A diet full of hamburgers, steaks and processed meats isn’t good for you. Worse: It may cause cancer. But a new study suggests that eating certain starchy foods can help offset that threat.
Red meat can damage DNA in the cells lining the digestive tract, explains Karen Humphreys. She is a cancer scientist at Flinders University in Adelaide, Australia. She led the new study, which was published August 4 in Cancer Prevention Research.
That DNA damage can happen in a number of ways. Certain compounds found in red meat can increase the rates at which mutations — genetic changes — occur. Mutations are a common trigger for pre-cancerous changes in the body. Also, iron in red meat can produce free radicals. These electrically charged molecules can damage DNA when they strip electrons from it.
Even the way meats are prepared can damage DNA. High heat (from grilling or frying) produces cancer-causing chemicals called heterocyclic amines (HET-er-oh-SY-klic Aa-MEENS). And processed meats (such as sausages, bacon and luncheon meats) are often cured with chemicals that have been linked with an increased risk of cancer. That’s why dieticians recommend limiting consumption of red meats, such as beef.
Previous studies in animals suggested that resistant starch might help combat the DNA damage caused by red meat. Resistant starch is found in foods such as potatoes, beans and whole grains. Like fiber, this type of starch cannot be digested. Instead, it moves into the large intestine, where microbes ferment it. Fermentation releases what chemists refer to as short-chain fatty acids. At least in animal studies, these chemicals help prevent cancer in cells of the gut. Whether starch could have such an effect in people remained unknown, however. So Humphreys and her team decided to find out.
They recruited a group of healthy volunteers between the ages of 50 and 75. Half of the volunteers ate a diet high in red meat for four weeks. That meant 300 grams (10.6 ounces) of beef or lamb each day. (For perspective, the average American eats less than half that amount.) The other people in the trial ate the same amount of meat plus 40 grams of resistant starch. Before the study, and again at the end of the 4-week period, the scientists collected feces from each participant. They also took tissue samples from the lining of each person’s rectum.
Afterward, the two groups of people switched to the opposite diet for an additional four weeks. The researchers then collected samples again.
When volunteers ate the low-starch diet, their rectal cells boosted their production of microRNAs. These molecules help regulate the production of proteins in the body. Some particular microRNA molecules are abundant in people with colon cancer. That disease affects the large intestine. And those particular microRNAs increased over the course of the 4-week, low-starch diet. So did the growth rate of cells lining the rectum. But when the volunteers ate the diet that boosted their intake of resistant starch, levels of both cancer-promoting microRNAs and rectal-cell growth fell.
“This is an interesting finding,” says Tim Key. He’s an epidemiologist at the University of Oxford in England. (Like a detective, an epidemiologist figures out what causes illness.) Key says this study is a first step in helping learn how diet can impact colon cancer in people.
Incorporating foods that contain resistant starch is important in promoting good health, says Humphreys. Slightly green bananas, cooked and cooled potatoes, whole grains and beans are good choices. (The potatoes must be cooled after cooking to alter the starch so that it is harder to digest.) These foods can boost levels of those helpful fatty acids and prevent an excess of those worrisome microRNAs.
And these findings could be important for people of all ages, says Humphreys. Colorectal cancer takes many years to develop. “It is the result of a gradual accumulation of mutations within the cells,” she explains. That means foods eaten today can set the stage for health or disease many years from now.
You don’t have to skip red meat completely. Instead, opt for potato salad instead of fries. Load up on baked beans and have your burger on a whole grain bun. That balance of foods might help protect you from one type of cancer.
