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Original scientific article                       DOI: https://doi.org/10.35469/ak.2022.344
received: 2022-09-15                    UDC: 612-053.9
FAT MASS TO FAT-FREE MASS RATIO AND BODY 
COMPOSITION IN PARTICIPANTS WITH CHRONIC LEG 
PROBLEMS
Mihaela JURDANA1, Ana TOMAŽIČ1, Boštjan ŽVANUT1
1 University of Primorska, Faculty of Health Sciences, Izola, Slovenia
Corresponding Author: 
Boštjan ŽVANUT
University of Primorska, Faculty of Health Sciences, Izola, Polje 42, SI-6310 Izola, Slovenia 
Phone: +386 5 662 64 60; Fax: +386 5 662 64 80 
E-mail: bostjan.zvanut@fvz.upr.si
ABSTRACT
Background & aim: Low fat-free mass (FFM) or high fat mass (FM) in older adults 
are abnormal body composition phenotypes associated with morbidity. The dual bur-
den of excessive FM and low FFM may lead to a higher risk of cardiovascular and 
other diseases and disability and lower physical performance compared with individu-
als with normal body composition or low FFM or high FM alone. Both conditions lead 
to poorer health and can be identified by a high FM/FFM ratio. Because few studies 
have examined the effects of abnormal body composition using the FM/FFM ratio, the 
aim of this study is to determine the FM/FFM values by gender in patients with knee 
and hip pathologies and compare them with body composition parameters. This condi-
tion is important when surgery is required because abnormal body composition leads 
to an increase in infections and other postoperative complications.
Methods: A total of 93 participants aged 64.71 ± 7.71 years in women and 65.9 ± 
9.9 years in men with a body mass index (BMI) ≥25 kg/m2 were recruited from the Val-
doltra Orthopaedic Hospital (Ankaran, Slovenia). The waist circumference (WC) and 
anthropometric data (BMI), as well as body composition data (FM, FFM, % of body 
water, estimated by bioelectrical impedance - BIA) were measured. In addition, the 
body composition phenotypes (FM/FFM ratio) were calculated. 
Results: The FM/FFM index was higher in females. The female group with FM/
FFM (0.68) and those with a higher FM/FFM (0.92) correlated positively with WC. 
Male participants were excluded for further analysis because only two males had FM/
FFM > 0.80. The FM/FFM of the remaining male participants was 0.53 ± 0.09.
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Conclusions: FM/FFM provide information on patient body composition and iden-
tify individuals at risk of body composition abnormalities in women only. 
Keywords: Fat-mass, fat-free mass, waist circumference, older adults, knee, hip.
RAZMERJE MED MAŠČOBNO MASO IN PUSTO MIŠIČNO 
MASO TER SESTAVA TELESA PRI BOLNIKIH S KRONIČNIMI 
TEŽAVAMI SPODNJE OKONČINE
IZVLEČEK
Izhodišča: Zmanjšan delež puste mišične mase (FFM) ali povečan delež maščevja 
(FM) vodi pri starejših odraslih do sprememb v telesni sestavi, ki odstopajo od me-
jnih vrednosti, kar je povezano z obolevnostjo. Sočasna prisotnost povečane FM in 
zmanjšane FFM lahko v primerjavi s posamezniki z normalno telesno sestavo ali samo 
z nizko FFM ali visoko FM poveča tveganje za razvoj srčno-žilnih in drugih bolezni, 
invalidnost in nižjo telesno zmogljivost. Oba dejavnika poslabšata zdravstveno stanje 
posameznika, ki ga lahko identificiramo s pomočjo povečanega razmerja FM/FFM. 
Zato je namen te raziskave določiti vrednosti FM/FFM pri preiskovancih s patologijo 
kolena in kolka  ter jih primerjati s parametri telesne sestave. Poznavanje vrednosti 
FM/FFM je ključnega pomena v primerih, ko je potreben operativni poseg, saj telesna 
sestava, ki odstopa od mejnih vrednosti vodi do večjega tveganja za razvoj poopera-
tivnih zapletov.
