Br J Nutr.2014 Mar;111(5):933-943

Gender differences in the composition of weight gain and loss in overweight and obese adults.

Millward D Joe1,  Truby H1a, Fox KR2 Livingstone MBE3, Macdonald IA4 and Tothill P5

 

1 Department of Nutritional Sciences, Faculty of Health and Medical Sciences, University of Surrey, Guildford, Surrey GU2 7XH,

2Centre for Exercise, Nutrition and Health Sciences, University of Bristol, Bristol BS8 1TP,

3Northern Ireland Centre for Food and Health, University of Ulster, Coleraine, BS52 1SA,

4School of Biomedical Sciences, University of Nottingham, Nottingham, NG7 2UH,

5Department of Medical Physics, University of Edinburgh

aCurrent address Department of Nutrition and Dietetics, Monash University, Level 1, 264 Ferntree Gully Road, Notting Hill  Vic  3168 Australia

 

Abstract

Sex differences in the ratio of fat mass (FM):fat-free mass (FFM) during weight change should differentially affect the extent of weight change during energy imbalance in men and women. In the present study, we determined FM and FFM contents by dual-energy X-ray absorptiometry and calculated the P-ratios (protein energy/total energy) of excess weight and weight loss during a randomised controlled trial of four commercial weight loss regimens. Overweight and obese women (n 210) and men (n 77) were studied at baseline and at 2 and 6 months during weight loss on four dietary regimens: Dr Atkins’ New Diet Revolution; The Slim-Fast Plan; Weight-Watchers programme; Rosemary Conley’s Diet and Fitness Plan. At baseline, the percentage of FFM (%FFM) and P-ratios of excess weight were 40 % and 0·071 for men and 27 % and 0·039 for women. At 2 months, men had lost twice as much weight as women and three times more FFM than women, indicating higher FFM content and P-ratios of weight loss for men, 0·052, than for women, 0·029, with no dietary effects. Between 2 and 6 months, the rate at which weight was lost decreased and the %FFM of weight loss decreased to similar low levels in men (7 %) and women (5 %): i.e. P-ratios of 0·009 and 0·006, respectively, with no dietary effects. Thus, for men compared with women, there were greater FFM content and P-ratios of weight change, which could partly, but not completely, explain their greater weight loss at 2 months. However, protein-conserving adaptations occur with increasing weight loss and over time, more extensively in men, eventually eliminating any sex difference in the composition of weight loss.

PMID: 24103395

 

Supplement:

In this study, Diet Trials, a large scale UK randomised controlled study of the clinical effectiveness of commercially available weight-loss programmes we found that each regime was equally effective in terms of losses of weight, FM or FFM loss over the six month trial, when analysed with no gender separation. However although gender was not a consideration in the study plan and more women were recruited than men we found that weight loss was significantly higher for men than women at 2 months (7.34kg v 3.80kg) and at 6 months (10.3kg v 6.27kg). Although weight loss in our trial depended on individual compliance to the regimes and gender differences in psychosocial and behavioural aspects of weight change are known to occur, because we had conducted detailed analysis of body composition by DEXA we were able to investigate whether there was any physiological basis to the gender difference in weight loss.

One potential explanation was a gender difference in the composition of weight loss. FFM and FM differ by 9-fold in their energy content/kg, so that the FFM: FM ratio of the weight loss will markedly influence its extent per unit energy imbalance. However there have been several dynamic models of weight change which take into account this marked difference in tissue energy content but they have differed in terms of the way that the composition of the weight change influenced its extent so that it was not a straightforward issue. The protein:fat ratio of weight change can be expressed as a phenotypic energy partitioning P-ratio, (protein energy as a fraction of total energy gained or lost). Although the gender dimorphism in relative FM and FFM for adults of normal body weight had long been known, the likely influence of a gender difference in the P-ratio for weight change had not been considered before.

Our DEXA data at baseline indicated considerably higher P-ratios of the excess weight in men than women. Clearly we could not show if this gender dimorphism had influenced their relative extent of weight gain but it is interesting that within Europe and Australia overweight and obesity is more prevalent in men than women. This suggests that in addition to the psychosocial and behavioral gender differences which influence weight gain, the gender difference in the P-ratio should be considered as another determinant, at least in terms of a simple model in which a higher % FFM and P-ratio of weight gain results in greater weight change.

