The water content of subcutaneous abdominal fat as estimated by a novel coaxial probe measuring the adipose tissue dielectric constant increased during a 9-week VLCD and remained so throughout the 1-y weight maintenance period. Changes in abdominal fat tissue hydration correlated with changes in body weight and insulin sensitivity.

The increase in subcutaneous abdominal fat water volume as estimated by changes in the fat dielectric constant indicates an increase in the intravascular or extravascular extracellular fluid volume. The methodology does not distinguish between these two fluid compartments. An increase in subcutaneous fat intravascular volume would reflect increased nutritive blood flow.5,11,12 An increase in overall subcutaneous abdominal blood flow as measured by xenon washout3 and in nutritive subcutaneous abdominal fat blood flow as measured by the microdialysis technique18 has also been shown previously in obese subjects after a 4-week VLCD. Extravascular or interstitial fluid volume changes in response to weight loss have not been previously reported. We found no evidence of attenuation of the increased subcutaneous fat water content as estimated by degree of hydration during 1 y of successful weight maintenance. To our knowledge, this is the first report on the changes of adipose tissue fluid volume during extended weight maintenance after weight loss.

The increase in abdominal subcutaneous water content as measured by the coaxial probe in this study most probably results from a weight-loss-induced and possibly insulin-mediated augmentation of nutritive blood flow or interstitial fluid volume. Insulin acutely increases adipose tissue blood flow, but in insulin resistant states such as obesity, blood flow is decreased.2,4 Other studies have shown that weight loss improves not only insulin sensitivity, but also increases adipose tissue blood flow.3,18 Extravascular or interstitial fluid volume in relation to insulin physiology has not been well studied, but muscle interstitial fluid volume appeared to increase acutely in response to insulin, and was related to the increase in glucose uptake.14 Skin water content or blood flow does not influence the results, because skin hydration changed little during the study. Furthermore, controlling for changes in the skin water content did not qualitatively alter correlations between changes in subcutaneous fat water content and changes in measures of adiposity or insulin sensitivity.

The increase in abdominal subcutaneous fat water content relative to baseline was fairly consistently correlated with improvements in insulin sensitivity and with decreases in fasting C-peptide levels, especially at 6 and 9 months. Changes in abdominal subcutaneous fat water content were also inversely associated with changes in body weight and percent body fat during the weight maintenance period, particularly towards the end. These findings again suggest that the increase in subcutaneous abdominal fat water content is induced by weight loss and mediated by insulin sensitivity. There were inconsistent correlations between changes in subcutaneous fat water content and HDL cholesterol and triglycerides.

There was no correlation between changes in abdominal subcutaneous fat water content relative to baseline and changes in weight, insulin sensitivity or other characteristics related to the metabolic syndrome immediately after the VLCD, even though subcutaneous fat water volume as estimated by the coaxial probe increased. It should also be noted that the correlation of changes in weight loss or percent body fat with changes in insulin sensitivity immediately after the VLCD were also uncorrelated (r=−0.09–0.02, not shown). A VLCD acutely induces numerous changes in neurohumoral regulation, including salt balance, renin–aldosterone axis function and sympathetic nervous system function,28 which may weaken the correlations between changes in fat tissue water content, weight loss and insulin sensitivity.

We used a novel open-ended coaxial probe that allows a noninvasive measurement of subcutaneous fat water content and thereby blood and extravascular water volume.23 The dielectric technique applies a high-frequency electromagnetic field to estimate the water content of a biological tissue.20,21,22 Measurement with the probe is simple and noninvasive, and does not require the use of radioisotopes. The probe cannot distinguish between tissue free and bound water, but it can be expected that fat tissue has less bound water per unit dry volume than tissues with high water content.20

The subjects of this study were abdominally obese, with evidence of disturbed lipid and glucose metabolism. All fulfilled the recently published NCEP criteria for the metabolic syndrome.25 These individuals thus represent a relevant high-risk subgroup of obesity for the study of weight loss and weight maintenance on abdominal subcutaneous fat water content.

This is a substudy of a larger and still ongoing randomised controlled double blind trial on the effects of the lipase inhibitor orlistat on weight maintenance after VLCD in the abdominally obese with the metabolic syndrome. We do not know which patients have been randomised to orlistat, but this is unlikely to influence our results. Orlistat has been shown in large long-term clinical trials to be an effective weight-loss drug that also lowers LDL cholesterol.29,30 Orlistat has not been shown in these large studies to have effects on other major cardiovascular or metabolic risk factors independently of weight loss. Furthermore, orlistat is only minimally absorbed into the blood stream.31

The noninvasive measurement of changes in water content of subcutaneous adipose tissue, although as yet not well characterised, may provide important insight on the metabolic changes that occur in subcutaneous fat tissue with weight loss and weight maintenance, and on the mechanisms that regulate those changes. The increased water content in abdominal subcutaneous fat tissue most probably indicates increased blood flow. Increased adipose blood flow could have implications not only for the storage and breakdown of triglycerides in fat tissue, but also in the regulation and release of numerous hormones and peptides, including leptin, resistin, adiponectin and tumour necrosis factor-α.

In what is to our knowledge the first study assessing the effects of weight loss and long-term weight maintenance on adipose tissue water content, subcutaneous abdominal fat water content as estimated by a novel dielectric probe increased after rapid weight loss and weight maintenance in abdominally obese persons with the metabolic syndrome. Changes in abdominal fat tissue water content furthermore correlated with changes in bodyweight and insulin sensitivity, suggesting that increased insulin sensitivity or other factors related to the metabolic syndrome may mediate the increase in subcutaneous fat nutritive blood flow that apparently occurs with weight loss and weight maintenance in the obese.