Metode: V raziskavo je bilo vključenih 93 preiskovancev iz Ortopedske bolnišnice 
Valdoltra (Ankaran, Slovenija), starih 64,71 ± 7,71 let (ženske) in 65,9 ± 9,9 let (moški) 
z indeksom telesne mase (ITM) ≥25 kg/m2, napotenih na ortopedsko specialistično 
obravnavo. Izmerjeni so bili antropometrija (ITM), obseg pasu (WC) ter telesna ses-
tava (FM, FFM, % telesne vode, ocenjeni z bioeletrično impedančno analizo-BIA). S 
pomočjo indeksa FM/FFM so bili določeni fenotipi telesne sestave.
Rezultati: Indeks FM/FFM je bil višji pri ženskah. Tako skupina žensk s FM/FFM 
(0,68), kot skupina z višjim FM/FFM (0,92) sta pozitivno korelirali z obsegom pasu. 
Ker sta imela samo dva moška FM/FFM >0,80 smo moške preiskovance iz nadaljnje 
analize izključili. FM/FFM preostalih moških preiskovancev je znašal 0,53 ± 0,09.
Zaključek: Razmerje FM/FFM nam poda pomembno informacijo o značilnostih 
pacientove telesne sestave ter pomaga identificirati bolnike, pri katerih obstaja tveg-
anje za spremenjeno telesno sestavo. V naši študiji smo s pomočjo indeksa ugotovili, 
pomembne značilnosti v spremembi telesne sestave pri preiskovankah ženskega spola.
Ključne besede: maščobna masa, pusta mišična masa, obseg pasu, starejši odrasli, 
koleno, kolk.
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INTRODUCTION
The ageing of the population and the rise in obesity prevalence worldwide are the 
two major risk factors for non-communicable diseases, including degenerative diseas-
es, sarcopenia, frailty, increased morbidity and mortality, with negative implications 
for the public health system (Merchant et al., 2021). Elevated body mass index (BMI), 
waist circumference (WC), and/or waist-to-hip ratio are commonly used to define obe-
sity. BMI needs to be interpreted with caution in older adults as loss of physiological 
body height may lead to over-interpretation and lack of correlation with the percentage 
of body fat, the distribution of fat, or body composition (Batsis et al., 2016). Addition-
ally, BMI has a limited ability to distinguish between muscle mass and fat accumulation 
and does not provide information on body composition (Prado et al., 2012). Therefore, 
a better understanding of body composition may require more specific measurements, 
such as fat mass (FM) and fat free mass (FFM), which can be obtained by bioelectri-
cal impedance analysis (BIA) (Prado et al., 2012). The fat mass to fat-free mass ratio 
(FM/FFM) is an integrated metabolic index for assessing body composition, which 
evaluates the combined effect of the ratio between FM and FFM (Godziuk, Woodhouse, 
Prado, & Forhan, 2020; Rugila et al., 2022) In other words, FM/FFM indicates whether 
the amount of FM is adequate for the amount of FFM in an individual. Theoretically, 
higher values of the ratio indicate a less favorable balance between FM and FFM, and 
a high FM/FFM ratio already has negative effects on physical abilities and contributes 
to morbidity and mortality in many patients (Prado et al., 2012). Despite these clinical 
associations previously found with FM/FFM in chronic disease, this ratio has not been 
studied in patients with chronic knee or hip pain. The clinical impact of higher versus 
lower FM/FFM in these patients requires increased attention due to the accelerated loss 
of FFM associated with pain, mobility limitations and obesity-related inflammation 
(Godziuk et al., 2020). In addition, a loss of muscle mass is associated with prolonged 
hospital stays, infections, and other complications and disability (Prado et al., 2012). 
Thus, the aim of this study is to determine the FM/FFM values and compare the body 
composition of patients with higher and lower FM/FFM values by gender. Screen-
ing of this ratio may be important in order to identify patients with abnormal body 
composition, especially if surgery is indicated, to avoid complications after surgery. 
