Tempted by a cream bun, you talk yourself out of it with thoughts of all that unhealthy fat clogging up your arteries. You opt for a low-fat yoghurt instead, with skimmed milk in your tea, congratulating yourself on your sensible self-control.
Think again. According to a ground-breaking new book about breast cancer (which kills over 600 women in Ireland annually), dairy products, whether low-fat or full cream, should be off everyone's menu overnight. (They are also culpable with regard to prostate cancer, so that really means everyone).
Prof Jane Plant CBE, author of Your Life in Your Hands, was diagnosed with breast cancer 13 years ago. She was 42, a successful geochemist (she is now chief scientist of the British Geological Survey), and led, she thought, a healthy life. There was no history of breast cancer in her family. She discovered that "only five to 10 per cent of breast cancers are the result of inherited genes, and the disease may not always develop, even in those carrying the mutated gene." Bamboozled by jargon and frozen with panic, she fell back on her scientific training to try and figure out how she had developed the disease, and how best to cure herself.
She went on the Bristol diet, she had a mastectomy, she had radiotherapy, she had her ovaries irradiated (to induce menopause and eliminate oestrogen), she asked questions and did lots of research. To no avail.
By the time of the cancer's fifth recurrence (it spread into the lymph), she was given a course of chemotherapy and three months to live. She had an egg-sized tumour on the side of her neck. Brainstorming one night with her fellow scientist husband about why, in the West, one in 10 women get breast cancer (one in 14 in Ireland), while in China it's only one woman in 10,000, the pair came up with the simple answer: Chinese people don't eat dairy products. Plant eliminated all dairy products (including goat and sheep) from her diet. Six weeks later, the tumour had disappeared.
When I meet her she is a youthful-looking woman in her mid-fifties, quaffing mint tea and eating a tuna sandwich (no butter or mayonnaise). She has stayed on her dairy-free diet and has remained clear of cancer.
Giving up dairy products was only part of a healthy regimen she had been following throughout her cancer, including taking folic acid and zinc supplements, drinking filtered water and never consuming anything that had been packaged in plastic (phthalates, harmful carcinogenic chemicals, leak from soft plastic into food).
In spite of her best efforts it was only after she gave up all dairy products that the cancer disappeared. Sixty-three other women who had breast cancer and who came to her for advice, also recovered after giving up dairy products.
So how, I ask, can dairy products - beloved of both the Irish and British alike, not to mention the Americans whose diet is 40 per cent dairy - have such a lethal effect? "Milk is designed as the perfect food for newborn animals. They can't eat ordinary food, they are dependent on milk to keep development and cell differentiation going. But milk contains a chemical - insulin-like growth factor, or IGF-1 - which girls have naturally as teenagers to help their breasts develop. This chemical - which is designed to stimulate cell growth - can send the wrong signal to adult breast tissue."
She quotes studies in the US and Canada in 1998 which found that premenopausal women with the highest IGF-1 concentration in their blood had a far higher risk of developing breast cancer (similar studies have found a link between IGF-1 and prostate cancer). The drug Tamoxifen, prescribed for women with breast cancer, is thought to work by reducing circulating IGF-1 levels.
"Over 70 per cent of the world's population are unable to digest the milk sugar, lactose," she observes. "Lactose intolerance may be nature's early warning system: perhaps nature is trying to tell us that we're eating the wrong food." Homogenisation apparently only enables cancer-producing chemicals to reach the bloodstream quicker.
Plant has done her homework: "Epidemiological studies have indicated a positive correlation between dairy product consumption and breast cancer risk going back two decades. Studies have found an increase in breast cancer risk among women who consumed milk (especially whole milk) and/or cheese." In 1977 scientists examining the incidence of breast cancer in Japan found "a significant increase in both the consumption of dairy products and the occurrence of breast cancer in urban areas".
She quotes more research to suggest that "free oestrogens" - found in commercial pasteurised whole cow's milk and in skimmed milk - may stimulate expression of IGF-1 resulting in "indirect long-term tumour growth". She lists dioxins and other damaging environmental chemicals, some of them carcinogenic, which are often fat soluble and end up "particularly concentrated" in milk.
As for the argument that we need dairy products because they contain calcium, Plant quotes the World Health Organisation's finding that countries which have low intakes of calcium do not have an increased incidence of osteoporosis: "Scientific studies into calcium absorption have shown that only 18 to 36 per cent of the calcium in milk is taken up by the body."
Now that we're convinced, what should we be eating instead? Plant recommends soya milk, herbal tea, houmous, tofu, nuts and seeds, non-farmed fish, organic eggs and lean meat (not minced beef, which tends to be dairy cow) and plenty of fresh organic fruit and vegetables (in salads, juiced, or lightly steamed). But how can the average woman afford the time and energy it takes to source and prepare such food?
"Your priority should be good food, not glop," she stresses. "Put organic food first. Your health is more important than a new car. Anyway, I don't find it too costly - after all, I don't buy any processed food, which is very expensive." Her husband and two children have no problem following her diet. And although she travels a lot for her job, she finds that she is able to manage - she includes many tips in her book about what to bring with you on a trip (dried soya milk, herbal tea bags, kelp tablets for iodine, etc). She is about to start writing a new book, a guide for busy women who want to stay healthy.
She advocates thorough and frequent self-examination of your breasts, and, if you do develop breast cancer, self-empowerment by working with your doctor "as a partner, not as a victim". She is not a fan of the Louise Hay You Can Heal Your Life philosophy: "I do believe in positive thinking, but I'm also a scientist and I wanted a rational explanation. I have friends with diseases like MS who have read Hay's books and feel guilty because they can't adapt their mental attitude; or, if they have adapted, and the disease doesn't go away, they become distressed."
Plant, who is an advocate of acupuncture, has varying opinions of alternative therapies. She is suspicious of aromatherapy, found visualisation didn't work, but took much comfort from cognitive therapy and hypnotherapy (both of which helped her to reduce the stress and anxiety caused by having cancer).
Overall, however, it was her professional research as a geochemist into the links between disease and trace elements (such as selenium) in the environment in China and Korea that led to her insight about the role of dairy produce in her cancer. She finds the medical profession particularly shortsighted about the influence of environmental factors - such as pollution and industrialisation - on disease: "I think public health has done a lot for the elimination of infectious diseases, but looking at the environment and nutrition could do the same for a lot of degenerative diseases."
Plant started writing Your Life in Your Hands for her daughter Emma (now 25). Emma's teen years were dominated by the fear that her mother was going to die: "The book's original title was What I Want My Daughter to Know," recalls Plant. "The 63 women with breast cancer who followed my diet and survived their cancer encouraged me to publish the book. I was reluctant at first - I knew I'd get flak for it, because science is an adversarial process. But morally, I felt if I had done the research and I had the information, I should share it with others. Men and women have the right to know what I know, and to draw their own conclusions."
Your Life in Your Hands by Jane Plant is published by Virgin at £16.99 in UK