This study is a part of an ongoing research that aims at determining the effects of selected environmental, clinical, and demographic variables upon HM constituents. We found that the carbohydrate content was significantly higher in the summer season (May to September) than in the winter season (October to April) in the colostrum of the HM of lactating Israeli mothers of healthy term infants. Our findings are contrary to our initial hypothesis.
The effects of seasonal variation on composition and physio-chemical properties of raw milk had been described decades ago in animal studies (14–18). Saadi et al. compared the milk composition in cattle fed either on green grass or on concentrates during winter and summer periods. Those authors showed that the highest percentage of fat was found in the milk of cattle fed on green grass in the winter (16). Heck et al. reported lower fat and protein contents in bovine milk in summer than in winter (17). Few studies, however, have examined the effect of seasonal variation on human breast milk output and macronutrient contents (20, 21). A recent study by Davis et al. assessed the HM oligosaccharides (HMO) composition of breast milk in 33 rural Gambian mothers. They reported that mothers nursing in the wet season (July to October) produced significantly less HMO compared to those nursing in the dry season (November to June) (22). Similarly, Bates et al. documented seasonal variations for ascorbic acid serum levels (23). However, all of these studies had been conducted in countries with a poorly developed economy, unlike the current work.
Seasonal variation in macronutrients in the milk of animals seems obvious. Animals are more vulnerable to changes in weather conditions that can affect workload, food availability, and disease burden (14–17). Moreover, seasonal variability affects the light-to-dark ratio that, in turn, can affect bovine milk production and chemical ingredients through its effect on prolactin concentration (19). However, seasonal variation also affects human physiology and health. In a study by Prentice et al that was conducted four decades ago among rural African mothers, breast milk fat concentrations were affected by season in a manner that correlated with seasonal changes in maternal subcutaneous fat stores, and were unrelated to seasonal variations in maternal energy intake and breast milk output (20). Seasonality in nutrient intakes and meal patterns have also been documented in several studies (24–27). Although daily total caloric intake did not vary significantly by season in some of them (24, 25), others showed differences related to season in HM macronutrient composition (27) that was accompanied by change in specific food ingredients consumption, such as milk products, oils and fats, fruits, and vegetables (25).
Moreover, the effect of seasonal variation upon HM macronutrients may be mediated by a third factor, such as serum vitamin D levels, which has pronounced seasonal variations and a known systemic effect on glucose homeostasis and insulin sensitivity (28). Cross-sectional studies, including large-scale population studies (29, 30), have shown a significant positive relationship between serum 25- hydroxyvitamin D (25(OH)D) and measures of insulin sensitivity. This relationship has been corroborated in a variety of populations, including pregnant women (31). Since the vitamin D receptor is expressed in normal mammary glands (32), it is possible that variations in the levels of sun exposure affects the serum levels of 25 OH and 1,25-dihydroxycholecalciferol (1,25OHD) which modulates multiple cellular pathways, including those related to energy metabolism and glucose utilization and production.
The mechanism and biological significance of the increase in carbohydrates in colostrum during the summer period remain to be determined. We are aware that this effect seen in our study cannot be attributed to seasonal food shortage or morbidity, as described in developing countries (20–22). We consider that changes in the mothers’ diet or vitamin D serum levels in the summer season might have been instrumental in leading to increased carbohydrate content in their milk’s colostrum.
The present study has several limitations. First, HM was expressed either manually or by pump and the sampling time varied throughout the day. While we and others have shown that there are circadian variations in HM composition (9), and that fat content is affected by mode of expression (33), there remains the possibility that this inconsistency in sampling had masked small macronutrient content differences despite the large number of milk samples included in the study. Furthermore, three different investigators performed the milk analysis with the HMA, which might have led to significant scattering of the data, as discussed in a recently published study by Kwan et al (34). Lastly, we only measured macronutrients, and it would be valuable to determine whether micronutrients (vitamins and minerals) and other biologically active components (enzymes, hormones, antioxidants, and so on) of HM exhibit a seasonal variation in their distribution. The major strength of this study lies in its contribution to the sparse knowledge on the effect of seasonal variations of macronutrient content in the colostrum obtained from mothers of healthy term infants in a developed country. If confirmed in larger, prospective studies, these findings may encourage different dietary and supplements recommendations for lactating women in the winter months.