Obesity is a disease in which body fat excessively accumulates to the extent that health is negatively affected.¹ Obesity is reported to be associated with diseases including diabetes, hypertension, hypercholesterolemia, asthma, arthritis, heart disease, stroke, endometrial cancer, breast cancer, and colorectal cancer.^2,3 According to a 2014 report by the World Health Organization (WHO), the obese population has more than doubled since 1980, and now comprises approximately 13% of the global adult population (11% and 15% of the male and female populations, respectively).³ This is also the case in Korea, where the obese population is steadily increasing at a higher rate than that noted previously. In 2013, the prevalence of adult obesity, defined as body mass index (BMI) ≥25 kg/m², was approximately 32.5% (37.6% for men and 27.5% for women). Hence, a third of the Korean adult population is obese.⁴

Generally, BMI and waist circumference are used as indexes for obesity.¹ Obesity is defined as a BMI ≥30 kg/m² in Western countries¹ and ≥25 kg/m² in Asia-Pacific countries⁵; the latter definition is used in Korea. Additionally, waist circumference and waist-hip ratio are used as indexes for abdominal obesity.⁶ The Korean Society for the Study of Obesity gives the definition of abdominal obesity as waist circumference ≥90 cm for men and ≥85 cm for women.⁷ BMI does not provide information regarding body fat mass or fat-free mass⁸, but recent studies have reported that metabolic syndrome is associated with body fat and fat-free masses.⁹ Of several body composition measures, fat-free mass in particular has an inverse J-shaped association with all-cause mortality, and the level of fat-free mass has been suggested as a predictor of the all-cause mortality rate.¹⁰ Previous studies have reported sex, age, height, weight, and exercise and dietary habits to be factors that affect fat-free mass.^11-15

A number of studies on fat-free mass in obesity have been conducted to investigate a high-protein diet and the maintenance of fat-free mass; these studies seem to show that a high-protein diet may help obese people maintain fat-free mass.^16-20 However, diverse protein levels and various intervention periods were used in the literature¹⁵, and some studies have reported that a high-protein diet was ineffective in maintaining fat-free mass.^21,22 Aside from the effect of fat-free mass maintenance, a high-protein diet in obesity was also found to be effective in improving body composition by reducing body weight and body fat mass, improving blood lipid composition by decreasing serum triglycerides and increasing high density lipoprotein cholesterol (HDL-C)^20,21, and reducing the levels of inflammatory cytokines such as interleukin-6 (IL-6) and tumor necrosis factor alpha (TNF-α).²² However, bone mineral density, bone mineral count, and bone metabolic index were not affected by protein intake level.^16,23

Previous studies on fat-free mass were interventional research investigations of high-protein intake and fat-free mass maintenance in obese people on a low calorie diet to reduce body weight. Cross-sectional studies have never been conducted to compare muscle mass indexes according to protein intake level. Therefore, in the present study we hypothesized that muscle mass index would differ in obese people depending on their protein intake level. We compared fat-free mass and muscle mass indexes by protein intake level in obese men and women who were not on a low calorie diet. We additionally examined bone indexes, blood lipid composition, and the level of inflammatory cytokines according to protein intake level.

1. Study participants

Obese adults (aged between 19 and 65 years) with a BMI ≥25 kg/m² or waist circumference ≥90 cm for men and ≥85 cm for women were recruited through newspaper advertisements between May and July 2014. Of those interested in the study, breastfeeding or pregnant women were excluded. Additional exclusion criteria were as follows: weight reduction of 10% or more in the last 3 months, having taken a pharmaceutical product (e.g. anti-obesity or lipid-lowering drugs) or health foods that may affect weight within the last 4 weeks, treatment with antipsychotic medication within the last 2 months, history of drug or alcohol abuse, acute severe cardiovascular disease (e.g. heart failure, myocardial infarction, or stroke), current or previous immune system disease, severe liver or kidney failure, previous gastrointestinal surgery (with exceptions of simple appendectomy and hernia repair surgery), and participation in another study within the last 2 months. Lastly, exclusion criteria based on clinical laboratory test results were as follows: serum creatinine ≥176.8 μmol/L, systolic blood pressure ≥180 mmHg, and diastolic blood pressure ≥110 mmHg. Of the 133 obese adults initially recruited, 53 were excluded after screening and a final total of 80 subjects (50 men and 30 women) participated in the study. The study was approved by Hanyang University Institutional Review Board (HY-14-041), and was conducted according to the Declaration of Helsinki. All participants signed a written consent form.

