Obesity and childhood obesity are increasing and etiology, epidemiology and treatment should be adressed 1. After fetal nutritional experiences and early breastfeeding in the first months of life, the introduction of complementary feeding along with either breastfeeding or formula feeding is important for long-term infant development 234. A high infant protein intake has been suggested to be related to childhood overweight 5678, although not confirmed in hispanic children 9. The suggested mechanism is an accelerated growth during infancy via stimulation of insulin and IGF-1 2. Sugar containing beverage intake in childhood has been shown to be related to overweight in childhood 10, and this effect seems to extend into adulthood 11. The suggested mechanism is that higher sugar containing beverage intake is not compensated for by lower energy intake from other sources 12. More specifically it is suggested that beverages from fruit sources or corn syrup contain a high level of fructose 1213. Fructose does not increase insulin levels after intake as glucose does, which results in relatively high levels of plasma triglycerides as a result of unsuppressed lipolysis 13. However, there is no conclusive evidence for fructose as the central cause of obesity 1415. Also very early postnatal high carbohydrate complementary feeding is suggested to be related to later overweight 3. These effects might imprint metabolic changes that last into later years of life and predispose for obesity and cardiovascular illness 2.

That other factors play a role in the etiology of overweight is undoubted, including overweight parents and inactive lifestyle of child 161718. However, we were able to retrieve only one study on beverage sugar intake during infancy and risk of overweight development 19. The European Society of Pediatric Gastroenterology, Hepatology, and Nutrition (ESPGHAN) states that there are few data on the effects of specific complementary foods on growth 20.

The aim of the present pilot study was to determine whether high beverage sugar intake or high animal protein intake during infancy is associated with an increased risk of overweight at the age of 8 years.

A total of 120 (64 boys, 56 girls) infant BMI were used for analysis. Mean infant (sd) ages of responders at the time of completing the questionaire were 4.8 (0.3), 6.9 (0.3), 8.8 (0.3), 10.9 (0.3), and 12.4 (0.2) months (see also Additional file 1, Table S1). Mean age at infancy was 8.7 (2.8; range 4.4-13.0) months and at follow-up 8.7 (0.3; range 8.3-9.1) years. Mean BMI and BMIsds at 8 years was 16.3 (2.1) and -0.12 (1.17). For the whole group with complete data at 8 years (n = 63) 48 (80%) of the children had ever been breastfed, and 22 (37%) still received breastfeeding at the age of 6 months. Further infant characteristics are presented in Table 1. Table 2 shows food, energy, protein, and sugar intakes in infancy for children being overweight or not at age 8 years 26. Both beverage sugar and animal protein intake from complementary foods like meat, milk and meals from a jar were higher in overweight children when adjusted for total energy intake. Overweight children consumed more drinks and less infant formula, and therefore also less protein from formula. The intake of beverage sugar and animal protein adjusted for total energy intake was fairly constant across age groups (see supplementay data file 1). Mean beverage sugar intake of users was not statistically different between age groups (one-way anova, with post-hoc Bonferroni). Protein from mothers milk is included in animal protein, but contributes a mean of about 10% to total animal protein in the three younger groups and only about 2, 5% in the two older age groups. Formula milk contributes a mean of 47% (sd 37) to animal protein intake. Interactions between beverage sugar and animal protein intake and age, gender, socio-economic status, and maternal overweight were tested, and all were found to be not significant.

A total of 20 out of 120 children (17%) were considered overweight (BMIsds > +1.0) and in the subsample 9 out of 63 children were overweight (14%), which is not significantly different (p = 0.675). Table 3 shows that the unadjusted odds ratio (95% CI) was 1.10 (1.02, 1.18) for beverage sugar intake, and 4.06 (1.50, 11.00) for the highest tertile of animal protein intake compared to the lowest two tertiles. Adjusted odds ratios (model 3) were 1.13 (1.03, 1.24) for beverage sugar and 9.67 (2.56, 36.53) for animal protein, thus both were still independently associated with overweight at 8 years. In the subgroup analysis (n = 63) model 4 showed that after addition of current maternal overweight, full breastfeeding for more than two months, current physical activity in hours per week, and current energy intake in kcal per day, the ORs for beverage sugar and animal protein were still significantly associated with overweight at 8 years.