We showed quite clearly that the % FFM and P-ratios of weight loss was a determinant of the rate of weight loss. Thus at 2 months the men had lost twice as much weight and 3 times more FFM than women. In fact the relationship between the rate of weight loss and the P-ratio of its composition was quite complex as shown in the Figure 1. This is a subset of subjects selected on the basis of sufficient loss of FFM and FM at 2 months to enable an accurate P-ratio to be calculated. Generally P-ratios were lower for women than for men at any weight loss, but within each gender greater weight loss was associated with a lower P-ratio resulting in a significant curvilinear inverse relationship between P-ratio and weight loss. Also the mean P-ratios for the entire cohort of men and women (calculated from ratios of average FFM and fat mass losses) for 0-2 months were lower for women than for men. However after the first two months very little further FFM was lost so that P-ratios for 2-6 months were very low and similar in men and women. Taken together the main characteristics of weight loss observed in our studies was a fall in the %FFM or P-ratio  with the extent of weight loss and with time, to very low values in both men and women. We know from metabolic studies of fasting that although tissue protein is rapidly lost initially to provide amino acids for gluconeogenesis, with time ketones replace glucose as the main fuel for the brain enabling protein conservation to occur. Although it is highly unlikely that our subjects were ketotic nevertheless protein conservation did appear to occur.

These findings raise several important questions. Firstly there is no obvious physiological explanation for the greater FFM content of the excess weight in men, The distribution pattern of the FM component of excess weight shows a clear gender dimorphism (e.g. bust and hips in women and waist in men), but this was unrelated to the additional FFM in our cohort. WC had no influence as an additional independent variable in the multiple regression of FM and FFM on weight and height for women but did capture a significant fraction of the variation in FM and exerted a negative influence on FFM in men. Thus the “male”  FM distribution pattern of central obesity is not associated with increased FFM elsewhere in the body. Increased skeletal muscle mass is likely to be part of the additional FFM with weight gain given that increased creatinine excretion was an early observation in obese women. A greater expansion of the skeletal muscle mass with obesity in men than women might be expected given their greater skeletal muscle mass/lower FM at healthy weights, but it is not obvious why this would occur. Clearly this is an important remaining research question.

Secondly the lower energy density of weight loss in men (higher p-ratio) and the greater weight loss is consistent with most other models in which weight change mainly reflects tissue energy density. However this is not a complete explanation because difference in weight loss between men and women was more than would have been expected from their respective P-ratios if tissue composition was the only explanation. Indeed the men lost more FM, as well as weight and FFM than women. The implication of this is that the greater weight loss for men within our studies reflects physiological factors which we have not identified and/or non-physiological factors such as greater dietary compliance/energy deficit or more exercise, which are more important in influencing weight loss compared with physiological factors such as a genotypic P-ratio. Clearly resolution of these questions will help our understanding of the factors which explain the marked variation in weight loss which is observed in practice.

 

fig1Figure 1. Relationship between the energy partitioning parameter (P-ratio) and the extent of weight loss.

Individual P-ratios for weight loss at 2 months calculated for a subset of subjects conforming to selection criteria for minimum detectable differences in loss of FM and FFM, plotted against weight loss men; ■ women □. The lines shown are power curves fitted separately to male (continuous line) and female values (dashed line). Mean P-ratios versus weight loss for all subjects calculated from a) baseline regressions, ▲ men, Δ women: i.e. mean values for gain; b) mean loss of FFM and FM at 2 months, ■ men; □women and at 2-6 months ● men;○ women, plotted against weight loss at 6 months.

 

Acknowledgements:  This study was supported by the BBC. The BBC had no role in the study design; collection, analysis, or interpretation of data or report writing.

 

Contact:

D Joe Millward PhD, DSc, RNutr.

Emeritus Professor of Human Nutrition

Department of Nutritional Sciences

School of Biosciences and Medicine

Faculty of Health and Medical Sciences

University of Surrey, Guildford GU2 7XH UK

e.mail: D.Millward@surrey.ac.uk

http://www.surrey.ac.uk/nutrition/People/professor_joe_millward/index.htm

 

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