 70% 2H2O (5 g/kg body water, calculated as 60% of body weight in men and 50% of body weight in women) at 22:00, 02:00, and 06:00. Subjects were allowed to drink 0.5% 2H2O ad libitum during the study. Between 08:30 and 09:00 subjects underwent measurement of their respiratory quotient (RQ) using a Delta Trak II indirect calorimeter (Sensormedics, Yorba , CA). An intravenous catheter was placed and subjects were given a 2.25 mg/kg bolus of [3,4-13C]glucose followed by a 2-hour infusion (0.0225 mg/kg/minute). At the end of the infusion period, a 50-cc blood draw was performed.

Low-Carbohydrate Diet.

Subjects assigned to the low-carbohydrate diet were instructed by the GCRC dietician to limit carbohydrate intake to less than 20 g/day[18] and were provided with instructional handouts detailing allowed and disallowed foods. Participants were encouraged to eat three regular-size meals and to consume eight 8-ounce glasses of water per day. Subjects initiated this diet on their own for the first 7 days of the study, keeping a detailed dietary record. Food for the final 7 days was provided to subjects as frozen meals preprepared by the GCRC kitchen in accordance with the caloric intake documented in the dietary record.

Calorie Restricted Diet.

This diet was designed to place individuals in negative energy balance relative to their typically daily caloric intake. Subjects were asked to continue their regular diet without calorie restriction for the first 7 days of the study and record their food intake in a diet record to determine average daily caloric intake as well as dietary macronutrient composition. Based upon this diet record, the GCRC dietician and kitchen staff prepared all meals for the final 7 days of the study in accordance with the dietary composition recorded by the individual, but reduced in caloric content by 800 kcal/day.

Weight-Stable Diet.

Subjects were asked to continue their regular diet, but were placed on a diet that was 40% carbohydrate, 30% fat, and 30% protein for 3 days prior to the study The daily caloric value of the meals was 1,700 kcal for women and 2,000 kcal for men.Abstract

Carbohydrate restriction is a common weight-loss approach that modifies hepatic metabolism by increasing gluconeogenesis (GNG) and ketosis. Because little is known about the effect of carbohydrate restriction on the origin of gluconeogenic precursors (GNG from glycerol [GNGglycerol] and GNG from lactate/amino acids [GNGphosphoenolpyruvate {PEP}]) or its consequence to hepatic energy homeostasis, we studied these parameters in a group of overweight/obese subjects undergoing weight-loss via dietary restriction. We used 2H and 13C tracers and nuclear magnetic resonance spectroscopy to measure the sources of hepatic glucose and tricarboxylic acid (TCA) cycle flux in weight-stable subjects (n = 7) and subjects following carbohydrate restriction (n = 7) or calorie restriction (n = 7). The majority of hepatic glucose production in carbohydrate restricted subjects came from GNGPEP. The contribution of glycerol to GNG was similar in all groups despite evidence of increased fat oxidation in carbohydrate restricted subjects. A strong correlation between TCA cycle flux and GNGPEP was found, though the reliance on TCA cycle energy production for GNG was attenuated in subjects undergoing carbohydrate restriction Together, these data imply that the TCA cycle is the energetic patron of GNG. However, the relationship between these two pathways is modified by carbohydrate restriction, suggesting an increased reliance of the hepatocyte on energy generated outside of the TCA cycle when GNGPEP is maximal. Conclusion: Carbohydrate restriction modifies hepatic GNG by increasing reliance on substrates like lactate or amino acids but not glycerol. This modification is associated with a reorganization of hepatic energy metabolism suggestive of enhanced hepatic beta-oxidation.Results

Subjects.

Subjects in the low-carbohydrate group were matched to those in the low-calorie group (Table 1). The majority of participants (10/14) were obese (BMI > 30 kg/m2), with the remainder classified as overweight (25 kg/m2 < BMI < 30 kg/m2). All subjects in the low-carbohydrate arm of the study demonstrated urinary ketones at the end of the study except for one. Despite the absence of ketosis, this individual had an RQ of 0.84 at study-end and experienced a 3.5-kg weight loss over the period of the study. Intervention.

