Q. Can people who are already overweight reduce their cancer risk if they lose weight?

A. Most adult cancers develop over a 10 to 20 year period or longer. Although researchers can't say for sure that losing weight will lower cancer risk, the evidence looks promising.

For example, two large studies of post-menopausal women found that those who lost weight after menopause reduced their risk of breast cancer substantially. That's important since a recent report estimated that overweight and obesity in the U.S. account for 14 percent of cancer deaths among men and 20 percent among women.

When overweight people lose weight, they may not be able to undo cancer-causing damage that's already done, but shedding excess fat can reduce elevated levels of insulin, insulin-related growth factors and certain hormones like estrogen. As these compounds are all associated with the process of cancer development, we expect that their cancer-promoting effects also decrease with weight loss. Each loss or gain of excess body fat seems to change cancer risk.

Even if you can't reach what you consider your ideal weight, researchers say that any excess body fat that you can lose and keep off should lower cancer risk.

Q. Are mushrooms a good source of vitamin D?

A. Most mushrooms supply only a trace amount of vitamin D. However, emerging research suggests that exposing mushrooms to ultraviolet light for a few hours before harvest (or 5-15 minutes after harvest) can trigger production of vitamin D within the mushroom.

"Enriched mushrooms" treated in this manner contain 23 to 85 micrograms (mcg) of vitamin D per 3-ounce serving, about one raw portabella mushroom piece or four large white mushrooms.

When converted to International Units (IU) - the measure most commonly used to record vitamin D levels in the U.S. - the figures are equivalent to roughly 900 to 3000 IU. It's an amount that satisfies most people's vitamin D requirements for a few days (based on current U.S. recommendations) and is even enough to meet the possibly increased standards being debated by health professionals.

• Karen Collins, registered dietitian, writes this column for the American Institute for Cancer Research. Write to Ask the Nutritionist, c/o the Daily Herald, 1759 R St., N. W., Washington, D.C. 20009. Questions will not be responded to personally.