I can drink dairy milk with impunity. I can eat all the saturated fat I want without gaining extra weight, although it would probably clog my arteries. And I have at least some of the muscle power of an elite athlete, even if I haven’t made good on that genetic promise.

Genetic tests promising to help you guide your diet and lifestyle are becoming widely available. Thousands of Americans bought them as holiday presents over the winter, and some are now being marketed as the perfect Father’s Day gift.

But what can they really deliver?

To report this story, I took four of the tests on the market, sold by 23andMe, Vitagene, Helix and Orig3n. I can only report on the results from 23andMe and Vitagene; Helix and Orig3n say they lost my results.

DNA tests can tell people if they have rare genetic conditions or if their children risk developing certain diseases, such as cystic fibrosis. But genetic experts say these tests really can’t tell people much of use when it comes to day-to-day living. They’re just not that precise yet.

That's because dozens or even hundreds of genes contribute to traits such as eye color, hair color and height. The risk of heart disease, cancer, and your propensity to gain weight, are even more complicated. There’s no such thing as a single “fat gene.”

Last February, a team at Stanford University found that genes cannot predict who might lose more weight on certain diets.

“It’s not definitive,” says Larry Brody of the National Human Genome Research Institute (NHGRI), one of the National Institutes of Health.

Brody said there are good genetic tests for telling if a person will be helped by a certain drug. One panel of tests can show whether a breast cancer patient needs chemotherapy, for example.

“Where we are still in a gray area is health and lifestyle,” Brody said.

No test can tell you if you eat fewer carbohydrates, you’ll lose weight, or if you do a certain type of exercise you’ll flatten your belly. Human genetics just are not that simple.

Nonetheless, some genetic testing companies tell consumers that they can tailor a diet, supplements and other advice to their genes. I took several of these tests to compare notes on what they found.

There were a few surprises. First, the good.

DNA testing company 23andMe provides a comprehensive, science-based test. To its credit, this pioneer of direct-to-consumer genetic testing doesn’t make promises that don’t hold water.

It does not claim to tell you what to eat or how much to exercise, and provides a huge amount of medical information as a bonus. I was happy to learn I don’t have common genes that predispose to breast cancer, Alzheimer’s or serious genetic diseases such as thalassemia.

One piece of diet guidance the company does offer is on eating saturated fat.

“Some people are likely to gain more weight than others when they eat a high-saturated-fat diet,” said Alisa Lehman, senior product scientist at 23andMe.

I am not one of them, the test tells me. But Lehman, and my detailed 23andMe report, both advise me to take it easy on saturated fat anyway. This fat, found in some meats, cheese and palm oil, can help clog arteries, raising blood pressure and the risk of stroke and heart attack.