Differences in fat, carbohydrate, and protein metabolism between lean and obese subjects undergoing total starvation

The effect of rate and extent of weight loss on urea salvage in obese male subjects

It is well established that in human subjects a proportion of urea production undergoes hydrolysis in the gastrointestinal tract with release of N potentially available for amino acid synthesis. Previous studies have suggested adaptive changes in urea kinetics, with more urea-N retained within the metabolic pool during reduced dietary intakes of energy and protein. We therefore investigated the effect of rate and extent of weight loss on adaptive changes in urea kinetics in two groups (each n 6) of obese men (mean age 43 (sd 12) years, BMI 34.8 (sd 2.9) kg/m(2)) during either total starvation for 6 d or a very-low-energy diet (2.55 MJ/d) for 21 d. Subjects were resident in the Human Nutrition Unit of the Rowett Research Institute (Aberdeen, Scotland, UK) and lost 6 and 9 % initial body weight within the starvation and dieting groups respectively. Changes in urea-N metabolism were assessed by stable isotope tracer kinetics using [(15)N(15)N]urea infused intravenously for 36 h before, during and after weight loss. In response to weight loss, urea production decreased (P<0.01) by 25 % from 278 to 206 micromol urea-N/h per kg within the dieting group only. However, no changes were observed in the proportion of urea being hydrolysed in the gastrointestinal tract (range 20-25 %) or in the proportion of N retained for anabolic purposes (80-85 % urea-N from gastrointestinal hydrolysis) within either group. It was concluded that no adaptive changes in urea kinetics occurred in response to either the different rate or extent of weight loss.

Distinguishing starvation from cachexia

The poor response to hypercaloric feeding in ill adults may be caused by failure to distinguish cachexia from starvation (Table 1). The chief difference between starvation and cachexia is that refeeding reverses starvation but is less effective for cachexia. The ineffectiveness of refeeding in treating cachexia may explain some of the poor results from direct nutritional interventions in clinical trials. Simple starvation should respond to voluntary or involuntary hypercaloric feedings. The failure to demonstrate a more positive response may be caused by underlying cachexic states.

Sexual dimorphism in the energy content of weight change

BACKGROUND

The energy content of weight change is assumed to be sex- and age-neutral at 3,500 kcal/pound or 32.2 MJ/kg.

OBJECTIVES

As sexual dimorphism in body composition generally exists in mammals, the primary hypothesis advanced and tested was that the energy content of weight change differs between men and women.

DESIGN

The energy content of 129 adult men and 287 women was measured by neutron activation analysis. Cross-sectional energy content prediction models were developed and then evaluated in two longitudinal samples: one that used the same methods in 26 obese women losing weight; and the other a compilation of 18 previously reported weight change-body composition studies.

RESULTS

Multiple regression modeling identified weight, sex, age and height as total energy content predictor variables with significant sex x weight (P<0.001) and age x weight (P<0.001) interactions; total model r(2) and s.e.e. were 0.89 and 107.3 MJ, respectively. The model's predictive value was supported in both longitudinal evaluation samples. Model calculations using characteristics of representative adults gaining or losing weight suggested that the energy content of weight change in women (approximately 30.1-32.2 MJ/kg) is near to the classical value of 32.2 MJ/kg and that in men the value is substantially lower, approximately 21.8-23.8 MJ/kg. The predicted energy content of weight change increases by about 10% in older (age approximately 70 y) vs younger (approximately 35 y) men and women.

CONCLUSIONS

Sexual dimorphism and age-dependency appears to exist in the estimated energy content of weight change and these observations have important clinical and research implications.

Waging war on physical inactivity: using modern molecular ammunition against an ancient enemy

A hypothesis is presented based on a coalescence of anthropological estimations of Homo sapiens' phenotypes in the Late Paleolithic era 10,000 years ago, with Darwinian natural selection synergized with Neel's idea of the so-called thrifty gene. It is proposed that humans inherited genes that were evolved to support a physically active lifestyle. It is further postulated that physical inactivity in sedentary societies directly contributes to multiple chronic health disorders. Therefore, it is imperative to identify the underlying genetic and cellular/biochemical bases of why sedentary living produces chronic health conditions. This will allow society to improve its ability to effect beneficial lifestyle changes and hence improve the overall quality of living. To win the war against physical inactivity and the myriad of chronic health conditions produced because of physical inactivity, a multifactorial approach is needed, which includes successful preventive medicine, drug development, optimal target selection, and efficacious clinical therapy. All of these approaches require a thorough understanding of fundamental biology and how the dysregulated molecular circuitry caused by physical inactivity produces clinically overt disease. The purpose of this review is to summarize the vast armamentarium at our disposal in the form of the extensive scientific basis underlying how physical inactivity affects at least 20 of the most deadly chronic disorders. We hope that this information will provide readers with a starting point for developing additional strategies of their own in the ongoing war against inactivity-induced chronic health conditions.

