Creatine phosphokinase tests are used to measure trauma to the muscles, including the heart. Abnormal levels may signify heart disease, severe injury or stress, or may be used to verify that a heart attack is occurring. Your diet is unlikely to affect your CPK levels directly, because your body only produces as much CPK as it needs -- but your diet may play a role in the rate at which your body uses CPK, which would affect how much it produces. For the most accurate test results, follow your doctor's pre-test instructions.
Creatine, the base amino acid in the formation of creatine phosphokinase, is found in protein foods like meat, fish and dairy. Even if you eat a high-protein diet, your diet is unlikely to affect your CPK levels because you don't get CPK from your food -- you only get parts of it. During digestion, your body breaks the protein down into its individual amino acids, then reassembles it later into whatever enzyme is needed. No matter how much creatine you eat, your body won't build more CPK unless it's needed. A high CPK level is not a thing to be fixed -- it's more an indicator that something else needs to be fixed, and that's what the doctor is looking for.
If you've been eating less in an effort to lose weight, your CPK levels may drop. A 2008 study in the "Journal of Animal Physiology and Animal Nutrition" found that Shetland ponies put on a calorie-restricted diet for weight loss had lower CPK levels after 17 weeks. This isn't a bad thing -- it means that your weight isn't stressing your heart and muscles like it used to and is seen as a sign of progress. If you've lost weight, tell your doctor about it before your test. The opposite is also true -- if you overeat to the point where you're gaining weight, your CPK levels may rise as your body becomes more stressed.
Dietary supplements may slightly affect CPK levels if used long term. Athletes that use creatine-enriched protein shakes may have elevated CPK levels, especially if the shakes are used for recovery -- recovery shakes contain carbohydrates, and MedlinePlus says that creatine taken with carbs can result in a 60 percent increase in muscle creatine. Athletes also stress their muscles, so when extra creatine is available in the body, the CPK levels may rise. The CPK increase wouldn't occur unless the muscles were stressed; if you use a protein shake for extra calories as part of a non-athlete lifestyle, your CPK levels aren't likely to be affected. If you are a serious athlete who uses creatine, however, alert your doctor to the fact before the test.
Just because your diet probably won't affect your CPK test doesn't mean you're in the clear -- there are additional factors that may result in inaccurate readings. Alcohol may affect your CPK levels, especially if you've been using it heavily long term. Certain medications like statins, anesthetics and dexamethasone can artificially raise your reading, as can illicit drugs like cocaine. Concurrent medical conditions like abnormal thyroid function and rhabdomyolysis can also skew your results, so be sure your doctor has your complete medical history so she can accurately analyze your test results.