2. Procedure

3. Statistical analysis

All statistical analyses were performed using SPSS 21.0 (SPSS, Inc., Chicago, IL, USA). For each variable, all descriptive statistics were presented as the mean and standard deviation (in the format of mean±standard deviation) and percentage. Continuous variables were tested by performing an independent t-test, and nominal variables were tested by performing a chi-square test after the data were cross-tabulated. Male and female participants were each divided into 2 groups with the median value of the daily average protein intake as the cut-off; an analysis of covariance (ANCOVA) was conducted to compare the 2 groups within each sex. In the ANCOVA, age and exercise habit were adjusted in the analysis of body composition, and age, BMI, and exercise habit were adjusted in the analysis of blood biochemical measures. Furthermore, energy intake was adjusted in the analysis of the intake of macronutrients and major protein sources by food group. Statistical significance was defined as a P value less than 0.05.

1. General characteristics

Participants’ general characteristics are summarized by sex in Table 1. Out of a total of 80 participants, 50 were men and 30 were women. The mean age, arm, leg, and trunk fat masses, and total body fat mass were significantly higher in women than in men. In contrast, men had significantly higher measurements than women for the following: height, weight, systolic and diastolic blood pressures, arm, leg, and trunk fat-free masses as well as total fat-free mass, ASM, ASM/height², skeletal muscle index, total bone area, bone mineral count, and bone mineral density. More men were smokers and drinkers, but more men exercised at least once a week compared to women. Additional analyses conducted to compare the intakes of energy and macronutrients showed that men had significantly higher energy, protein, and lipid intakes. No significant sex differences were found in BMI, waist and hip circumferences, waist-hip ratio, or carbohydrate intake.

Table 1 . Basic characteristics of obese men and women.

	Men (n=50)	Women (n=30)	P value
Age (year)	27.1±6.6	36.4±10.2	<0.001
<30, n (%)	11 (22.00)	22 (73.33)	<0.001
≥30, n (%)	39 (78.00)	8 (26.67)
Height (cm)	173.84±5.68	160.63±4.49	<0.001
Weight (kg)	84.27±7.77	72.08±6.83	<0.001
BMI (kg/m2)	27.85±1.70	27.91±2.05	0.890
WC (cm)	94.82±5.23	92.74±6.04	0.110
HC (cm)	105.05±3.99	103.34±3.95	0.065
WHR (%)	0.90±0.04	0.90±0.05	0.726
SBP (mmHg)	138.96±11.09	127.13±14.83	<0.001
DBP (mmHg)	83.52±9.95	78.30±10.47	0.029
Body fat (kg)
Arm	2.74±0.70	4.13±0.83	<0.001
Leg	4.65±1.18	7.13±1.72	<0.001
Trunk	7.41±2.01	9.77±2.24	<0.001
Total	14.87±3.38	21.04±4.14	<0.001
Fat-free mass (kg)
Arm	8.37±0.98	5.19±0.70	<0.001
Leg	21.40±2.28	14.72±1.57	<0.001
Trunk	29.00±2.53	22.58±1.77	<0.001
Total	58.77±5.24	42.48±3.63	<0.001
ASM (kg)	29.77±3.05	19.90±2.07	<0.001
ASM/height2 (kg/m2)	9.84±0.78	7.71±0.67	<0.001
SMI (%)	35.35±2.19	27.65±1.85	<0.001
Total bone area (cm2)	2,204.5±126.0	1,921.1±121.4	<0.001
BMC (g)	2,664.7±265.8	2,164.6±242.8	<0.001
BMD (g/cm2)	1.21±0.07	1.13±0.09	<0.001
Smoking, n (%)	20 (40.00)	1 (3.33)	<0.001
Alcohol, n (%)	43 (86.00)	14 (46.67)	<0.001
Exercise, n (%)	40 (80.00)	16 (53.33)	0.012
Energy (kcal)	1,906.5±407.0	1,609.7±380.6	0.002
Carbohydrate (g)	231.67±56.45	229.48±43.81	0.856
Protein (g)	76.43±19.64	62.28±20.75	0.003
Lipid (g)	62.83±17.23	48.56±19.69	0.001

Data are reported as means±SD for continuous variables and number (percentage) for categorical variables..