Regional muscle and adipose tissue amino acid metabolism in lean and obese women

The effect of obesity on regional skeletal muscle and adipose tissue amino acid metabolism is not known. We evaluated systemic and regional (forearm and abdominal subcutaneous adipose tissue) amino acid metabolism, by use of a combination of stable isotope tracer and arteriovenous balance methods, in five lean women [body mass index (BMI) <25 kg/m(2)] and five women with abdominal obesity (BMI 35.0-39.9 kg/m(2); waist circumference >100 cm) who were matched on fat-free mass (FFM). All subjects were studied at 22 h of fasting to ensure that the subjects were in net protein breakdown during this early phase of starvation. Leucine rate of appearance in plasma (an index of whole body proteolysis), expressed per unit of FFM, was not significantly different between lean and obese groups (2.05 +/- 0.18 and 2.34 +/- 0.04 micromol x kg FFM(-1) x min(-1), respectively). However, the rate of leucine release from forearm and adipose tissues in obese women (24.0 +/- 4.8 and 16.6 +/- 6.5 nmol x 100 g(-1) x min(-1), respectively) was lower than in lean women (66.8 +/- 10.6 and 38.6 +/- 7.0 nmol x 100 g(-1) x min(-1), respectively; P < 0.05). Approximately 5-10% of total whole body leucine release into plasma was derived from adipose tissue in lean and obese women. The results of this study demonstrate that the rate of release of amino acids per unit of forearm and adipose tissue at 22 h of fasting is lower in women with abdominal obesity than in lean women, which may help obese women decrease body protein losses during fasting. In addition, adipose tissue is a quantitatively important site for proteolysis in both lean and obese subjects.

Value of VLCD supplementation with medium chain triglycerides

BACKGROUND

Medium chain triglycerides (MCT) are energetically less dense, highly ketogenic, and more easily oxidised than long chain triglycerides (LCT). MCT also differ from LCT in their digestive and metabolic pathways.

OBJECTIVE

To test the effects of MCT supplementation during a very low calorie diet (VLCD).

SUBJECTS AND METHODS

Three groups of tightly matched obese women with body mass index (BMI)>30 kg/m(2) received an isoenergetic (578.5 kcal) VLCD (Adinax, Novo Vital, Sweden) enriched with MCT or LCT (8.0 and 9.9 g/100 g Adinax respectively) or a low-fat (3 g/100 g) and high-carbohydrate regimen. The diets were administered over 4 weeks. Body composition was measured with DEXA and appetite/satiety-according to Blundell. Beta hydroxybutyric acid concentration in plasma and nitrogen excretion in urine was measured during consecutive days of VLCD. The study was performed in a randomised double-blind manner.

RESULTS

The MCT group showed a significantly greater decrease in body weight during the first 2 weeks. The contribution of body fat to the total weight loss was higher while the contribution of fat-free mass (FFM) was lower. The MCT group had a higher concentration of ketone bodies in plasma and a lower nitrogen excretion in urine. Hunger feelings were less intense while satiety was higher. These differences were observed during the first 2 weeks of treatment and gradually declined during the third and fourth weeks.

CONCLUSIONS

Replacement of LCT by MCT in the VLCD increased the rate of decrease of body fat and body weight and has a sparing effect on FFM. The intensity of hunger feelings was lower and paralleled the higher increase of ketone bodies. These effects gradually declined, indicating subsequent metabolic adaptation. Further studies are required to confirm the protein-sparing and appetite-suppressing effects of MCT supplementation during the first 2 weeks of VLCD treatment.

Hunger disease

This paper examines three aspects of hunger disease: the effect of initial fat stores on macronutrient fuel selection during total starvation (no energy) and how it influences survival; the effects of different rates of weight loss on tissue and body function; and the importance of appetite sensations, including hunger, during malnutrition and during enteral and parenteral nutritional support. Long-term starvation studies in humans reveal major differences in fat carbohydrate and protein metabolism between lean and obese subjects, including a 2-4-fold lower contribution of protein oxidation to energy expenditure in obese subjects, which ensures that more of the excess body fat is oxidized. The rate of weight loss, determined by recent dietary intake, can have major effects on tissue and body function, including wound healing, the acute phase protein response, muscle fatigue and psychological/behavioural function in both clinical and non-clinical settings. In depleted states uncomplicated by disease, changes in appetite sensations can result in energy intakes as high as 6000 to 10,000 kcal/day ( 25-42 MJ/day). Long-term enteral tube feeding and parenteral nutrition are associated with frequent disturbances in appetite sensations, and in those able to eat normally they tend to add rather than replace oral intake to an extent that appears to depend on the regimen. It is concluded that 1) differences between lean and obese subjects in macronutrient fuel selection during starvation are adaptive because they optimize survival in both groups of subjects; 2) the rate of weight loss in health and disease has a major effect on certain tissue and body functions, independently of the magnitude of weight loss; and 3) clinically relevant disturbances in appetite sensations are common subjects receiving long-term enteral and parenteral nutrition. The clinical modulation of all these variables would be aided by greater knowledge of the mechanisms involved.