A recently published study in the BMJ has supported a synergistic effect between low birthweight and poor lifestyle on the risk of developing type 2 diabetes in adults.

The study used pooled data from three continuing long-term prospective cohort studies that had gathered data on health and lifestyle. The Health Professionals Follow-Up Study, the Nurses’ Health Study and Nurses’ Health Study ll were started in 1986, 1980 and 1991 respectively. Participants completed a food frequency questionnaire at the beginning of each study that was used as a baseline for this study. The food frequency questionnaires were repeated at approximately four-year intervals.

The study was concerned with adult modifiable lifestyle habits on type 2 diabetes, so the scoring for risk did not include raised blood pressure, or adverse lipid profiles. The five factors selected were: diet, body mass index, physical activity, smoking, and alcohol consumption. Patients were scored 0 or 1 according to absence or presence of the risk factor.

New incident cases of diabetes were obtained by self-reporting, validated on a supplementary questionnaire. This methodology has been shown to be accurate. Birthweight was also based on self-reporting at the commencement of the studies.

Self-reporting of birthweight is not quite as accurate as the self-reporting of diabetes, but the average error margin was known and adjustments could be made.

In total the pooled studies gave 149,794 participants who were free of cardiovascular disease, cancer or diabetes at entry to the study, and with no missing data on lifestyle.

For each of the three cohorts the participants were divided into categories according to birthweight. Prevalence of the lifestyle variables was similar across each weight category in all three cohorts.

In the 20-30 years of follow-up, 11,709 new cases of diabetes were documented.

The authors found a consistent association between low birthweight and risk of type 2 diabetes in all three cohorts. After adjustment for current BMI, the relative risk for those with the lowest birthweight (< 2.5 kg) compared with those in the middle birthweight category (3.18-3.82 kg) was 1.55 (95% CI:1.46-1.64).

After adjusting for confounding variables, the results showed a consistent relationship between each unhealthy risk factor and the risk of developing diabetes, which was again consistent across all three cohorts.

For patients with unhealthy lifestyle scores and low birthweight, the risk of developing new type 2 diabetes was increased by more than the simple sum of the two individual risk factors.

Across the cohorts studied, the new cases of type 2 diabetes could be attributed as follows:

• Low birthweight alone 22% (95% CI: 18.3-26.4%)

• Poor lifestyle alone 59% (95% CI: 57.1-61.5%)

• Additive interaction between low birthweight and unhealthy lifestyle 18% (95% CI: 13.9-21.3%)

Low birthweight is a surrogate for intrauterine stress, which is known to produce hyperglycaemia and increase the risk of diabetes. Poor lifestyle, especially obesity, an unhealthy diet and lack of exercise, is already strongly linked to the current high incidence of type 2 diabetes. This study shows that a combination of the two increases the risk by more than the addition of the risks associated with each individual factor.

The authors conclude: ‘The finding suggests that most cases of type 2 diabetes could be prevented by the adoption of a healthier lifestyle, but simultaneous improvement of both prenatal and postnatal factors could further prevent additional cases.’

This is another brick in the wall of evidence that change at population level is needed to stem the current flood of new diabetes cases that underlie the statistics.

Dr Matthew Lockyer