Longer duration of breastfeeding is associated with lower BMI among individuals aged 8-49 months in Poland.

Objectives: Breastfeeding in the first years of life seems to be an important factor affecting the shaping the body mass, and body mass/height proportion during first months of life, that importantly differentiates body mass and body proportion at the later stages of the human ontogenesis. The aim of this study was to evaluate if duration of breastfeeding affects BMI in later life. Material: In the analysis 470 children born in time, aged 8-49 months were included. The data came from the nurseries in Lodz (central Poland) where children' parents filed the questionnaires and was collected in years 1993-1997. In the analysis were included information about: duration of breastfeeding, parental educational level, birth body weight, birth body length. The BMI was standardized on sex and age of the children. The stepwise regression models were used to evaluate the hypothesis. The obtained residues for BMI were used to verify only breastfeeding impact using the regression analysis. Results: The regression model included parental education level and birth parameters were used to adjust BMI on them and revealed that low educated mothers had children with higher BMI (Beta=0.213; p<0.015). Additionally heavier newborns had higher BMI at the later stages of ontogenesis (Beta=0.207; p<0.001). There were made two regression analyses using BMI residues. The first one treated breastfeeding as categories, and revealed that children breastfed over 6 months vs. not breastfed or breastfed no longer than 1 month had lower BMI (Beta=-0.098; p<0.030). The second model treated duration of breastfeeding as a continuous variable, and showed that breastfeeding duration was inversely correlated with BMI residues (Beta=-0.092; p=0.042). Conclusions: Duration of breastfeeding differentiates the BMI among Polish children aged 8-49 months and seems to reduce BMI in further life. Trial registration: This study was conducted according to the guidelines laid down in the Declaration of Helsinki and all procedures involving research study participants were approved retrospectively by the Ethical Commission at the University of Lodz (NR21/KBBN- UŁ/I/2018). Written informed consent was obtained from all subjects' parents.

For decades breastfeeding has seemed to be an important factor which protects against later risk of overweight and obesity [1]. The mechanisms which affect later metabolism probably stem from composition of chemical compounds, especially hormones such as: adiponectin, ghrelin, leptin and IGF-1, of maternal milk [2] and regularity of the feeding and self-control by newborns of amounts of the consumed meal [3]. Moreover, the comparison between breastfeeding and formula meals showed that formula food had mainly higher energy density than human milk [4]. There are numerous of researches which indicated valuable impact of breastfeeding on body mass in the latest stages of the human ontogenesis. Chiasson et al. (2016) [5] presented that breastfeeding was associated with lower probability of obesity in children at age 3. On the other hand, the investigation conducted between 2001 and 2007 among individuals aged 2-6 years in South California (US) showed that those who were breastfed longer than 6 months had slightly lower BMI [6]. Longitudinal studies carried out in Japan in years [2001][2002][2003][2004][2005][2006][2007][2008][2009] showed that breastfeeding was associated with decreased risk of overweight and obesity among children aged 7-8 years [7]. Additionally, Moss et al. (2014) [8] presented that children aged 2-4 years who were breastfed had lower likelihood of obesity than not breastfed individuals. In contrary, Vafa et al (2012) [9] reported that duration of breastfeeding is not associated with BMI among Iranian children aged 7 years.
Taking into account current worldwide problem with obesity, especially during childhood that act long term effect on adulthood obesity [10], it is crucial to determine range of the factors which can be connected with the body mass gaining. Beside importance of breastfeeding, familial factors such as parental education seem to be important covariates for body mass and body proportion. In the study which tackled the problem of familial factors and body composition among Polish children aged 6-13 years it was showed that children whose mothers had basic educational level had excess body fat and BMI [11].
Moreover, birth body parameters have important impact on later BMI and body mass. The newborns characterized by lower birth body weight are significantly lighter during the next years of ontogenesis compared with heavier newborns [12,13]. There was also observed negative association between birth length and BMI in the future life [14].
The aim of this study was to evaluate if duration of breastfeeding affects BMI in later life taking into account parental education and birth parameters.

