Low-Carbohydrate vs. Low-Fat Diets and Mortality

Study Questions:

Do dietary patterns low in carbohydrates or low in fat reduce mortality among US adults?

Methods:

This was a prospective cohort study design using data from the US National Health and Nutrition Examination Survey from 1999 to 2014, which included adults ≥20 years of age who completed 24-hour dietary recalls. Diets were scored by unhealthy and healthy low-carbohydrate and low-fat diet patterns based on the percentage of energy as total and subtypes of carbohydrate, fat, and protein. The unhealthy low-carbohydrate score was calculated from the percentage of energy from high-quality carbohydrates, animal protein, and saturated fat. The healthy carbohydrate score was calculated from the percentage of energy from low-quality carbohydrate, plant protein, and unsaturated fat. A similar approach was used for the unhealthy and healthy low-fat dietary scores. The primary outcome of interest was all-cause mortality from baseline until December 31, 2015, linked to National Death Index mortality data.

Results:

A total of 37,233 adults (mean age 49.7 years, 52.6% female) were included in the present analysis. During the follow-up (297,768 person-years), 4,866 deaths occurred including 840 deaths from heart disease and 1,068 from cancer. Low-carbohydrate and low-fat diet scores were not associated with total mortality. The multivariable-adjusted hazard ratios for total mortality per 20-percentile increase in dietary scores were 1.07 (95% confidence interval [CI], 1.02-1.11; p = 0.01 for trend) for unhealthy low-carbohydrate diet score, and 0.91 (95% CI, 0.87-0.95; p < 0.001 for trend) for healthy low-carbohydrate diet score. For unhealthy low-fat diet score, the hazard ratio was 1.06 (95% CI, 1.01-1.12; p = 0.04 for trend), and 0.89 (95% CI, 0.85-0.93; p < 0.001 for trend) for healthy low-fat diet score. The associations remained similar in the stratification and sensitivity analyses.

Conclusions:

The investigators concluded that overall low-carbohydrate and low-fat diet scores were not associated with total mortality. Unhealthy low-carbohydrate and low-fat diet scores were associated with higher total mortality, whereas healthy low-carbohydrate and low-fat diet scores were associated with lower total mortality. These findings suggest that the associations of low-carbohydrate and low-fat diets with mortality may depend on the quality and food sources of macronutrients.

Perspective:

These data, although observational, suggest that dietary quality is more important than a dietary pattern low in carbohydrates or low in fat. Not accounting for quality of carbohydrates or fats in prior studies may account for the heterogeneity of findings. Perhaps clinicians may best serve their patients by advising them regarding the difference in healthy versus unhealthy carbohydrates and healthy versus unhealthy fats.

Keywords: Diet, Carbohydrate-Restricted, Diet, Fat-Restricted, Diet, Protein-Restricted, Dietary Carbohydrates, Fats, Unsaturated, Heart Diseases, Neoplasms, Nutrition Assessment, Plant Proteins, Primary Prevention


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