When protein from mothers milk was excluded from animal protein, this did not change the OR's. The OR's for becoming overweight for animal protein become slightly lower but demonstrate very significant independent effects (beverage sugar: model 2, OR 1.11 (1.03, 1.19) p = 0.006; model 3, 1.12 (1.02, 1.22) p = 0.014; animal protein: model 2, 3.27 (1.16, 9.19) p = 0.025; model 3, 4.66 (1.47, 14.8) p = 0.009).

Figure 1 shows that for infants in the lower two tertiles for both beverage sugar and animal protein, only 3 out of 58 infants (5%) were categorised as overweight. Infants in the highest tertile for either beverage sugar or animal protein had 5/23 (22%) or 7/27 (26%) infants with overweight at 8 years, respectively. Infants that were in the highest tertile for both beverage sugar and animal protein had 5/12 (42%) overweight at 8 years. The percentage overweight between the four groups of Figure 1 was significantly different (Chi square test, p = 0.005). Table 4 shows that linear regression analysis provided an effect size of beverage sugar on BMIsds of 0.05 per energy percent of increase in beverage sugar intake, which was not altered by adjustment for confounders. A change in beverage sugar intake of 10 percent of energy intake, might therefore be responsible for an increase in BMIsds of 0.5. For the highest tertile of animal protein intake the increase was 0.5-1.0 BMIsds compared to the lower two tertiles.

To our knowledge this is the first study to show two independent effects of both beverage sugar and animal protein intake during infancy and overweight at age 8 years. Beverage sugar intake during infancy increases the risk of overweight at 8 years by more than 10% extra risk per energy percent beverage sugar increase. For animal protein infants with the highest tertile of intake had a more than 9 times higher risk of becoming overweight at 8 years.

In 1995 Rolland-Cachera suggested the early protein hypothesis; a high protein intake in excess of metabolic requirements enhances weight gain in infancy and increases the risk of obesity later in life 5. This hypothesis has been confirmed in similar small studies 678, although not in hispanic youth 9. The period of dietary exposure, the timing of overweight assessment and the definition of overweight might in part explain different outcomes. Gunther et al. suggested that particularly animal protein might be responsible for the association 27. When the present analysis was performed with two dummies for second and third tertile of animal protein intake (data not shown), the second tertile did not show a significant relationship. In line with this observation is the hypothesis that animal protein above a certain level of intake might deregulate insulin and IGF-1 with an impact on preadipocyte differentiation and multiplication 227. High protein intakes may also decrease human growth hormone secretion and reduce lipolysis, which might aid fat accumulation 2. Lower protein levels in infant formula seems to reduce weight up to age 2 years 28, and might therefore contribute to reduction of later overweight.

A recent review by Gibson 14 showed an overwhelming amount of studies on sugar sweetened soft drinks and obesity. Gibson included all cold beverages containing added sugars, carbonated or not. The evidence seems to be inconclusive, although a small effect of sugar sweetened soft drinks on BMI is suggested. A recent study by Herbst et al. 19 is the single study that prospectively investigatedinfant beverage sugar intake during the first year of life. Herbst et al. 19 show no or possibly a protective effect of sugar from beverages and sweets, however the level of beverage sugar intake is very low. At 1 year of age the beverage sugar intake is 0 percent of total energy intake in the Donald study 19 compared to more than 5 percent in the present study. A reasonable explanation for this difference is that the Donald study has selected infants from a higher socio-economic level: > 80% of infants was breastfed for more than 4 months versus 41% in our study, > 60% of parents had a high socio-economic status versus one third in our study, and maternal overweight was only 25% versus > 40% in our study). In line with our study, the authors conclude that when added sugar intake increases to higher levels this might be detrimental to BMI development.