P values were analyzed by independent t-test or χ² test..

BMI, body mass index; WC, waist circumference; HC, hip circumference; WHR, waist hip ratio; SBP, systolic blood pressure; DBP, diastolic blood pressure; ASM, appendicular skeletal muscle mass; SMI, skeletal muscle index (skeletal muscle mass (kg)/weight (kg)×100); BMC, bone mineral count; BMD, bone mineral density; Exercise, ≥once a week..

2. Body composition

Participants of each sex were divided into 2 groups according to the sex-specific median value of daily protein intake as a cut-off point, i.e., group 1 participants consumed a sub-median level of protein (men: 0.76 g/BW kg/day, women: 0.65 g/BW kg/day), and group 2 consumed more than the median amount of protein (men: 1.08 g/BW kg/day, women: 1.11 g/BW kg/day) (Table 2). Comparison of body compositions between the groups defined according to protein intake level showed the following results. In men, arm and leg fat-free masses, total fat-free mass, ASM, and ASM/height2 were significantly higher in group 2 than in group 1. However, BMI, waist and hip circumferences, waist-hip ratio, arm, leg, and trunk fat masses, total body fat mass, trunk fat-free mass, skeletal muscle index, total bone area, bone mineral count, and bone mineral density did not show significant differences between the groups. In women, none of the body composition measures were significantly different between groups 1 and 2.

Table 2 . Body composition according to dietary protein intake in obese men and women.

	Men		P value*	Women		P value†
	1 (≤75.0 g) (n=25)	2 (>75.0 g) (n=25)		1 (≤61.4 g) (n=15)	2 (>61.4 g) (n=15)
BMI (kg/m2)	27.56±1.45	28.14±1.91	0.231	28.23±2.32	27.58±1.76	0.355
WC (cm)	93.92±4.75	95.71±5.63	0.247	92.94±7.33	92.55±4.65	0.907
HC (cm)	104.27±3.97	105.84±3.94	0.204	104.10±3.65	102.57±4.21	0.093
WHR (%)	0.90±0.03	0.90±0.04	0.617	0.89±0.06	0.90±0.05	0.384
Body fat (kg)
Arm	2.65±0.66	2.82±0.75	0.483	4.25±0.95	4.02±0.72	0.383
Leg	4.61±0.92	4.69±1.41	0.905	7.40±1.95	6.86±1.47	0.197
Trunk	7.31±1.87	7.51±2.17	0.700	10.28±2.65	9.27±1.67	0.178
Total	14.73±2.70	15.02±4.00	0.806	21.93±4.86	20.14±3.18	0.131
Fat-free mass (kg)
Arm	8.00±0.78	8.73±1.03	0.005	5.25±0.59	5.12±0.81	0.505
Leg	20.62±1.75	22.18±2.51	0.017	14.59±1.66	14.84±1.53	0.925
Trunk	28.43±2.69	29.58±2.27	0.108	22.67±2.10	22.49±1.44	0.631
Total	57.05±4.65	60.50±5.30	0.021	42.52±3.96	42.45±3.41	0.743
ASM (kg)	28.62±2.30	30.91±3.31	0.008	19.84±2.08	19.96±2.14	0.871
ASM/height2 (kg/m2)	9.60±0.52	10.08±0.92	0.033	7.69±0.57	7.72±0.78	0.959
SMI (%)	34.89±1.93	35.82±2.36	0.140	27.30±1.70	28.00±1.98	0.332
Total bone area (cm2)	2,183.9±121.1	2,225.2±129.7	0.265	1,929.3±141.8	1,913.0±101.2	0.382
BMC (g)	2,599.2±209.1	2,730.1±302.6	0.086	2,140.5±197.9	2,188.8±285.8	0.726
BMD (g/cm2)	1.19±0.07	1.22±0.08	0.107	1.11±0.06	1.14±0.11	0.230

Data are reported as means±SD for continuous variables and number (percentage) for categorical variables..

^*P values were analyzed by ANCOVA after adjusting for age and exercise in obese male;

^†P values were analyzed by ANCOVA after adjusting for age and exercise in obese female.

