Obesity has increased significantly since 1980 [1] with global prevalence doubling in the past three decades [2]. The WHO reports that in 2014, 1.9 billion adults worldwide were obese or overweight [2]. The epidemic is thought to be costing $2 trillion globally every year [3] as it notably increases the incidence of diabetes, cancer and endocrine diseases [4]. Research since the 1980s has reported that students gain a significant amount of weight during their first year of university. The phenomenon has been called the ‘Freshman 15’, in reference to the perception that students gain 15lbs (6.8 kg) during their first year in university [5]. However, a mix of cross-sectional, cohort and randomised studies have shown weight change to actually be between −0.6 kg and 4 kg [6–35] with no consensus on differences between sexes in weight change over this time [36]. The literature on university student weight change shows that weight gain is linked to poor eating habits, stress and lower physical activity following the transition from a structured secondary education environment to university [37, 38]. Indeed, for first year university students, at 17–19 years old, it is often the first time they live away from home, have greater access to alcohol and are in an environment with many catering facilities [36]. Meta-analyses published on the topic between 2008 and 2015 have slightly varying estimates with pooled mean weight change of first year undergraduates found to be between 1.36 and 1.75 kg [36, 38, 39]. The most recent meta-analysis, from 2015, further investigated weight trends and found that more than 60 % of students gained weight and that weight gainers gained on average 3.38 kg (7.5lbs) (95%CI: 2.85–3.92) over the first academic year, supporting the argument that weight change should be considered beyond overall population means. More specifically, studies should perform subgroup analyses within weight gainers and weight losers and should investigate weight change as a rate (kg/week) to account for variability in follow-up lengths, to increase comparability between terms/semesters and to ensure standardisation across studies.

Unlike the US and Canada, the English (UK) context of student weight change over the first year of university has not been thoroughly investigated. In England, approximately 50 % of the youth (17–30 years old) attend higher education [40] and according to the National Union of Student, 20 % of students live at home during their undergraduate studies [41]. In contrast to the US, students can usually drink from their first year as the legal age is 18 years. In the most recent meta-analysis, only one longitudinal study on the subject was identified where Finlayson et al., found a weight change of 0.83 kg over three months in 120 students [42]. However, this study did not explore weight change during the second academic term nor analyse weight change in weight gainers and losers. It is worth further investigating students in England to extend our understanding of weight change in university students, especially as we are seeing increases in childhood and adult obesity in the country [43]. Evidence indicates that adolescent weight gain is highly linked to overweight and obesity in adulthood [44] and poor life habits gained during university may persist throughout adulthood. Particularly, it is important to explore weight change in relation to baseline weight, investigate weight change in rates and to conduct subgroup analyses amongst susceptible groups such as weight gainers and weight losers. Understanding weight change better at a large scale in England, in many universities, can provide insights as to the extent of the problems and shed light into possible interventions.

To answer these gaps on weight change in first year undergraduates in England, we aimed at sampling students across all universities in England. After data cleaning for eligibility, we gathered longitudinal data from students in 23 universities in England and obtained estimates on the mean weight change, the percentage of students who gained/lost weight and the amount of weight change stratified by these categories. We further tested the hypotheses of differences based on sex, academic terms and baseline BMI. For the first time, we also examined weight gainers and weight losers within baseline BMI categories and conducted analyses on individual relative weight change within these subgroups. Following recommendations from the 2015 meta-analysis to increase the quality of studies on the Freshman 15 [36], we analysed our data as a complete case approach, longitudinally and reported all essential data with standard deviations, including weight change in rate.

In our study, the overall mean weight change over 7–8 months was 0.98 kg (95 % CI 0.49–1.47) at a rate of 0.029 kg/week (SD = 0.10). There was no difference by sex or in the rate by term. We found that 12.5 % of the sample gained over 4.5 kg and that 52 % of the sample gained over 0.5 kg by the end of the academic year. The average weight gain in weight gainers was 3.46 kg (95%CI: 2.98–3.94). Within weight gainers, males gained significantly more weight and significantly more relative weight than females (p < 0.05). Female weight gainers had significantly lower baseline weight than female non-weight gainers. Thirty students gained over 0.5 kg during both the first and second term and finished the academic year with a large weight gain of 5.28 kg (SD = 2.93). From our sample, 25 % of the students lost weight with an average of −3.21 kg, with males losing significantly more relative weight and at a significantly faster rate than females.