Subjects in the low-carbohydrate arm experienced greater total weight loss over the 2-week period than those in the low-calorie arm (Table 2). There was no significant change in weight in the weight-stable group (data not shown). As expected, RQs were significantly lower and total plasma ketone bodies were significantly higher among carbohydrate-restricted subjects. Insulin, lactate, and triglyceride levels did not differ between the two dietary-restriction groups after 2 weeks. There was no significant difference in caloric intake between the two groups, although a trend toward lower caloric intake in the low-carbohydrate group was noted. The dietary macronutrient proportions were significantly different in the two arms of the study; however, absolute dietary macronutrient intake was only significantly different with regard to protein and carbohydrate, and absolute daily fat intake in the dietary groups was similar. Concordant with this, the absolute daily intake of saturated, monounsaturated, and polyunsaturated fats also did not differ between the two groups. Table 2. Treatment Group Characteristics at Study EndDiscussion

In this study, the effect of carbohydrate restriction on flux through the metabolic pathways of hepatic glucose production and the TCA cycle were simultaneously assessed by isotopomer analysis of glucose using 2H and 13C NMR spectroscopy. We found that carbohydrate restriction increased the rate of GNG and decreased the rate of glycogenolysis. However, the observed increase in GNG in the low-carbohydrate group was solely the result of increased GNGPEP rather than GNGglycerol. Despite the energetic investment required to increase GNGPEP, TCA cycle flux in the low-carbohydrate group was similar to the low-calorie group, indicating similar rates of energy generation. Interestingly, in the groups consuming carbohydrate as a significant proportion of their diet (weight-stable, low-calorie), the TCA cycle alone provided sufficient energy to drive GNG regardless of whether the gluconeogenic substrate was assumed to be lactate or alanine. This was not the case in individuals undergoing carbohydrate restriction, indicating that a reorganization of hepatic energy metabolism occurred in tandem with the changes in hepatic carbohydrate metabolism.

Among previous studies of carbohydrate restriction, it remained unclear which gluconeogenic precursors were primarily responsible for increased GNG. Evidence of a negative correlation between alanine conversion to glucose and dietary carbohydrate content suggests that anaplerosis and GNGPEP are increased with decreased dietary intake of carbohydrate.[28][29] However, increased fat oxidation during carbohydrate restriction might be expected to increase availability of the gluconeogenic precursor glycerol.[10] In the present study, we showed that the increase in GNG associated with carbohydrate restriction is due to the induction of GNGPEP. This suggests that in fasted human subjects undergoing weight loss, the elevated GNG associated with carbohydrate restriction is driven by substrates such as lactate or amino acids. While it seems likely this increase is due to amplified protein turnover, we could not rule out enhanced cycling of lactate from the periphery back to the liver (Cori Cycle) as a source of increased GNG. Plasma lactate levels were similar between the two weight loss groups (Table 2); however, these were static measurements and gave no insight into the rate of production of lactate by muscle and uptake by liver. Likewise, protein turnover measurements were not performed so the contribution of amino acids also remains unknown. However, it is interesting to note that individuals on a low-carbohydrate diet increased their protein intake in favor of fat (Table 2), possibly as method to stave off nondietary protein breakdown for the formation of glucose.

The contribution of glycerol as a substrate for GNG was surprisingly unresponsive to dietary macronutrient composition. Though GNGglycerol appeared to be numerically higher in the low-carbohydrate group, this failed to reach statistical significance. It is possible that the small sample size of the study and/or the sensitivity of our technique limited our ability to detect modest changes in this measure. However, prior data in fasting man suggests that GNGglycerol occurs at a relatively fixed rate.[30] Our findings would further support this observation. Indeed, insulin levels were similar between the groups undergoing dietary restriction, suggesting that rates of peripheral lipolysis were also similar (Table 2). This was somewhat surprising as prior data in lean individuals clearly demonstrates a reduction in insulin levels and increase in free fatty acid levels as a result of carbohydrate restriction.[31] However, the present data is akin to that of Allick et al.[32] in which overweight/obese individuals with diabetes maintained similar insulin and free fatty acid levels regardless of dietary macronutrient composition. Further studies are needed to verify this finding.