And to further limit the cancer risk posed by those heterocyclic amines, or HCAs — which form on many grilled meats — consider adding that potato right into the ground meat. A Swedish study found that blending the starch right into the meat prior to cooking reduced the chance that those HCAs would form. And the good news: The bonus starch didn’t alter the meat’s taste.
amino acids Simple molecules that occur naturally in plant and animal tissues and that are the basic constituents of proteins.
cancer Any of more than 100 different diseases, each characterized by the rapid, uncontrolled growth of abnormal cells. The development and growth of cancers, also known as malignancies, can lead to tumors, pain and death.
chemical A substance formed from two or more atoms that unite (become bonded together) in a fixed proportion and structure. For example, water is a chemical made of two hydrogen atoms bonded to one oxygen atom. Its chemical symbol is H2O.
colon (in biology) The majority of the large intestine, it runs between the cecum (a pouch below the small intestine) and the rectum. Foods are not digested in the colon, although this tissue lubricates wastes that will be excreted. Some liquids and salts, however, will be removed from materials stored in the colon before excretion.
digest (noun: digestion) To break down food into simple compounds that the body can absorb and use for growth. Some sewage-treatment plants harness microbes to digest — or degrade — wastes so that the breakdown products can be recycled for use elsewhere in the environment.
digestive tract The tissues and organs through which foods enter and move through the body. These organs include the esophagus, stomach, intestines, rectum and anus. Foods are digested — broken down — and absorbed along the way, Any materials not used will exit as wastes (feces and urine).
DNA (short for deoxyribonucleic acid) A long, spiral-shaped molecule inside most living cells that carries genetic instructions. In all living things, from plants and animals to microbes, these instructions tell cells which molecules to make.
electron A negatively charged particle; the carrier of electricity within solids.
epidemiologist Like health detectives, these researchers figure out what causes a particular illness and how to limit its spread.
fatty acids Large molecules made of up chains of carbon and hydrogen atoms linked together. Fatty acids are chemical building blocks of fats in foods and the body.
fermentation The metabolic process of converting carbohydrates (sugars and starches) into short-chain fatty acids, gases or alcohol. Yeast and bacteria are central to the process of fermentation. Fermentation is a process used to liberate nutrients from food in the human gut. It also is an underlying process used to make alcoholic beverages, from wine and beer to stronger spirits.
free radical A charged molecule (typically highly reactive and short-lived) having one or more unpaired outer electrons. It will attempt to steal electrons to make itself whole again through a process known as oxidation.
heterocyclic amines A class of at least 17 chemicals that can form when meat is cooked at high temperatures. In 2005, a U.S. government review of published data concluded that at least some of these cause colon cancer in animals, and likely do the same in humans. These compounds form at high temperatures in chemical reactions between several food constituents, such as the simple sugar glucose, the amino acid creatinine and additional free amino acids. Meat contains all of those ingredients.
microbe Short for microorganism. (see microorganism)
microorganism A living thing that is too small to see with the unaided eye, including bacteria, some fungi and many other organisms such as amoebas. Most consist of a single cell.
microRNA Short pieces of RNA that do not code for proteins. Instead, they influence whether and how proteins are produced by interfering with normal production processes.
molecule An electrically neutral group of atoms that represents the smallest possible amount of a chemical compound. Molecules can be made of single types of atoms or of different types. For example, the oxygen in the air is made of two oxygen atoms (O2), but water is made of two hydrogen atoms and one oxygen atom (H2O).
mutation Some change that occurs to a gene in an organism’s DNA. Some mutations occur naturally. Others can be triggered by outside factors, such as pollution, radiation, medicines or something in the diet. A gene with this change is referred to as a mutant.
red meat A term used to describe beef, lamb or other meats that appear red when uncooked — and not light colored (as chicken breast is) when they are cooked.
proteins Compounds made from one or more long chains of amino acids. Proteins are an essential part of all living organisms. They form the basis of living cells, muscle and tissues; they also do the work inside of cells. The hemoglobin in blood and the antibodies that attempt to fight infections are among the better known, stand-alone proteins. Medicines frequently work by latching onto proteins.
resistant starch A form of starch that is not broken down in the gut. Instead, it is fermented by microbes, releasing fatty acids. Some of those fatty acids may help to protect against cancer.
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Original Journal Source:K. Humphreys et al. Dietary manipulation of oncogenic microRNA expression in human rectal mucosa: A randomized trial. Cancer Prevention Research. Published online August 2014. doi: 10.1158/1940-6207.CAPR-14-0053.
Original Journal Source:C.R. Daniel et al. Trends in meat consumption in the United States. Public Health Nutrition, Vol. 14, April 2011, p. 575. doi: 10.1017/S1368980010002077.