• Note that this dietary-intervention trial revealed that a tomato-rich diet increased serum concentrations of adiponectin among women at elevated risk of breast cancer.
• Be aware that, as incidence of breast cancer was not captured, it would be premature to conclude that such an intervention actually reduces breast cancer risk.

Eating a diet that includes plenty of tomatoes may prove important for lowering breast cancer risk, findings from a preliminary intervention study suggested.

When postmenopausal women at high risk for breast cancer ate tomatoes-rich foods every day for 10 weeks they had significant increases in serum adiponectin concentrations, with the strongest effect observed among lean women.

Adiponectin is an adipocyte-specific protein that plays an important role in regulating glucose homeostasis and fatty acid metabolism. The link between adiponectin levels and cancer has not been proven, but a recent meta-analysis of 17 studies concluded that high levels of the hormone may decrease the risk of breast cancer in postmenopausal women.

When the women in this study were switched to a diet that included daily soy without tomatoes for ten weeks, adiponectin levels decreased, reported Rutgers University assistant professor of epidemiology Adana Llanos, PhD, and colleagues.

Reductions in serum leptin were seen following the soy intervention in lean, but not obese women, the researchers wrote in The Journal of Clinical Endocrinology & Metabolism, published online Dec. 18.

The finding that a subset of at-risk women showed increases in adiponectin during the tomato phase of the intervention study suggests that lycopene or some other phytonutrient in tomatoes may improve insulin sensitivity and reduce breast cancer risk, Llanos told MedPage Today. “In postmenopausal women who are at increased risk for breast cancer, eating a diet high in fruits and vegetables, particularly tomatoes, high in vitamins, minerals and phytochemicals like lycopene could be an important practical strategy for promoting cancer prevention,” she said.

The aim of the study was to assess the effects of lycopene- and isoflavone-rich diets on circulating biomarkers for obesity in postmenopausal women at high risk for developing breast cancer.

The 26-week, two-arm, longitudinal, crossover trial included 70 women with a mean age of 57.2 years and a mean BMI of 30.0 kg/m².

The intervention included 10 weeks eating a tomato-rich diet, that included ≥25 mg of lycopene daily and a 10-week soy-based diet, with no tomato, that included ≥40 grams of soy protein daily. There was also a 2-week washout period in between.

The researchers measured changes in serum adiponectin, leptin, and the adiponectin-to-leptin ratio (A/L) for each intervention through linear mixed models, with ratio estimates corresponding to post-intervention adipokine concentration relative to pre-intervention concentrations.

The analysis revealed that:

• Following the tomato intervention, among all women, adiponectin concentrations increased (1.09; 95% CI 1.00-1.18), with the strongest effect seen among nonobese women (1.13, 95% CI 1.02-1.25).
• Following the soy intervention, adiponectin decreased overall (0.91, 95% CI 0.84-0.97), with a larger reduction observed among nonobese women (0.89, 95% CI 0.81-0.98).
• Overall, no significant changes in leptin or the A/L ratio were seen following either intervention, although nonobese women did have an 18% reduction in serum leptin (0.82, 95% CI 0.68-0.99). 

“While (other) studies have examined the effects of carotenoids in general, ours is the first to examine the effects of a lycopene-rich diet among postmenopausal women,” the researchers wrote.” In our study, women consumed roughly 30 mg/day of lycopene during the 10-week tomato arm, which is 6- to 15-times the average intake in the U.S.”

They added that this translated into a 9% increase in serum adiponectin.

The women did not lose weight during the study, and obese women did not experience a significant lowering of adiponectin.

“Lack of change in adiponectin or A/L following the tomato arm among obese women may indicate that a longer duration of tomato consumption is necessary to modify these biomarkers,” the researchers wrote. “Further studies involving longer-term lycopene interventions among postmenopausal women are necessary to clarify its potential as a breast cancer chemopreventive agent.”

The study also had its limitations, including a relatively short intervention period, a restricted study population, and nonrandom allocation of the dietary intervention.