One of the main strengths of this study was its longitudinal design across several universities in England. The final longitudinal sample was composed of 215 students from 23 different universities, in all regions of England, representing 0.2 % of the first year students in the sampled universities. According to a recent meta-analysis [36], this study would be the largest longitudinal study in England and the sixth largest in the world on the topic. The data we collected allowed us to stratify mean weight gain by sex, weight gainers and weight losers to better understand the Freshmen 15. By calculating weight change per week, we were able to investigate the rate of weight change over the academic year. We also analysed data as complete case and for the first time, presented relative weight change and relative weight change by BMI category. Some of the limitations of this study include the use of varied follow-up lengths, self-reported weight and below 50 % retention rate. The varied follow-up length is due to the nature of the English university system with unequal terms and academic years. We accounted for this by using rates of weight change. We tested for the impact of different follow-up lengths and found no significant effect. With regards to self-reported weight, there is a tendency to under-report weight and over-report height [47], though the possible bias has been shown to be minimal and consistent across time points in the student population [48, 49]. Since we are looking at weight change from longitudinally reported weights, the impact of self-report is reduced. Two meta-analyses showed no significant difference between mean weight change in studies when comparing studies that had measured weight and self-reported weight [36, 39]. One of the meta-analyses also found no significant effect of low, medium and high retention rates [36]. Our study was longer than the average of 5 months for studies on the topic and had a slightly lower retention rate than the average of 57 %. Nevertheless, we acknowledge there may be some reporting bias and selection bias, notably healthy responder bias, leading to misclassification and potentially conservative estimates. Another limitation of this study is that it is a descriptive study as we have no control groups of non-university students. Furthermore, we had wished to perform multi-level analyses of students nested within universities but the sample per university was not large enough. Overall, our study managed to account for recommendations of past meta-analyses by reporting the percentage of weight gainers/losers, stratifying analyses by these subgroups, reporting standard deviations and analysing from a longitudinal complete-case perspective.

This study further supports evidence that students in their first year of university gain a significant amount of weight and at a faster rate than the general population. The weight gain found in a 5 year follow-up study of approximately 20,000 health-conscious UK residents was 0.4 kg/year [50]. Our overall student sample mean weight gain over the academic year was within the range of similar studies published on this topic. It is statistically similar to results from a 2015 meta-analysis which found a pooled mean of 1.36 kg (95%CI 1.15–1.57) [36]. Similarly to the majority of comparable studies, we did not find a statistical difference based on sex for the overall mean [36]. In our study, this is likely due to lack of power and the imbalance in the male – female ratio (30:70). Our surveyed population of first year university students comprised 16.7 % of overweight or obese at baseline, lower than the country average of 30 to 36 % for 16–24 year old in England [51]. This is likely due to our sample being purely university students, being of 17 and 18 years of age along with potential volunteer bias.

We found a comparable proportion of weight gainers to the pooled estimate of 60.7 % from the meta-analysis. Across the literature, the range in percentage of weight gainers is between 22 % and 93 %, notably studies do not have the same definition of weight gainers [36]. The mean weight gain in weight gainers was statistically similar to the 3.38 kg (95%CI: 2.84–3.92) found in the meta-analysis of five studies [36]. This weight change in weight gainers is 4.6 times faster than the average weight gain in Scottish young adults (20–25 years old) [52] and 3.5 times the 1 kg average annual weight change in young adults in the US, according to the CARDIA study which followed adults for 14 years [53]. We found that 25 % of our sample lost weight, similar to a weighted average of 22 % from a range of 6 to 31 % found in studies identified by the meta-analysis [36]. The mean weight loss in weight losers of −3.21 kg was similar to the mean found by another study [27]. We are the first study to thoroughly investigate weight change in weight gainers and weight losers in rates and to explore relative weight change within subgroups.

As discussed in the meta-analysis, interpreting the overall mean results of a large population can be misleading and neglect the aspect that some individuals gain a lot while some lose a lot. It was 75 % of our sample that had a greater weight change than 0.5 kg. Within both weight gainers and weight losers, it seems apparent that there is an important and meaningful weight change of 3.46 kg (5.6 % of baseline weight) and −3.21 kg (4.6 % of baseline weight) respectively, at rates of 0.10 kg/week. Within the discussion of the Freshmen 15 which usually has a weight gain association, we must recognise that within this population, a large number of first year students lose weight, with a high averaged weight lost over the academic year. Interestingly, we found that for many individuals, a large weight loss during the first term did not necessarily mean a large academic year weight loss. Similarly, some students who had a large first term weight gain had small weight loss during the second term. This suggests that it is crucial to better understand the trajectory of this weight increase/decrease to attenuate this trend and prevent health consequences. Previous authors have identified physical inactivity, poor eating habits, stress and baseline weight linked to weight gain in the overall sample [37, 38]. Within weight gainers/losers, these influencers may have an exacerbated effect or other influencers pay a key role.

Our study is an important further steps into providing greater knowledge on the weight change trends in first year students. Students do experience important weight changes and universities need to focus on increasing health promotion to help them maintain/adopt healthy behaviours. Beyond this, it is important to understand the individual, interpersonal and environmental factors which influence weight gainers and weight losers, especially in the English context. Future studies should aim for large sample sizes to allow stratification and identification of significant correlates and studies should span more than one year to track trends following the first year of university. A study adopting the socio-ecological framework would allow for a holistic understanding of weight change in students.