BMI, body mass index; WC, waist circumference; HC, hip circumference; WHR, waist hip ratio; ASM, appendicular skeletal muscle mass; SMI, skeletal muscle index (skeletal muscle mass (kg)/weight (kg)×100); MFR, muscle to fat ratio; BMC, bone mineral count; BMD, bone mineral density..

3. Blood biochemical measures

Sex-specific comparison of blood biochemical measures between group 1 and 2 showed the following results (Table 3). In men, BUN was significantly higher and triglycerides significantly lower in group 2 than group 1. Aside from those, there were no significant differences between the groups in any of Hb, Hct, WBC, RBC, PLT, T-bilirubin, FBG, T-protein, albumin, creatinine, T-cholesterol, LDL-C, HDL-C, IL-6, or TNF-α. In women, BUN was significantly higher in group 2 than group 1. In contrast, no significant difference between the groups was found in any of the following measures: Hb, Hct, WBC, RBC, PLT, T-bilirubin, FBG, T-protein, albumin, creatinine, T-cholesterol, LDL-C, HDL-C, triglycerides, IL-6, and TNF-α.

Table 3 . Blood biochemical measurements according to dietary protein intake in obese men and women.

	Men		P value*	Women		P value†
	1 (≤75.0 g)	2 (>75.0 g)		1 (≤61.4 g)	2 (>61.4 g)
	(n=25)	(n=25)		(n=15)	(n=15)
Hb (g/L)	158.32±8.31	158.28±7.87	0.941	137.07±5.86	134.07±5.57	0.071
Hct (%)	45.78±2.05	45.54±2.27	0.820	40.47±1.41	39.73±1.44	0.105
WBC (×109/L)	6.76±1.64	7.05±1.54	0.523	6.11±1.24	6.89±1.47	0.172
RBC (×1012/L)	5.23±0.32	5.24±0.44	0.984	4.68±0.24	4.60±0.19	0.226
PLT (×109/L)	240.12±45.17	219.08±50.72	0.099	281.40±67.30	263.53±56.09	0.429
T-bilirubin (μmol/L)	12.84±3.29	11.76±4.38	0.379	10.47±4.58	10.53±4.90	0.889
FBG (mmol/L)	5.03±0.42	5.04±0.45	0.954	4.99±0.59	4.87±0.56	0.526
T-protein (g/L)	75.68±3.11	75.52±4.10	0.909	74.00±4.42	73.13±3.78	0.331
Albumin (g/L)	48.92±1.80	49.24±2.18	0.359	46.80±3.10	46.53±2.13	0.384
BUN (mmol/L)	4.88±1.10	5.52±0.94	0.025	4.13±0.99	4.99±1.08	0.013
Creatinine (μmol/L)	103.16±10.85	107.04±13.45	0.220	79.60±8.58	78.87±8.92	0.450
T-Cholesterol (mmol/L)	5.84±1.16	5.33±0.88	0.129	6.32±0.88	5.79±1.33	0.290
LDL-C (mmol/L)	3.07±0.93	2.86±0.70	0.498	3.45±0.79	3.08±1.04	0.362
HDL-C (mmol/L)	1.42±0.18	1.39±0.17	0.733	1.48±0.18	1.56±0.13	0.256
Triglyceride (mmol/L)	2.31±1.61	1.52±0.89	0.032	1.90±1.16	1.41±0.87	0.312
IL-6 (pg/mL)	5.67±2.40	6.33±3.39	0.390	5.82±1.77	5.13±0.85	0.183
TNF-α(pg/mL)	17.68±10.34	20.16±11.03	0.415	15.04±1.53	15.69±1.77	0.737

Data are means±SD..

^*P values were analyzed by ANCOVA after adjusting for age, BMI, and exercise in obese male;

^†P values were analyzed by ANCOVA after adjusting for age, BMI, and exercise in obese female.

Hb, hemoglobin; Hct, hematocrit; WBC, white blood cell; RBC, red blood cell; PLT, platelets; T-Bilirubin, total bilirubin; FBG, fasting blood glucose; T-Protein, total protein; BUN, blood urea nitrogen; LDL-C, low density lipoprotein cholesterol; T-Cholesterol, total cholesterol; HDL-C, high density lipoprotein cholesterol; IL-6, interleukin 6; TNF-α, tumor necrosis factor α..