Material And Methods
The study material comprised 470 individuals born in time (37th-42nd week), aged 8-49 months. The study material was collected in years 1993-1997 in the nurseries in Lodz (central Poland) and consisted of the questionnaires, which were filed by children' parents and included information about parental educational level and breastfeeding duration. Questions regarding birth parameters and gestational age were obtained from children's medical records kept by parents, which were provided by medical staff immediately after delivery. Current anthropometric measurements: body weight (kg) and body height (cm) were performed by staff of the Department of Anthropology. According to current body weight (kg) and body height (cm), BMI was calculated for each individual. The BMI was standardized on sex and age of the children. Z-score values for current BMI were calculated for each group. All the investigated individuals were divided into separate groups with the same calendar age and sex. Additionally, the z-scores for birth length and weight were calculated. In each case the following equation was used: z-score = (x -µ)/σ, where: x -the individual's value for the analyzed parameter; µ -the mean of the analyzed group; σ -the standard deviation of the created group.
The mothers' and the fathers' level of education was categorized as: (1) basic vocational education (2), secondary education and (3) higher education. Information about breastfeeding duration was reported as full months. In the analysis the breastfeeding duration was treated as a continuous variable (months) and categorized as: (1) not breastfed or no longer than 1 month, (2) longer than 1 month since 6 months and (3) over 6 months.

Statistical analysis
To check the differences between categories of breastfeeding duration the Chi 2 test (for all included categorical variables) and the ANOVA (for continuous variables) were used.
The forward stepwise multiple regression model was applied to estimate the importance of the breastfeeding as a continuous variable and categorical variable on BMI z-score.
The Statistica ver. 13.0 software was used to perform all calculations.

Results
Among 470 investigated individuals aged 8-49 months the mean duration of the breastfeeding was 5.47 (SD = 5.71) months. In Table 1 there were described basic statistics for breastfeeding duration.
The parental education statistically significantly stratified duration of breastfeeding (Table 2a- (Table 2a). The birth weight and length and BMI z-score were not statistically significantly associated with breastfeeding duration (Table 2b).
The regression model included parental education level and birth parameters were used to calculate residues for BMI necessary to design model for breastfeeding duration effect. The results were the following: lower educated mothers had children with higher BMI (Beta = 0.213; p < 0.015). Additionally heavier newborns had higher BMI at the later stages of the ontogenesis (Beta = 0.207; p < 0.001) ( Table 3).
There were made two regression analyses using BMI residues. The first one treated breastfeeding as categories, and revealed that children breastfed over 6 months vs. not breastfed or breastfed no longer than 1 month had lower BMI (Beta=-0.098; p < 0.030). The second model treated duration of breastfeeding as a continuous variable, and showed that breastfeeding duration was inversely correlated with BMI residues (Beta=-0,092; p = 0.042). However, in both cases the BMI was explained only approximately in 0.5% by breastfeeding duration.

Discussion
Breastfeeding is a factor which undeniably is associated with further metabolism and promotes, lower body fat levels, lower body mass and body proportion in later life. Simultaneously, chemical complexity of the human milk and abundance in hormones such as leptin or ghrelin, but also behavioral process of controlling the amount of food intake resulted in prominent impact on body mass and proportion in later life.
The worldwide discussion considering breastfeeding and later development also involves information about regulation of glucose blood level by skin to skin contact that occurs during breastfeeding [15].
Additionally, it is known that breastfeeding leads to enhancement of the intestinal bacterial flora and may be protective against overweight and obesity [16].
We pointed out that breastfeeding may protect against excess body mass in the later life. However, it has been not investigated yet how long breastfeeding should last for optimal BMI development. In our study, when we presented breastfeeding as categories, the statistically significant effect was for the We also presented that maternal education and body birth weight may be an important factor for proper shaping BMI in later life. In other investigations which tackle the problem of maternal education impact on the offspring BMI the results were similar -the better educated mothers the lower BMI of their offspring [11,17,18]. In the case of birth body parameters we presumed that higher birth body weight is connected with higher BMI values in later stages of life, since 4-56 months of life to adulthood [11,13,14,19].
It is worth to add that also skin to skin contact which is a part of breastfeeding process may lead to a proper brain development including shaping the HPA axis [20]. The important organ in the HPA axis is hypothalamus which regulates hunger and satiety. In the light of these connections we also suspect that breastfeeding may regulate adequate hypothalamus action in the terms of hunger and satiety in later life.

Conclusions
In conclusion we underline that breastfeeding additively slightly differentiates the BMI among Polish

Consent for publication
Written informed consent was obtained from all subjects' parents.

Availability of data and materials
The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.

Competing interests
The authors declare that they have no competing interests.