Yet nutrition too often is an afterthought until someone's already in trouble.
A move is on to change that, from hospitals that hire fancy gourmet chefs to the American Cancer Society's dietitians-on-call phone service.
With cancer, you've got to "bring a lot more nutrients to each spoonful of food," Certified Master Chef Jack Shoop is learning. A former restaurateur, he's newly in charge of the kitchen at the Cancer Treatment Centers of America in Philadelphia.
Don't underestimate the added temptation should the result resemble Bon Appetit: "The visual heartiness, and the actual heartiness, of these foods has to be understood for them to embrace it," Shoop insists.
Tempting the palate is a huge hurdle: At diagnosis, up to a quarter of patients already have their appetite sapped, and most treatments can bring side effects that worsen the problem. Aside from the well-known nausea, vomiting and diarrhea, some cancers inhibit absorption of the nutrients patients force down. Not to mention strangely altered taste, mouth sores, dry mouth, difficulty swallowing and constipation.
About half of all cancer patients eventually suffer serious weight loss and malnutrition, a wasting syndrome called cachexia where they don't just lose excess fat but vital muscle. A healthy person's body adjusts when it doesn't get enough calories, slowing metabolism to conserve nutrients. A cancer patient's body doesn't make that adjustment; metabolism even may speed up.
The National Cancer Institute estimates cachexia is the immediate cause of death for at least 20 percent of cancer patients, although advanced cancer might have eventually claimed many of them.
How much weight loss is too much? The institute defines patients as at-risk when they've lost more than 10 percent of their usual weight. Other research suggests that patients who lose more than 5 percent of their pre-cancer weight have a worse prognosis than people who can hang onto the pounds.
For their best shot at doing that, the American Cancer Society urges patients to ask to be assessed by a registered dietitian up front, right at diagnosis. While that's common at designated cancer centers where dietitians work on-staff, it's not routine elsewhere and surveys suggest just a third of patients have access to cancer nutritionists where they're being treated.
"Patients who are well-nourished as they're going through treatment have shorter hospital stays, are better able to tolerate treatment," not to mention have better quality of life, says Colleen Doyle, nutrition chief at the society, which offers nutrition advice through its hot line at 1-800-ACS-2345.
Desperate patients often hunt their own nutrition advice on the Internet but can't tell the good from the bad. The No. 1 Web-perpetuated myth: that sugar feeds tumor cells. Not true, says Maureen Huhmann, who chairs the American Dietetic Association's oncology nutrition group. In fact, protein-packed milkshakes and smoothies can literally be lifesaving for some patients.
"I don't want people to start losing weight because they cut foods out of their diet when they don't really need to," says Huhmann.
Then there are people overweight when diagnosed who delight at shedding pounds - until they learn they're not just losing fat but muscle, too.
"You can be overweight and even obese and still be malnourished. It's a dilemma," says Carolyn Lammersfeld, the Cancer Treatment Centers' nutrition director who works with Shoop, the chef, to help patients find palatable options.
Typically, a cancer patient needs as much as twice the protein of a healthy person and about 10 percent more calories. Some tumors bring bigger nutritional threats than others: Gastrointestinal and lung cancers tend to cause more weight loss than breast cancer.
Anti-nausea medications developed in the past decade bring relief to many patients, although they're not always covered by insurance. Among options are a synthetic version of an ingredient from marijuana; cancer experts don't promote smoking marijuana although some advocates claim it helps. Doctors also can prescribe appetite stimulants and, for worst cases, feeding tubes.
But eating by mouth is best, and dietitians can offer tips to help: Snacking throughout the day instead of trying to force down large meals can help, and high-fat or high-fiber foods make nausea last longer.
In his Philadelphia hospital cafeteria, Shoop gives taste tests to introduce patients to healthful foods they may never have tried: Quinoa, a grain with the same amino acids of meat, or Arctic char, a salmon-like fish but less fatty.
Doing his own butchering allows Shoop to make stocks and sauces with the bones to add even more protein to meat dishes. Garnish with mushrooms, he advises, for a bit more.
And he teaches caregivers how to add 400 extra calories and 20 grams of protein to a simple smoothie, milkshake or oatmeal - using whole milk or yogurt, some protein powder, and grinding up fruits, nuts and flax seed.
"If you're not getting answers, keep searching," says Lammersfeld. "People need to know that weight loss and not being able to eat is not a good thing during cancer treatment."
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