In a case-control study of 1,866 women in Mexico, those who derived 57 or more per cent of their total energy intake from carbohydrates incurred a risk of breast cancer 2.2 times higher than women with more balanced diets. Dietary patterns in Mexico are characterised by higher consumption of carbohydrates and lower intake of fat and animal protein than those in more affluent western countries.
The team of researchers from the Instituto de Salud Pública in Cuernavaca, Mexico, and the Harvard School of Public Health in Boston, suggest that the association between carbohydrates and breast cancer may be related to elevated levels of insulin and insulin-like growth factor binding proteins in the blood.
"Scientists have long suspected that diet was among the factors contributing to breast cancer," said study co-author Walter Willett at the Harvard School of Public Health. "Now, with studies like ours, we are beginning gradually to understand what elements of diet specifically are associated with the disease, and to grasp the chemical and biological processes that contribute to it at the cellular level."
According to the researchers, of all the carbohydrate compounds, sucrose and fructose demonstrated the strongest association with breast cancer risk in the study. Sucrose is derived from sugar cane, sorghum and sugar beet and is most commonly found in table sugar and sweetened prepared foods and beverages. Fructose is a component of sucrose and is also found in fruit.
"Eating sweets and starches causes a rapid rise in the body's blood sugar levels, which in turn cues the production of insulin and triggers a biological process that ultimately can influence carcinogenesis by causing cells to proliferate," report the scientists.
Insulin and an insulin-like growth factor also may contribute to higher circulating levels of biologically active estrogens, a risk factor for breast cancer in pre-menopausal women, they continue, adding that ninety per cent of breast tumours are insulin-receptor positive and over-express the insulin-like growth factor.
But scientists were quick to respond to, and to question, the study published in the 1 August issue of Cancer Epidemiology, Biomarkers & Prevention (CEBP). Ritva Butrum at the American Institute for Cancer Research (AICR ) claimed, "you cannot, on the basis of this or any single study, draw conclusions about carbohydrates and their effect on cancer risk."
To warrant dietary change, scientific findings must first be reviewed, considered against the bulk of previous evidence and replicated by different researchers using studies of different types, the scientist added.
Butrum also warned that "you cannot make generalisations about "carbohydrates," because the category contains an enormous variety of foods that have vastly nutritional profiles and vastly different effects on the body."
Carbohydrates are the dietary mainstay of most cultures around the world. The category encompasses unprocessed, fibre-rich foods such as vegetables and fruits as well as highly processed crisps, biscuits and cupcakes. 'Thus, it is unlikely that any generalisations about "carbohydrates" will stand up to scrutiny.'
Still with reference to the study, Butrum points out that what has been largely overlooked by media reports on this latest carb study is that the researchers also analysed how consumption of insoluble fibre - found in vegetables, fruits, whole grains and beans - affected cancer incidence. Women in the study who ate the most insoluble fibre had lower breast cancer risk.
"The distinction between refined and unrefined carb's is shaping up to be an important one, and one that will figure largely in future studies of diet's link to cancer and other diseases. We at AICR believe that every effort must be made to keep that distinction clear in the public's mind," concluded the American Institute for Cancer Research, one of the largest cancer charities in the US.
Popular in the UK and the US, recent market research from the Opinion Dynamics finds that 11 per cent of Americans - about 30 million - are still following the low carbohydrate dietary regime, a figure that seems to have stabilised and implying that the growing trend may well be tailing off.
According to the research firm, although the number of low-carb dieters has stayed the same, the number of people claiming a low-carb "lifestyle" - those who are not on a formal low-carb diet but who attempt to watch their carbohydrate intake - has dropped significantly from 32 per cent in April to 21 per cent in July.
"The key take-away is that low-carb dieters are a stable group, and that opportunities exist for manufacturers, retailers and restaurants to serve them better," said Larry Shiman, a director at Opinion Dynamics."But, opportunities are not unlimited and many of the smaller players will not survive. It's important for all businesses serving this segment to carefully consider costs versus opportunity before investing in the low-carb market."