Dietary guidelines say that we should eat fat in moderation and include lots of fruit, vegetables and grains in our meals. The recent results of the largest ever dietary intervention trial did not demonstrate a beneficial effect of such recommendations on the risk of developing cancer and cardiovascular disease (CVD). Or did it? In reality, the interpretation of this milestone study needs nuanced careful interpretation.
The case against fat and for vegetables and fruit
Scientists have long suspected that a high level of dietary fat increased the risk of breast cancer, based on laboratory studies and observations that groups of people consuming less fat had lower incidence of the disease. Other types of studies, although being less conclusive, tended to lead to the same conclusion. Similarly, it has been hypothesized that a diet low in fat and high in vegetables, fruit and fibre protects from colorectal cancer. However, the evidence for this is slim.
The case for a negative role of fat in heart disease is strong, but the culprits are saturated fat and trans-fat; unsaturated fat and in particular fish oils are protective.
In search for a definite answer
Despite the body of evidence for a role of diet in cancer and CVD, few studies are available to prove a causal effect. The most convincing proof
is obtained through a controlled intervention trial where one group changes their diet (intervention group) and a comparison group does not (control group). Any effect observed has to be due to the intervention.
This is why the Women’s Health Initiative Dietary Modification (WHI) trial is so important. This controlled trial was conducted in 48,835 women aged 50 to 79, followed up for 8 years. The intervention group participated in an intensive behavioural modification programme. The group’s baseline dietary fat intake represented 38% of the energy needs. The intervention decreased by a quarter the baseline fat intake (to 27-30%) for at least 6 years, relative to the control group. It also led to moderate increases in dietary vegetables, fruit and grains consumption.
Having a closer look
Rates of breast cancer, colorectal cancer and cardiovascular events such as coronary heart disease (CHD) and stroke did not differ significantly between the two groups.
Despite the lack of statistical significance of the results, one should not conclude that diet has no effect on cancer or CVD.
Overall, the results were consistent with the previous evidence, i.e. that women who have high fat intake at baseline and decrease it while aiming for the recommended amount of vegetables and fruit, decreased the risk of some breast tumours. The risk reduction was stronger for breast cancer (and was almost significant for all participants of the intervention group) than colorectal cancer.
Women who reduced their intake of saturated fat the most tended to have a greater reduction in CHD risk. The conclusions of the study, regarding CHD, should be interpreted carefully as the dietary intervention in the trial does not fully reflect the current dietary recommendations – for example, no attempt was made to reduce saturated fat specifically while maintaining or increasing the intake of beneficial fats. However, the results reinforce the rationale that changes in dietary fat intake resulting in lowering LDL-cholesterol are good for your heart.
Possible reasons for a modest effect
The dietary intervention in the trial was sizeable since it led to cutting dietary total fat by a quarter and increased intakes of vegetables and fruit by a third. The observed effects on only certain types of breast cancer and CHD may thus seem modest, if not disappointing. The reasons for this could be the non-optimisation of the intake of specific nutrients (e.g. type of fat), as this may prove to have more powerful effects. Another is that the benefits of diet may cumulate over a life time and/or dietary pattern has more impact when it happens in the first half of life.
- Prentice RL et al. Low-Fat Dietary Pattern and Risk of Invasive Breast Cancer: The Women's Health Initiative Randomized Controlled Dietary Modification Trial. JAMA. 2006;295:620-642.
- Beresford SAA et al. Low-Fat Dietary Pattern and Risk of Colorectal Cancer: The Women's Health Initiative Randomized Controlled Dietary Modification Trial. JAMA. 2006;295:643-654.
- Howard BV et al. Low-Fat Dietary Pattern and Risk of Cardiovascular Disease: The Women's Health Initiative Randomized Controlled Dietary Modification Trial. JAMA. 2006;295:655-666.