The Cancer Research
Messages circulating on the Internet about asparagus preventing and curing cancer is an unfortunate result of misinterpreting and over-extending laboratory study results. This non-starchy vegetable is, however, an excellent food to enjoy as part of a plant-forward diet to help lower cancer risk.
Interpreting the data
After a systematic review of the global scientific literature, AICR/WCRF analyzed how vegetables and their nutrients affect the risk of developing cancer.
“Convincing” or “probable” evidence means there is strong research showing a causal relationship to cancer—either decreasing or increasing the risk. The research must include quality human studies that meet specific criteria and biological explanations for the findings.
A convincing or probable judgement is strong enough to justify recommendations.
- There is probable evidence that non-starchy vegetables and fruit combined DECREASE the risk of:
- Aerodigestive cancers overall (such as mouth, pharynx and larynx; esophageal; lung; stomach and colorectal cancers)
“Limited suggestive” evidence means results are generally consistent in overall conclusions, but it’s rarely strong enough to justify recommendations to reduce risk of cancer.
- Limited evidence suggests that non-starchy vegetables may DECREASE the risk of:
- Estrogen receptor-negative (ER-) breast cancer
- Limited evidence suggests that non-starchy vegetables and fruits combined may DECREASE the risk of:
- Bladder cancer
Ongoing Areas of Investigation
- Laboratory Research
Flavonols (such as the quercetin in asparagus) influence gene expression and cell signaling in ways that increase antioxidant, anti-inflammatory and carcinogen-deactivating enzymes in studies of isolated cells and limited animal studies. These compounds inhibit cancer cells’ growth and ability to spread, and activate signaling that leads to self-destruction of abnormal cells. Flavonols dial down expression of oncogenes (genes that have potential to cause increased cell growth that can lead to cancer) and increase expression of tumor suppressor genes.
Folate helps maintain healthy DNA and keep cancer-promoting genes “turned off.” Animal studies, however, suggest that exceptionally high amounts or intervention after cancer cells have formed might promote development of cancer.
Inulin is a type of dietary fiber called a fructan that functions as a prebiotic, which means that it nourishes health-promoting bacteria in the body. Bacteria in the gut ferment inulin, forming short-chain fatty acids. In rodent studies, these short-chain fatty acids (such as butyrate) help strengthen the barrier protecting cells lining the colon and reduce markers of low-grade inflammation in the body. In cell and mice studies, butyrate also affects gene expression and cell signaling in ways that could reduce development of colorectal cancer.
Saponins seem to inhibit oxidative stress, inflammation and growth of cancer cells, and promote their self-destruction (apoptosis) in cell studies. However, these compounds seem to be poorly absorbed out of the digestive tract, and more research is needed about their effects when consumed.
- Human Studies
Human studies related to asparagus and cancer risk compare groups of people who consume relatively high and low amounts of total vegetables, green vegetables and/or levels of dietary folate.
People who eat more vegetables and fruits have lower risk of a wide range of cancers, and eating more green-yellow vegetables in particular has been linked to lower overall risk of cancer. Results like these probably reflect combined protection from many different nutrients and compounds these foods contain.
Folate: Research on folate and cancer is challenging to interpret, since effects may differ based on time in the cancer process, amount consumed and individual genetic differences. Consuming too few folate-rich foods and having low blood levels of folate are linked with greater risk of several forms of cancer in some population studies. However, in randomized controlled trials, consuming excess folic acid (the form of folate in supplements and fortified foods) from supplements has increased risk of some cancers. More research is needed to understand the amounts that are high enough to pose risk.
Inulin: Prebiotics such as inulin can promote growth of gut microbes linked with lower markers of inflammation and better health in humans. This could help protect against development of colorectal and other cancers. But more human studies are needed to understand whether specific types and sources of prebiotics actually reduce occurrence of cancers or cell changes.
Glutathione: Human research identifies additional cautions about trying to translate laboratory studies of glutathione to advice about glutathione-containing foods, such as asparagus. First, glutathione is a short chain of amino acids (the building blocks of protein). Enzymes in the mouth and digestive tract break it down, so it’s not clear whether glutathione in food even reaches body cells. Limited randomized controlled trials of glutathione from isolated supplements have shown inconsistent results about effects on body levels of glutathione, immune function and markers of oxidative stress, and have used daily doses many times greater than amounts found in a serving of asparagus.
- Questions We Hear
Q: Is it true that a substance called glutathione in asparagus can prevent cancer?
A: Glutathione is an antioxidant found in asparagus that you may see identified as a reason to focus on asparagus for protection against cancer. Studies of isolated cells show that glutathione helps decrease formation of free radicals, regenerate other antioxidants, repair DNA and detoxify carcinogens. Actually, our own body cells produce glutathione and raising levels as high as possible does not seem beneficial. That’s because large amounts of glutathione in cancer cells can protect those cells from destruction and promote resistance to chemo and radiation cancer treatment.
Q: Can eating blenderized asparagus every day cure cancer?
A: Statements you see proclaiming asparagus as a protector against cancer often reflect anti-tumor effects of asparagus extracts in cell culture or rodent studies. These studies don’t indicate effects in humans. Some of these studies even use extracts prepared from a varietal of asparagus used in Ayurvedic medicine, which is not the asparagus we get from gardens and grocery stores.
On the other hand, laboratory studies may raise panic about potential for asparagus as a risk. But these do not represent effects of people eating asparagus either. For example, one study found that high levels of asparagine, an amino acid found in asparagus (but also in other foods), caused breast cancer to spread (metastasize) in mice. And decreased levels limited spread of the cancer. However, as AICR’s vice president of research explains, this study involved cancer cells grown in a laboratory and implanted into mice with no immune system. So for many reasons, results don’t apply to people.
In fact, even if people reduce asparagine consumption, our bodies produce it.
- Tips for Selection, Storage and Preparation
- Choose stalks that are firm yet tender, with closed deep green or purple tips. Avoid limp or wilted stalks.
- Narrow stalks tend to be more tender than thick ones.
- Green asparagus is by far the most common, but you may also see asparagus that is white or purple. All are healthy choices. Compared to green asparagus, white varieties do not seem to be as high in phenolic compounds being studied for health-protective potential, but some analysis suggests vitamin C content may be higher. Purple asparagus owes its color, which is only on the outside of its stalks, to anthocyanin phytochemicals. It tends to be smaller, sweeter and more tender than green asparagus.
- Asparagus can dry out quickly, so refrigerate in a plastic bag for no more than four days.
- Wrap bottom ends of the asparagus stalks in a damp paper towel to provide extra help against drying out.
- Wash asparagus in cool running water.
- To remove tough ends, common advice is to snap the stalks at their natural breaking point. To minimize the amount you waste, however, simply chop off a small amount at the bottom where the stalks seem to start getting tougher.
- Asparagus stalks do not require peeling, especially when they are thin. If some have a tough outer layer, you can remove it with a vegetable peeler.
- Many of asparagus’ nutrients are water-soluble, so don’t cook it in a big pot of boiling water. For best nutrition and taste, cook asparagus by steaming lightly in a steamer or microwave, grilling or stir-frying.
- Add raw or blanched and cooled asparagus to your favorite salad.
- Asparagus is also a wonderful addition to pasta with mixed vegetables, a chicken or tofu stir-fry or an omelet.
- Grill or broil after brushing lightly with olive oil for a delicious side dish.
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