It may never be too late to improve your health by changing your diet, a new study suggests.

Health professionals who began eating more healthy foods such as whole grains, fruits, vegetables, and fish were able to significantly lower their risk for death in as few as 8 years, according to Mercedes Sotos-Prieto, PhD, a research fellow in the Department of Nutrition at Harvard University in Cambridge, Massachusetts, and colleagues.

"Our study indicates that even modest improvements in diet quality could meaningfully influence mortality risk, and conversely, worsening diet quality may increase the risk," Dr Sotos-Prieto told Medscape Medical News in an email. "In addition, these findings underscore the importance of maintaining a high diet quality over a long term in reducing mortality risk."

She and her colleagues reported their findings in an article published online today in the New England Journal of Medicine.

Many studies have shown an association between diet and mortality, and the 2015 Dietary Guidelines for American recommends the Alternate Healthy Eating Index, the Alternate Mediterranean, and the Dietary Approaches to Stop Hypertension (DASH).

Although the three diets differ in detail, all three emphasize food groups that contribute to healthy diets, such as whole grains, vegetables, fruits, and fish or omega-3 fatty acids, Dr Sotos-Prieto and colleagues write.

To find out what happens when people switch to these diets after eating less healthy diets, the researchers examined data from the Nurses' Health Study, which enrolled 121,700 registered nurses who were aged 30 to 55 years of age in 1977, and the Health Professionals Follow-up Study, which enrolled 51,529 US health professionals who were 40 to 75 years of age in 1986.

Participants filled out questionnaires at baseline and every 2 years after that, with follow-up rates better than 90% in both studies.

Dr Sotos-Prieto and colleagues looked at changes in diet from 1986 to 1998, and mortality and morbidity data through 2010. They excluded participants who died before 1998, had a history of cardiovascular disease or cancer that year, had missing information regarding diet and lifestyle, or had very low or high caloric intake. The average participants had a poor diet in 1986, Dr Sotos-Prieto said.

The researchers calculated three "diet-quality" scores based on criteria used previously. The scores differed, depending on the diet.

For the Alternate Healthy Eating Index, 11 food components each rank from 0 to 10, with 10 the healthiest, for a total possible score of 110.

The Alternate Mediterranean Diet included nine components, each scored as unhealthy (0) or healthy (1), depending on whether the participant's consumption was above or below the cohort-specific median level. The total possible score was 9.

The DASH score included eight components from 1 to 5, with 5 the healthiest, according to the participant's quintile of intake. Total DASH scores ranged from 8 to 40 points.

The researchers were able to determine 98% of the deaths in each study, using the National Death Index, information from the participants' families, and the US Postal Service. They determined causes of death from death certificates and, when appropriate, from medical records.

Participants whose diets improved the most were younger, had worse diets at baseline, exercised more, and drank alcohol less than those whose diets changed the least. Those with consistently healthy diets were older, had lower body mass indices, were less likely to be current smokers, and were more physically active than those with consistently poor diets.

Overall, an increase of 20 percentile points in diet quality during the first 8, 12, or 16 years was associated with a reduction of 8% to 17% in the risk for death from any cause in a multivariable analysis. The effect of the changes varied among the three diets in the study.

Changes over longer durations had bigger effects. For example, for the Alternate Healthy Eating Index, a 20 percentile increase for 8 years was associated with an 11% (95% confidence interval [CI], 6% - 15%) reduction in mortality. A 20 percentile increase over the course of 16 years resulted in a 26% (95% CI, 21% - 31%) reduction in mortality.

A 20 percentile improvement in diet reduced the risk for death from cardiovascular disease in the Alternate Healthy Eating Index and the Alternate Mediterranean Diet, but not the DASH diet, perhaps because the DASH diet does not include alcohol and does not emphasize fish or omega-3 fatty acids as much as the other two diets, the researchers write.

In contrast, only an improvement in the DASH diet was associated with a reduced risk for death from cancer, and this was mainly attributable to a reduced risk for death from lung cancer.

As an example of how a participant might improve a score by 20 percentile points, the researchers described an increase in consumption of nuts and legumes from no servings to 1 serving per day, and a reduction in consumption of red and processed meats from 1.5 servings per day to "little consumption."

Participants who improved their diets lowered their risk for death regardless of whether they got more mammograms or physical checkups, and regardless of other major confounding factors such as smoking and alcohol consumption.

A worsening diet over the course of 12 years was associated with an increased mortality of 6% to 12%, the researchers found.

Those who stayed consistently on a healthy diet starting at baseline had a 9% to 14% lower risk for death than those who stayed consistently on a poor diet.

The data showed a lower mortality risk for participants who changed from a bad diet to a good one than for those who maintained a healthy diet throughout the study, but Dr Mercedes-Prieto said this difference was not statistically significant. "Based on our results, both modest improvement and maintaining a high quality diet over a long term are beneficial for decreasing mortality risk," she said.

This study was supported by the National Institutes of Health. Researchers reported financial relationships with Metagenics, the California Walnut Commission, and Fundacion Alfonso Martin Escudero.

N Engl J Med. 2017;377:143-153. Abstract

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