Commentary by Terry M. Gibbs, DO, CCD, NCMP
Many researchers have linked dietary habits, particularly fat intake, to risk of cancer. Here, Thomson and colleagues present the first long-term randomized controlled trial on the subject to date; the current study is a continuation of the Women’s Health Initiative Dietary Modification trial. The authors hypothesized that reducing fat intake to less than 20% of total energy intake would positively impact the incidence of invasive breast cancer and colorectal cancer in these women. While the initial reports suggest modest reductions in risk of certain cancers associated with this dietary change, the long-term follow-up data indicate that the risk reduction previously appreciated had not been sustained. Importantly, adjustments for change in body weight and body mass index did not affect hazard ratios, implicating dietary intake rather than body fat makeup as the true mediating process investigated in this study.
Though the authors report that the statistics do not support their overall hypothesis, one must remember that the intervention itself did result in an appreciable decrease in certain cancer rates, particularly in the estrogen receptor positive/progesterone receptor-negative breast cancer tumor subtype. However, as has been observed in many studies of lifestyle modifications, temporary changes do not necessarily provide significant long-term benefits; similarly, in this study hazard ratios describing cancer risk returned to (or nearly to) pre-intervention figures after return to normal dietary habits. Interestingly, the risk of colorectal cancer increased in participants during the intervention phase and decreased during the postintervention phase. As a clinician, I see these results as supporting counseling of patients regarding lifelong lifestyle modifications as opposed to temporary changes. Additionally, the ability to quantify the risk reduction that one might enjoy upon committing to such lifestyle changes is quite important, as these data could be presented to patients, guiding conversations and possibly serving as motivation for women who wish to actively affect their risk of developing cancer. The present study included only women whose baseline fat intake was greater than 32% of overall energy intake, most of whom were overweight or obese, and the most drastic reductions in risk were observed in women who reported the highest baseline fat intake. Future studies to further characterize the effects of a low-fat diet on women with differing baseline fat intake could help clinicians to target their counseling efforts. Furthermore, the investigators have focused only on overall fat intake, rather than the type of fat being consumed. Because previous studies have implicated animal fat in cancer development, it would be of interest to determine whether the types of fat consumed in this population are associated with cancer incidence in these subjects. This may also be of significant consequence to clinicians counseling women on dietary habits and cancer risk. Patients may be more likely to achieve consumption of different, healthier types of fat rather than to decrease their intake drastically, in particular because it appears that women were not able to restrict their fat intake to the extent the investigators desired.