In the present study, MR spectroscopy was used to non-invasively quantify the intra- and extramyocellular lipid content in 159 obese children and youths included in a multidisciplinary obesity treatment.15
We found that a substantial proportion (74.2%) of these patients had an MFC of ≥5%. A high MFC was correlated with a higher BMI SDS and a higher VAT, but not with sex, age, SAT, SAT/VAT-ratio, pubertal stage, testicular size, menarche, PAS, PIS, or biochemical measures including liver enzymes. As expected, the MFC was strongly associated with both IMCL and EMCL. The IMCL was twice as high and the EMCL four times higher in patients with a high MFC compared with those with an MFC <5%, suggesting that the EMCL accumulates more readily than the IMCL.
An association between MRS-assessed muscular fat deposition and BMI has been reported in two smaller studies on children and adolescents (n=22 and n=29, respectively).19,20
We also found an association between MFC and BMI SDS in this larger group of obese children and youths. However, we used BMI SDS as a measure of obesity that also takes into account the growth and development in boys and girls.
Glycolytic muscles generally contain less fat than oxidative muscles.21,22
In the present study, the amount of fat deposited as IMCL measured in the mainly glycolytic psoas major muscle was higher than that reported by Bredella et al.
for the mainly glycolytic tibialis anterior muscle in 21 obese adult women (mean BMI 34 kg/m2
) assessed by MRS.23
Compared with studies in children and adolescents, the IMCL was slightly higher in our study than that measured in the mainly oxidative soleus muscle in two groups, each comprising 14 obese youth with normal glucose tolerance (mean BMI 35 kg/m2
; age 11–15 years and
mean BMI 39 kg/m2
; BMI SDS 2.54; age 13.5 years),19,24
and in 14 obese youth with impaired glucose tolerance (mean BMI 37 kg/m2
; BMI SDS 2.48; age 13 years).24
The IMCL in the present study was also higher than that reported in studies of 17 and 20 lean adults23,25
and in eight lean adolescents.19
These findings suggest that deposition of fat in skeletal muscle tissue is not related to age, but is promoted by the increased amount of lipids available in the natural course of severe obesity, as indicated by the high MFC in these young, severely obese patients in the present study.
Sinha et al.
found a significant association between VAT and both IMCL and EMCL in 14 obese (mean BMI 35 kg/m2
; age 11–15 years) and eight lean children.19
Similarly, Saukkonen et al.
found increased IMCL and VAT in eight obese children with impaired glucose tolerance and 13 obese children with normal glucose tolerance (mean BMI 36 kg/m2
; age 11–15 years and BMI 35 kg/m2
; age 11–15 years) compared with eight non-obese siblings.20
These relationships are similar to the association between MFC and VAT found in the present study, suggesting that the development of visceral fat depot and muscular fat content might not be independent of each other. However, MFC was not associated with liver enzyme concentrations in the present study, suggesting that fat deposition in the muscle is independent of the liver status.
Plasma TG concentration correlated positively with IMCL content, but not with EMCL content, in a study of 14 obese (mean BMI 35 kg/m2
; age 11–15 years) and eight lean children.19
However, we could not confirm this association between TG concentration and MFC, IMCL or EMCL contents even though we included a large group of children. This finding is intriguing and may suggest a great variability in plasma TG concentration in children and youths, even though blood samples were acquired in relative proximity to the initiation of treatment. A confounding factor is the time gap between blood samples and MR examinations in the present study, which was a median of 41 days in the present study. The MR examination was done after treatment initiation in the majority of the patients (96 of 119), which might tend to show a decreased MFC due to obesity treatment, which may have biased the relationships studied.
Although the role of puberty in accumulation of IMCL and muscular fat is not fully determined, one study has shown elevated IMCL content in 5 lean girls with premature adrenarche (mean BMI SDS 0.65; age 7.8 years) compared with prepubertal controls,26
which may suggest that the IMCL content is affected by pubertal development per se
. In the present study we did not find associations between pubertal stages and MFC or IMCL.
An association between physical activity and IMCL has been reported in healthy, lean adult males,27,28
but we did not confirm this association in the present study. However, it is difficult to compare these studies directly because of divergent methods, differences in exercise and workload duration, and the fact that the above-mentioned studies27,28
included only healthy, lean adult males whereas the obese children and youths in the present study may have great difficulty in attaining a high intensity of physical activity. Although the present study did not show a significant association between PIS and MFC (P=0.08), the data might indicate a tendency that children and youths performing high levels of inactivity exhibit a high MFC.
We acknowledge that several confounding factors may have biased the results in the present study. One limitation of our study was the lack of a control group, which would have been desirable for comparing the results in obese patients with normal-weight children and youths. A second limitation was that the extremely obese (>130 kg) were excluded from investigation because they could not enter the MR scanner; this precluded the most extreme obese patients from examination, even though they had the highest probability of having a high MFC. MFC and IMCL are dependent of many factors, such as diet,28
morning-to-evening changes in MFC,28
In the present study, we did not record dietary details, although the participants were given specific individual guidelines about food groups to eat or avoid as part of the obesity treatment protocol.15
These guidelines included the generally recommended diet low in carbohydrates and fat. The 5% limit in MFC was defined arbitrarily because no studies have defined a widely accepted MFC limit. We used the 5% limit because ectopic fat deposition is thought to be pathological and because this is similar to the limit used in the characterization of liver fat.17
The MRS examinations were not conducted in the fasting condition, which may have caused minor variations in the degree of MFC depending on the diet and time since the last meal consumed. To avoid morning-to-evening changes, the MRS examinations occurred between the hours of 9 and 11 in the morning. We had interview-reported approximate estimates of the duration of physical activity and inactivity, which makes the PAS and PIS rough estimates. Unfortunately, we did not record physical activity and inactivity on a daily basis or the exercise workload or intensity.
Finally, fat deposition in skeletal muscle is a patchy disease
as both IMCL and EMCL are distributed inhomogeneously.33,34
Therefore, some variation should be expected, depending on the placement of the spectroscopy VOI, which we did not adjust for by measuring several positions in the psoas muscle at the same time. Instead, measurements by MRS were made in the same muscle (psoas major) on approximately the same anatomical site in all participants. The present MRS data were not corrected for the effect of transverse relaxation time,35
but such adjustments have not traditionally been performed in previous studies.28,33
However, the calculated MFC%, IMCL%, and EMCL% in the present study may deviate somewhat from the objective values. Correction for longitudinal relaxation time was not necessary because of the long TR.
The strength of this study is the relatively large number of participants investigated using a non-invasive, non-ionizing, and precise technology, which showed that most of the patients had increased fat content in their muscles.