It should be noted that prior studies assessing the impact of carbohydrate restriction on hepatic glucose metabolism show that the main effect is a reduction in hepatic glucose output, predominantly via a reduction in glycogenolysis.[31][32] This is in contrast to the present study in which hepatic glucose production was similar between the dietary groups. The low-carbohydrate group was able to maintain hepatic glucose production at the levels observed for the weight-stable and low-calorie groups by increasing GNGPEP to match the reduction in glycogenolysis. This observation is reminiscent of hepatic autoregulation by which EGP remains unchanged in the setting of altered GNG or glycogenolysis because the two pathways tend to compensate for each other.[28][33] This finding may also be the result of the much larger intake of dietary protein in the low-carbohydrate group (about 34%) as compared to prior studies (11%-15%),[31][32] possibly yielding an enhanced supply of gluconeogenic substrate.

The multitracer approach used in the present study allowed for the simultaneous assessment of both hepatic glucose production as well as the TCA cycle flux. Knowledge of metabolic flux through these pathways provided insight into the relationship between hepatic glucose and energy metabolism (Table 3). Energetic coupling of GNGPEP and the TCA cycle was observed as a correlation between TCA cycle flux and PEPCK flux (Fig. 3), the metabolic pathway responsible for the delivery of substrate for GNG. It is, however, intriguing that the increased GNGPEP in the low-carbohydrate group was not associated with increased TCA cycle flux (that is, energy production). Indeed, assuming net glucose synthesis predominated (i.e., alanine or other amino acids acted as the gluconeogenic substrate as opposed to lactate), energy production in the TCA cycle would be unable to meet the energetic demands of GNG in the low-carbohydrate group. This would suggest a greater reliance on sources of energy upstream from the terminal oxidation of fat in the TCA cycle in this group, possibly -oxidation/ketogenesis. Prior data has demonstrated a relationship between GNGPEP and ketogenesis: in general, ketogenesis parallels the rate of GNG under most circumstances.[30] Indeed, static measurements of ketone bodies were markedly higher in subjects undergoing carbohydrate restriction, indicating that the availability of acetyl-coenzyme A to 3-hydroxy-3-methylglutarate-coenzyme A synthase may have been greater. However, absolute rates of fatty acid delivery to liver as well as ketone body production were not measured, limiting our ability to further interpret the above findings.

Every attempt was made to equalize caloric intake between the two dietary restriction groups. However, a trend was noted toward decreased caloric intake in the group undergoing carbohydrate restriction (Table 2). It is possible that the differences observed between these two groups are solely the result of differences in caloric intake and weight loss. However, prior data obtained under weight-stable, isocaloric conditions showed similar changes in hepatic glucose metabolism in lean individuals.[11] Likewise, fractional and absolute glucose production was similar between the weight-stable and low-calorie group despite the difference in energy balance between the two. It should also be noted that the present study was not designed to determine the effectiveness of these two weight-loss diets in weight reduction, but was simply designed to assess hepatic metabolism under the differing macronutrient compositions during negative energy balance. Additionally, it was our desire to examine changes in hepatic metabolism under conditions likely to be encountered in a clinical setting; hence, dietary choices of the subjects were more varied than what would be encountered in a strict physiologic study.

In conclusion, we have shown that the sources from which endogenous glucose is produced are dependent upon dietary macronutrient composition. Carbohydrate restriction yields a decreased rate of glycogenolysis and an increased rate of GNG compared to calorie restriction. We have shown for the first time that this increased rate of hepatic GNG is the result of an increased rate of utilization of substrates like lactate and amino acids, but not glycerol. Additionally, the TCA cycle appears to be the energetic patron of GNGPEP, as TCA cycle flux and PEPCK flux were highly correlated. Furthermore, it appears that the shift in glucose metabolism associated with a low carbohydrate diet leads to an increased contribution of energy generated outside of the TCA cycle to GNG. This shift is consistent with enhanced beta-oxidation/ketogenesis, which could be beneficial in individuals with NAFLD due to enhanced disposal of hepatic triglyceride. These findings may explain, in part, the correlation between carbohydrate intake and severity of liver disease in individuals with NAFLD.[7] Understanding the alterations to cellular energetics that occur with simple macronutrient manipulation may be important for understanding and treating NAFLD and other metabolic disorders associated with obesity.[34]