Our hypothesis is that higher FM/FFM values in these individuals are associated with 
abnormalities in body composition. 
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METHODS
Study Design
In this cross-sectional study, 93 participants (Caucasian origin), women with a 
mean age of 64.71 ± 7.71 years and men with a mean age of 65.90 ± 9.90 years with 
knee and hip problems, referred to an orthopedist for further diagnosis, were recruited 
at the Valdoltra Orthopaedic Hospital (Ankaran, Slovenia), excluding participants with 
cancer and acute illness, patients with pacemakers, and patients with a BMI ≤ 25 kg/m2. 
Participants were examined in 2019 in the departments of the mentioned institution and 
underwent anthropometric measurement and body composition analysis.
The National Medical Ethics Committee of Slovenia (code 0120-557/2017/4) ap-
proved the study. All the participants were fully informed about the procedures before 
written informed consent was obtained.
Anthropometric Data
At the study site, body height, body mass and WC were measured according to a 
standardized protocol. All the measurements were taken between 7 and 8 am after fast-
ing overnight. The body height (cm) of the subjects was measured to the nearest 0.1 cm 
while standing, without shoes, using a Leicester Height Measure (Invicta Plastics Lim-
ited, Oadby, England). The body mass (kg) of the participants wearing light clothing 
without shoes was measured with an accuracy of 0.1 kg. The WC (cm) was measured 
in the standing position halfway between the costal arch and the iliac crest with a non-
stretchable tape measure. The BMI (kg/m2) was calculated using the formula body mass 
(kg) / body height (m2).
Body Composition Measurements 
FM (kg) and FFM (kg) were estimated by bioelectrical impedance analysis (BIA) 
using the Tanita BC 418MA (Tanita Corporation, Arlington Heights, IL) and the data 
was analyzed with the software provided by the manufacturer. The FM to FFM ratio 
(FM/FFM) was calculated as an index of obesity combined with sarcopenia and the fol-
lowing cut-off values were used (Biolo et al., 2015; Prado et al., 2012): FM/FFM ratio < 
0.40 for metabolically healthy obese individuals in whom the increase in FM was minor 
compared to that in FFM; FM/FFM ratios between 0.40 and 0.80 for obese phenotypes 
in whom FM predominated FFM, but FFM is still adequately maintained; FM/FFM 
ratios > 0.80 for obese phenotypes with sarcopenia in whom FM was increased and 
FFM was reduced.
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Statistics
Statistical analyses were performed using IBM SPSS ver. 26. Univariate data 
analysis was performed by calculating the frequencies and percentages for qualitative 
variables, mean and/or median, standard deviation (SD) were calculated for quantita-
tive variables. To assess the normality of the distribution, Kolmogorov-Smironov and 
Shaphiro-Wilk tests were performed. Normality was additionally assessed by evaluat-
ing the histogram, skewness, and kurtosis.
The independent sample t-test was used to determine the statistically significant 
differences between the two groups because the variables in both groups followed the 
normal distribution. The Pearson’s correlation coefficient was calculated to determine 
the association between normally distributed variables.
RESULTS
The participant characteristics are shown in Table 1. The female and male partici-
pants were comparable in age and BMI. Male participants had higher body mass, % body 
water, FFM and WC whereas FM and the FM/FFM ratio were higher in females. We 
excluded male participants from further analysis because only two men had FM/FFM > 
0.80 (1.0 ± 0.13); the FM/FFM of the remaining male participants was 0.53 ± 0.09. 