4. Intake of macronutrients and major protein sources

The results of the analysis on the intake of macronutrients and major protein sources according to protein intake level are as follows (Table 4). In male participants, the percentage of energy from carbohydrates was significantly higher in group 1 than in group 2, whereas the percentage of energy from total protein, animal protein, total fat, and animal fat consumption was significantly higher in group 2 than in group 1. An analysis conducted to compare the consumption levels of major protein sources showed that male participants in group 2 consumed more grains and meat than those in group 1. No significant difference between the groups was found in the consumption of carbohydrates, plant protein and fat, beans, eggs, fish and shellfish, or milk and milk-based products. The analysis of female consumption of nutrients and major protein sources revealed that the percentage of energy from carbohydrates was significantly higher in group 1 than in group 2, whereas the percentage of energy from protein and fat was significantly higher in group 2 than in group 1. Otherwise, there were no significant differences in the consumption of carbohydrates, total animal and plant protein, total animal and plant fat, grains, meats, beans, eggs, fish and shellfish, or milk and milk-based products between groups 1 and 2. Lastly, protein intake per kg of bodyweight was significantly higher in group 2 than in group 1 in male and female participants (men: 0.76 g/kg vs 1.08 g/kg, women: 0.65 g/kg vs 1.11 g/kg).

Table 4 . Intake of macronutrients and protein major sources food group according to dietary protein intake in obese men and women.

	Men		P value*	Women		P value†
	1 (≤75.0 g)	2 (>75.0 g)		1 (≤61.4 g)	2 (>61.4 g)
	(n=25)	(n=25)		(n=15)	(n=15)
Energy (kcal)‡	1,653.2±309.5	2,159.8±329.8	<0.001	1,300.2±223.3	1,919.2±212.2	<0.001
Carbohydrate (% of energy)‡	54.75±5.30	48.43±7.43	0.001	62.94±6.41	53.82±6.02	<0.001
Protein (% of energy)‡	15.78±2.36	18.32±3.13	0.002	14.29±2.77	16.35±2.25	0.033
Lipid (% of energy)‡	29.47±4.18	33.25±5.41	0.008	22.77±6.02	29.83±4.31	0.001
Carbohydrate (g)	215.41±50.44	247.93±58.41	0.063	202.40±32.26	256.56±36.93	0.742
Protein (g)	60.83±9.75	92.04±13.62	<0.001	46.33±11.17	78.23±14.91	0.348
Plant protein (g)	27.15±7.49	31.59±10.64	0.283	24.94±5.49	34.10±8.04	0.927
Animal protein (g)	33.67±10.05	60.45±17.95	<0.001	21.39±9.33	44.13±18.34	0.525
Lipid (g)	50.65±8.99	75.00±14.71	<0.001	33.48±12.15	63.63±12.98	0.708
Plant lipid (g)	24.26±8.34	30.45±12.42	0.398	18.75±7.81	32.23±9.56	0.861
Animal lipid (g)	26.38±9.74	44.56±14.98	<0.001	14.74±7.59	31.39±12.04	0.639
Grain (g)	226.75±84.91	234.5±72.16	0.018	210.83±38.79	255.44±58.68	0.748
Beans (g)	24.61±31.67	18.86±27.23	0.225	21.69±21.63	35.24±38.81	0.414
Meats (g)	113.71±50.03	226.12±91.42	<0.001	53.33±33.48	130.40±90.48	0.686
Eggs (g)	32.99±22.30	55.68±41.85	0.183	27.21±33.98	46.63±28.01	0.533
Seafood (g)	36.23±38.09	37.09±35.37	0.446	40.38±52.05	43.34±39.33	0.535
Milk and dairy products (g)	73.78±93.81	122.93±149.57	0.610	93.10±100.35	175.10±249.02	0.753

Data are means±SD..

^*P-value were analyzed by ANCOVA after adjusting for energy in obese male;

^†P-value were analyzed by ANCOVA after adjusting for energy in obese female;

^‡P-values were analyzed by independent t-test.

