Nancy A. Melville
January 21, 2020
Low-fat diets show no significant difference compared with low-carbohydrate diets in terms of overall risk of death, however, healthy versions of each regimen are linked to lower mortality compared with diets that are high in unhealthy fats and carbohydrates, new US research shows.
"Our study extended the previous evidence and suggests that the health benefits of a low-carbohydrate diet or low-fat diet may depend not only on the types of protein and fat or carbohydrate but also on the quality of carbohydrate or fat...in the diet," the authors report.
They note that although previous studies have generally shown no difference between low-fat and low-carb diets in terms of weight loss goals, research on broader health outcomes has been lacking.
Experts not involved with the study say it represents well-conducted research, but they note there are some limitations, including the fact that the definitions of "low-fat" and "low-carb" were quite loose.
"This is a very well-conducted observational study. The authors used data from a representative survey of the US population, and the results therefore apply to the general public -- at least in the United States. However, their definition of 'low carb' is quite unusual, in that it isn't what we would think of as low," said Gunter Kuhnle, PhD, professor of nutrition and food science at the University of Reading, UK, in a statement issued through the UK Science Media Centre.
Similarly, Ian Johnson, PhD, nutrition researcher and emeritus fellow, Quadram Institute Bioscience, Norwich, UK, said: "This is a large and well-conducted observational investigation but it is difficult to see a very clear or novel message for consumers in this paper, other than that moderate variations in the proportions of fat and carbohydrate in our diets, within the range selected freely by an American population, were not associated with big differences in long-term health or risk of premature death."
New NHANES Data on Quality of Foods
"Diet plays an important role in the public health, and suboptimal diet is estimated as the first leading cause of death and the third leading cause of disability-adjusted life-years lost in the United States," writes Zhilei Shan, MD, PhD, of the Harvard T.H. Chan School of Public Health, in Boston, Massachusetts, and colleagues in their study, published online January 21 in JAMA Internal Medicine.
For the prospective cohort study, they evaluated data from the US National Health and Nutrition Examination Survey (NHANES) from 1999 to 2014, which included dietary details, allowing for the designation not just of low-fat and low-carbohydrate patterns of eating overall, but of unhealthy and healthy low-carbohydrate and low-fat diet scores.
Diet scores on 37,233 adults, who were a mean age of 49.7 years and 52.6% were female, were compared with National Death Index mortality data on all-cause mortality from baseline through December 31, 2015.
Researchers divided participants into 11 sex-specific strata each of percentage of energy from fat, protein, and carbohydrate as determined by 24-hour dietary recalls. For fat and protein, individuals in the highest stratum received 10 points and those in the lowest stratum received 0 points. For carbohydrate, the order of the strata was reversed. Points for the three macronutrients were then summed to create the overall score, which ranged from 0 to 30.
Additional scores were then allocated based on consumption of high- or low-quality carbohydrates or fat.
Examples of unhealthy carbohydrates included refined grains and foods with added sugars, whereas healthier sources included whole grains, nonstarchy vegetables, and whole fruits. For low-carbohydrate diets, the replacement of saturated fat with unsaturated fat was among the food choices leading to a healthier score.
Over the course of the study, overall scores on low-carbohydrate and low-fat diets showed no association with total mortality rates, with 4866 total deaths occurring over 297,768 person-years.
However, after multivariate adjustment, an increased total mortality risk was evident for unhealthy low-carbohydrate diet scores (hazard ratio [HR], 1.07 per 20-percentile increase in score; 95% CI, 1.02 - 1.11; P = .01 for trend) and similarly for unhealthy low-fat diet scores (HR, 1.06; 95% CI, 1.01 - 1.12; P = .04 for trend).
Conversely, total mortality rates per 20-percentile increase in dietary scores were reduced for healthy low-carb diets (HR, 0.91; 95% CI, 0.87 - 0.95; P < .001 for trend) and healthy low-fat diets (HR, 0.89; 95% CI, 0.85 - 0.93; P < .001 for trend).
Diets Similar, but Food Quality Important, Consistent With Guidance
"Despite variance in macronutrient composition, low-carbohydrate and low-fat diets have shown similar associations with weight loss and metabolic biomarkers, with similar intensity of energy restriction and adherence to the intervention," the authors write.
"However, the associations between two types of diets and long-term health outcomes were inconsistent."
The link between unhealthy carbohydrates and fats and mortality could be explained by a variety of mechanisms, the authors note.
"A high-saturated-fat diet is highly palatable and may have a weak effect on satiation, potentially leading to overconsumption and obesity," they write. The replacement of saturated fat with unsaturated fat is meanwhile associated with a lower risk of heart disease and mortality.
At the same time, "low-quality carbohydrates, such as refined grains and added sugars, provide limited nutritional value, and their high glycemic load could be associated with high postprandial glucose and insulin, inflammation, insulin resistance, and dyslipidemia," the authors explain.
"What does seem to matter" emphasized Johnson, is the quality of the carbohydrates and fats, "where quality has been defined according to some pretty conventional criteria."
"High-quality carbohydrates for example are lightly refined whole grain cereals, fruits, and nonstarchy vegetables, and high-quality fats would contain a higher proportion of polyunsaturated lipids from plant sources, and less fat from animal sources."
He concludes: "Overall, the results of this study seem consistent with the current dietary recommendations given by public health authorities in the UK and elsewhere."
The study was supported by the National Natural Science Foundation of China and National Institutes of Health. Kuhnle and Johnson have reported no relevant financial relationships. Disclosures for the other authors are listed in the article.