Table 1. General characteristics of the participants 
Parameters Men n=37 (45.8%)
Women 
n = 56 (54.2%)
Age [years] 64.71 ± 7.71 65.90 ± 9.90
Anthropometric measurements
Body mass [kg] 93.2 ± 16.0 78.9 ± 12.9
Body height [cm] 172 ± 7 160 ±7
WC [cm] 109 ± 14 101 ± 11
BMI [kg/m²] 31.4 ± 4.8 30.8 ± 5.0
Body composition measurements
FFM [kg] 59.7 ± 7.1 43.3 ± 4.6
Body water [%] 48.7 ± 3.2 41.1 ± 2.9
FM [kg] 30.4 ± 10.6 43.3 ±4.6
FM/FFM 0.50 ± 0.16 0.75 ± 0.15
Legend: All the values are mean ±SD, BMI - body mass index, WC - waist circumference, FM - 
fat mass, FFM - fat free mass, FM/FFM ratio - ratio between fat mass and fat free mass
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Therefore, we divided the females into two groups based on the cut-off values for 
the FM/FFM ratio: FM/FFM between 0.40 and 0.80 and FM/FFM ≥ 0.80 (Table 2).
Table 2. Body composition data in women with medium and higher FM/FFM (0.4 < 
FM/FFM < 0.8 and FM/FFM ≥ 0.8).
FM/FFM
0.4 < FM/FFM 
< 0.8 FM/FFM ≥ 0.8
Independent 
sample t-test 
(p)Parameters (n=41) (n=15) Cohen d
Age [years] 66.62 ± 9.82 65.85 ± 9.55 .704
Body mass [kg] 72.33 ± 8.11 91.31 ± 11.12 < .001 -2.112
Body height [cm] 159.05 ± 6.94 160.53 ± 7.58 .318
WC [cm] 97.07 ± 7.93 110.18 ± 9.07 < .001 -1.581
BMI [kg/m²] 28.66 ± 3.32 35.56 ± 4.79 < .001 -1.837
Body water [%] 42.38 ± 2.18 38.13 ± 1.76 < .001 2.044
FM [%] 38.95 ± 3.34 46.28 ± 2.68 < .001 -2.304
FM [kg] 28.3 ± 4.9 42.4 ±7.0 < .001 -2.481
FFM [kg] 41.63 ± 3.61 46.31 ± 4.63 < .001 -1.200
FM/FFM ratio .68 ± .09 .92 ± .11 < .001 -2.511
Legend: All values are mean ± SD, BMI - body mass index, WC - waist circumference, FM  - fat 
mass, FFM - fat free mass, FM/FFM ratio - ratio between fat mass and fat free mass
Forty-one of the women were classified in the obesity phenotype group: FM/FFM 
ratio (0.68 ± 0.09), while fifteen women had an obesity phenotype combined with a low 
FFM: FM/FFM ratio (0.92 ± 0.11). In addition, women with a higher FM/FFM ratio 
(≥ 0.8) exhibited significantly greater body mass, BMI, WC, FFM and FM and a sig-
nificantly lower % of body water when compared to the group with a lower FM/FFM. 
Pearson’s correlation analysis revealed a significant, positive correlation in both groups 
between FM/FFM and BMI (overall: r = 0.742, p < 0.01; for 0.4 < FM/FFM < 0.8: 
r = 0.659, p < 0.01; for FM/FFM ≥ 0.8: r = 0.676, p < 0.01, Figure 1a) and WC (over-
all: r = 0.768, p < 0.01; for 0.4 < FM/FFM < 0.8: r = 0.741, p < 0.01; FM/FFM ≥  0.8: 
r = 0.542, p < 0.01, Figure 1b). In addition, a significant, negative correlation was 
identified between FM/FFM and % of body water (overall: r = - 0.907, p <0.01; for 
0.4 < FM/FFM < 0.8: r = - 0.869, p < 0.01; for FM/FFM ≥ 0.8: r= - 0.874, p <0.01, 
Figure 1c). 
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Figures 1a, 1b: Scatter diagrams of the FM/FFM ratio and BMI (a) and WC (b).
a)
b)
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DISCUSSION
In this cross-sectional study, the majority of the participants with hip or knee joint 
problems and associated leg pain were obese with normal FFM, while obesity associ-
ated with lower FFM was only found in women. A higher FM/FFM (> 0.8) is signifi-
cantly associated with worse body composition parameters (body mass, BMI, WC, FM, 
FFM) in female patients. 