In the study, we conducted comparative analysis of fat-free mass and muscle mass indexes, bone health parameters, serum lipid profiles, and inflammatory cytokine levels of obese male and female adults, separated into 2 groups according to protein intake level. In male participants, arm and leg fat-free masses, total fat-free mass, ASM, and ASM/height² were significantly higher, and serum triglycerides were significantly lower in group 2 than in group 1. However, for female participants, we did not observe a significant difference between groups in fat-free mass index, muscle mass index, or serum lipid profile. Furthermore, in both male and female participants, there were no significant differences in bone health parameters and inflammatory cytokine levels between group 1 and group 2. The analyses on the consumption of macronutrients and major protein sources according to protein intake level showed that in men, the percentage of energy from carbohydrates was significantly lower and the intakes of total protein, animal protein, total fat, animal fat, grains, and meat were significantly higher in group 2 than that in group 1, whereas in women there were no significant differences in the intake of macronutrients or major protein sources between the groups.

Dietary protein requirements are described as the minimal level of protein necessary to maintain a short-term balance of nitrogen under controlled energy intake.²⁴ The daily recommended dietary allowance (RDA) of protein is 0.8 g/kg in the US.²⁵ When determining the RDA, it is important to consider that a certain amount of protein is directly used for structural and functional purposes, such as the growth, maintenance, and repair of bones and soft tissues, and the production of hormones, antibodies, and enzymes.²⁴ According to the Dietary Reference Intakes for Koreans (KDRI, 2015), the RDA of protein is 65 g and 55 g for men and women in their 20s, respectively, and 60 g and 50 g for men and women in their 30s to 50s, respectively.²⁶

In the present study, arm and leg fat-free masses, as well as total fat-free mass, were significantly higher in group 2 than in group 1 among obese adult males. This finding is due to the fact that fat-free mass increases as protein intake increases²⁷; the rates of energy intake from protein in this study were 18.32% and 15.78% for the male participants in group 2 and group 1, respectively. Studies by Lejeune et al.²⁸ and Westerterp-Plantenga et al.²⁹ have shown that a protein intake group (18%) maintained a higher level of fat-free mass than a control group (15%), and studies by Tang et al.¹⁷ and Leidy et al.¹⁸ have reported a higher level of fat-free mass in a high protein intake group (1.4 g/kg) compared to a normal protein intake group (0.8 g/kg), all of which are consistent with the findings of the present study.

ASM, ASM/height², and skeletal muscle index, which we examined in relation to fat-free mass, are typically used to differentiate sarcopenia from sarcopenic obesity in the elderly.^30,31 Sarcopenic obesity is a major risk factor for metabolic syndrome, and the risk of people with sarcopenic obesity developing metabolic syndrome is 12 times higher than those without sarcopenia and obesity. Therefore, sarcopenic obesity can be considered a major risk factor of metabolic syndrome, rather than sarcopenia or obesity alone.³² Until now, research had not been conducted to address the prevention of sarcopenic obesity in young obese adults; however, in the present study, ASM and ASM/height2 were significantly higher for men in group 2 than for those in group 1. Additionally, in comparison with a study by Kim et al.31, ASM and ASM/height² were significantly higher among men in both groups 1 and 2 than in the control group, and, in particular, they were much higher in group 2. Therefore, additional research is warranted to investigate the effect of protein intake level on the prevention of sarcopenic obesity among young obese adults.

In women, however, neither fat-free mass nor muscle mass index differed with protein intake level. According to Tipton et al.³³, male and female sex hormones affect muscle protein synthesis differently, and thus, we believe that the observed different patterns of protein intake and fat-free mass in men and women could be attributable to hormonal differences between the 2 sexes. In addition, we suspect that the number of female participants in the present study was too small and that the protein intake level was not high enough to show a difference in fat-free mass between the groups.

In the study, we did not find a significant difference in body fat mass depending on the protein intake level in either men or women. However, in previous studies^20,21 that compared a high carbohydrate group (0.8 g/kg) and a high protein group (1.6 g/kg), body fat mass significantly decreased in the high protein group and the authors argued that this was due to a low insulin response following a relative decrease in carbohydrate intake.²⁰

In men, the serum triglyceride level was significantly lower in group 2 than in group 1. It was within a normal range in group 2, whereas the level in group 1 was elevated compared to normal. In women, groups 1 and 2 did not show a significant difference in serum triglyceride levels, but the level in group 1 was higher than normal, showing the same trend as in men. In both sexes, the rate of energy intake from fat was significantly lower and that from carbohydrates significantly higher in group 2 than in group 1. The difference in the level of serum triglycerides seems to reflect the difference in carbohydrate intake in the diet.³⁴ According to a study by Kasim-Karakas et al.³⁵, a low-fat high-carbohydrate diet increases the level of serum triglycerides. Our finding is also similar to the result reported by Westerterp-Plantenga et al.²⁹; the group in which 18% of energy intake was from protein had a lower level of triglycerides in comparison to the control group in which 15% of energy intake was from protein.