Beverage sugar was consumed through apple and orange juice, most often from apple concentrate, and lemonade also from sugar concentrate. Apple juice has a high fructose content. Fructose consumption results in decreased circulating levels of insulin and leptin when compared with glucose 13. Because insulin and leptin function as key signals to the central nervous system in the regulation of energy balance, it has been assumed that over a longer period of time high fructose intake could lead to increased caloric intake or decreased caloric expenditure and weight gain 15. However, at this moment and from this study it is not possible to draw any firm conclusions about the mechanism involved.

Limitations of the study are the total number of infants, although similar studies were of similar size 678. The low response rate to the initial study is also a limitation, but each child serves as its own control. Confounders were not all available for the whole sample of 120 children, but for the subsample of 63. However, the primary effect of beverage sugar and animal protein did not seem to change when tested in the subsample (from model 3 to model 4). Therefore we assume that the effects we found are very robust and would not be different when the whole sample would have provided all information on confounders. Although a large random sample was the basis of the study, socio-economic status (maternal education) was slightly higher compared to Dutch average http://statline.cbs.nl/statweb/. The educational level of parents was very similar to the only other Dutch investigation into dietary intake of infants, the Nutrient Intake Research (VIO) study 29. This study used a representative sample of 300 infants for both the 9 and the 12 month old infants. We have evaluated the quality of the self reported dietary intake data in an earlier study as good 21, also based on the similarity with the VIO study. Two day food records at one time point is a limited assessment of longer-term exposure to food constituents. However, providing high levels of beverage sugar and/or food products high in animal protein are most likely related to long established food habits of (primarily) the mother. It should also be stressed that both beverage sugar and animal protein were not related to poor food habits. Beverage sugar is largely from fruit juices and from apple concentrate, which the parents judge as 'fruit' and therefore as 'good'. Animal protein is derived from normal healthy food groups and infant formula. The level of intake of percentage energy beverage sugar or animal protein is not significantly different between high and low socio-economic status groups. The relationship between high animal protein intake (highest tertile) with overweight at 8 years is even stronger in high compared to low socio-economic status group (p = 0.006 vs p = 0.102 respectively). This may have resulted in low underreporting as well as high consistency in food 'habits'. Comparing responders with non-responders, no differences appeared in socio-economic status, but parents with breastfed infants at the time of investigation in 2001 had responded to the follow-up call in 2009 significantly more often (26/38 vs 93/188, p = 0.009). Information on maternal smoking is missing, however it is not likely that smoking is associated with beverage sugar or animal protein intake at infancy.

Also, the intake of mother's milk may be more inaccurately estimated than the intake of formula milk.

However, considering the low contribution of mother's milk to the total consumption, it is unlikely to play a key role in effect size and/or significance.

The use of BMIsds as definition of overweight is not ideal but acceptable as an outcome measure; no definition of obesity is ideal at present 16. Both weight and height are self reported which might introduce some bias. However, recently it has been shown that BMI misclassification was almost absent in an adult population when self reported weight and height were evaluated 30. Children are more often subject to measurement of weight and height, therefore BMI misclassification is assumed to be limited. Although some BMI misclassification may have occurred it is not likely that the amount of BMI misclassification is related to protein and beverage sugar intake in infancy. It is unlikely this would have caused the significant effect found in this study since the effect of non-differential misclassification on estimates of relative risk will be to always reduce the true relative risk 31. Physical activity is also self reported, however self reporting is probably less important than selection of a method with a high internal and external validity 32. Another limitation is the observational nature of the design, which makes it diffecult to conclude a causal nature 2.

Provision of sugar containing beverages and a high animal protein diet in the first year of life appears to increase the risk of overweight at 8 years. Although childhood overweight is clearly multi-factorial, each significant and clinically meaningfull contributing factor that can be influenced by the parents should be addressed accordingly. Sugar containing beverages provided during infancy are much less difficult to address than lifestyle changes during later stages of life 3233.

Future studies with a larger sample size should confirm these observations.

The authors declare that they have no competing interests.

PW designed the study, LK, NvB, PW conducted literature research, PW, LK, NvB, SvdZ took part in food calculations and statistical analysis, all authors contributed significantly to writing of the manuscript and all authors read and approved the final manuscript.