The ageing process in the increasing population of older adults is associated with 
an increase in chronic health conditions and a high prevalence of functional limitations 
and disabilities associated with a decrease in muscle mass and strength (Delmonico & 
Beck, 2017). In addition, obese adults have greater skeletal muscle mass than normal 
body mass adults of the same age and sex, as we confirmed in this study. These dif-
ferences are the result of muscle anabolism induced by the loading effect of the higher 
body mass in obesity (Biolo et al., 2015; Morse & Soeldner, 1964).
In our study, the FM/FFM ratio in both female groups correlated strongly and posi-
tively with WC. The WC is a surrogate marker for assessing the risk of visceral/ab-
dominal fat accumulation (Ma et al., 2021). The association between abdominal fat and 
muscle mass loss is explained by several mechanisms. Abdominal obesity is character-
ized by systemic inflammation, oxidative stress and insulin resistance, which affect 
muscle proteolysis and inhibit protein synthesis (Biolo et al., 2015). Adipose tissue is 
actively involved in metabolic regulation and secretes pro-inflammatory cytokines such 
Figure 1c: Scatter diagrams of the % of body water.
c)
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as interleukin-6 (IL -6) and tumor necrosis factor-α (TNF-α), as well as the hormone 
leptin. The pro-inflammatory cytokines have effects on the brain, liver, and pancreas, 
which control appetite, carbohydrate and fat metabolism, and energy balance (Dhawan 
& Sharma, 2020; Roubenoff, 2000). The increase in FM and the accompanying release 
of pro-inflammatory cytokines lead to insulin resistance, which reduces the normal ana-
bolic effect of insulin on amino acid transport in muscle (Kim et al., 2000). In addition, 
there is evidence that leptin reduces growth hormone secretion, suppressing another 
important anabolic stimulus (Obradovic et al., 2021).
In addition, there are a number of explanations for the association between body fat, 
muscle mass and musculoskeletal pain, including the upregulation of pro-inflammatory 
cytokines released by adipose tissue (Younger, Kapphahn, Brennan, Sullivan & Ste-
fanick, 2016). Inflammation triggered by pro-inflammatory cytokines is known to be 
associated with physical pain, particularly in women (Walsh et al., 2018). In addition, a 
higher FM/FFM is a determinant factor of physical inactivity (Rugila et al., 2022). Ac-
cording to the FM/FFM ratio, obese women in this study had predominantly more fat 
mass and less muscle mass. This could be a result of the different distribution of body 
fat between the sexes (Prado et al., 2012). Physical activity became more difficult in 
this condition and their habitual level decreased further. The FM/FFM ratio may help 
reinforce the notion that body composition is a more informative measure of risk for 
musculoskeletal pain and immobility (Walsh et al., 2018). Taken together, these find-
ings support the hypothesis that a higher FM/FFM ratio (> 0.8) may associate obesity 
with an additional decrease of FFM due to immobility and inflammation. This could be 
important information for sarcopenia screening in participants scheduled for surgery to 
avoid complications.
This study should be considered in light of certain limitations. First, the study in-
cluded a relatively small sample size. Second, there is a lack of additional measure-
ments of inflammatory markers and muscle function, which would certainly be of inter-
est. Finally, this study is cross-sectional. 
CONCLUSIONS
A higher value of the FM/FFM ratio, studied for the first time in participants with 
chronic knee or hip pain, was significantly associated with abdominal obesity and is 
useful for identifying individuals at risk of body composition abnormalities. The clini-
cal implications of an FM/FFM > 0.8 in these participants require increased attention, 
as accelerated loss of FFM and complications may occur if surgery is indicated.
Acknowledgements
We thank the participants for their time and the entire staff of the Valdoltra Ortho-
paedic Hospital, Slovenia.
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