The level of HDL-C was not significantly different in either sex between group 1 and group 2. However, Layman et al.^20,21 have shown that, HDL-C was maintained or significantly increased in the high-protein group (1.6 g/kg), but was not in high-carbohydrate group (0.8 g/kg). In the present study, protein intake level adjusted for weight was 0.76 g/kg and 1.08 g/kg for men groups 1 and 2, respectively, and 0.65 g/kg and 1.11 g/kg for women in groups 1 and 2, respectively, which are lower than levels described in previous studies. Therefore, we suspect that a significant difference was not found in the HDL-C level between the groups due to the relatively low level of protein intake.

It has been suggested that obesity is detrimental to bone health because inflammatory cytokines such as IL-6 and TNF-α increase in obesity, and they in turn increase bone resorption.³⁶ Moreover, IL-6 and TNF-α are independent predictors of cardiovascular disease.³⁷ In the present study, there was no significant difference between the groups in IL-6 or TNF-α for either sex, although Kitabchi et al.²² reported a significant decrease of IL-6 and TNF-α in the high protein group (30%/kcal). In this study, the percentage of energy from protein in groups 1 and 2, i.e., 15.78% vs 18.32% for men and 14.29% vs 16.35% for women, was lower than those in a previous study.²² This may have resulted in an insignificant difference in inflammatory cytokines between the groups.

BUN level was significantly higher in group 2 than in group 1 in both sexes because BUN level reflects protein intake. This finding is consistent with that of Tang et al.¹⁶, who showed that BUN was higher in the high protein intake group than the normal protein intake group.

In men, after adjustment for energy intake, the meat (animal-derived foods) consumption level was higher in group 2 than in group 1, which suggests that the former group had a higher level of protein and fat intake. Furthermore, group 2 consumed more grains, indicating that they tended to consume more carbohydrates compared to group 1, although the difference between the 2 groups was not significant. In contrast, there was no significant energy-adjusted difference between women in groups 1 and 2 in the consumption of major protein sources or macronutrients. Accordingly, the difference in protein intake in women was due to the difference in energy intake, and the significant difference in protein intake disappeared after the level was adjusted for energy intake.

The comparison of the daily average intake of macronutrients between the Korean adult population and the study participants revealed the following. The daily average intake of carbohydrates among Korean adults is 340.5 g for men and 274.6 g for women, and thus, the study participants of both sexes consumed less than the daily average intake of the Korean adult population. With respect to protein, the daily average intake among Korean adults is 83.7 g and 61.3 g for men and women, respectively. Thus, for both sexes, group 2 of the present study had a higher daily average intake of protein than the Korean adult population. Finally, with respect to fat, the daily average intake by Korean adults is 56.9 g and 42.2 g for men and women, respectively, and again, in both sexes, group 2 of the present study had a higher daily average intake of fat than the Korean adult population.³⁸

This study was a cross-sectional study with a small sample size, and we found differences in fat-free mass and muscle mass indexes according to protein intake in men but not in women. Hence, it is difficult to generalize the study findings. Moreover, the 3-day dietary journals used to investigate nutritional intake were completed by the study participants themselves, who may have underestimated the intake of total energy, macronutrients, and major dietary protein sources. Furthermore, there were not many survey questions regarding exercise, and thus, information about exercise habits may not have been fully captured.

The present study found that in men, fat-free mass, ASM, ASM/height², and serum triglycerides were significantly different between groups defined by protein intake level, whereas in women there was no significant difference between the groups for any of the indexes. Accordingly, we conclude that, of all dietary intakes, protein intake positively influenced fat-free mass, ASM, ASM/height², and serum triglycerides in obese men in our study. Based on the findings, additional research should be conducted to investigate an appropriate level of protein intake necessary to prevent loss of muscle mass and further increase protein intake in obese people.