Association between healthy/unhealthy low fat and low carb diet and risk of death

By Dr. Ananya Mandal, MDJan 21 2020

A large study from the Harvard T.H. Chan School of Public Health in Boston has found that there are no significant associations between low carb and low fat diets and risk of death. However the converse was not true. Unhealthy low fat and low carb diets were associated with a raised risk of deaths.

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The team of researchers led by Zhilei Shan, M.D., Ph.D., from the Harvard T.H. Chan School of Public Health looked at around 37,000 American adults and their self reported diet data to come to these conclusions.

Their study titled, “Association of Low-Carbohydrate and Low-Fat Diets With Mortality Among US Adults,” was published in the latest issue of the journal JAMA Internal Medicine.

The team wrote that there is evidence in scientific literature that advice a low fat and a low carbohydrate diet. However the actual association between these healthy food options and reduction in risk of mortality has not been proven in large studies. Further the quality of the low fat and low carb diets have also never been studied in these association studies. Thus for this study the team included 37,233 adults who were aged over 20 years and were recruited in the US National Health and Nutrition Examination Survey cohort study between 1999 and 2014.

The team defined a healthy low carbohydrate diet as one with lower amounts of poor carbohydrates and high amounts of plant proteins and unsaturated fats. A healthy low fat diet meant fats less on saturated fats and high amounts of high quality carbohydrates and plant proteins.

The participants were divided into 11 different subgroups according to their diet preferences. These were sex specific wrote the researchers. For each of the strata, the amount of energy derived from fats, proteins and carbohydrates (the 3 macronutrients) were different. For example those in the highest strata were awarded 10 points and those in the lowest strata received 0 points. This was the system for fats and proteins. On the other hand for the carbohydrates, this number system was reversed. An overall “LCD score” or low carbohydrate diet score was calculated by summing up the scores from fats and proteins and carbohydrates for each strata. These LCD scores ranged between 0 and 30. If the scores were higher, it meant that the person was eating a healthy low-carbohydrate diet. To calculate unhealthy low carbohydrate scores, percent age of energy obtained from the “high quality carbohydrate, animal protein, and saturated fat” was assessed. Similarly to assess the healthy LCD score percentage of energy from “low-quality carbohydrate, plant protein, and unsaturated fat” was assessed. Same method was used to calculate overall, healthy and unhealthy low fat diet or LFD scores, wrote the researchers.

Each of the participants were provided with detailed diet questionnaires where they reported their previous day diets from memory. Once the data from the large cohort was gathered, the analysis was started from July 2019 and it ended in August 2019. For each of the participants scores were developed that analyzed the energy obtained from carbohydrate, fat, and protein as part of the whole energy obtained from food. Scores regarding unhealthy and healthy low carbohydrate and low fat diets were also analyzed. To assess the risk of mortality, the deaths due to any cause or all-cause mortality until 31st December 2015 was checked from the National Death Index mortality database.

The results showed that the participants were aged an average around 50 years and there were 52.6 percent or 19,598 females in the cohort of participants. The team assessed 29,7768 person-years of follow up. During this time there were a total of 4,866 deaths, they wrote. Overall the low fat and low carbohydrate diet scores were not associated with a lowered risk of deaths due to any cause, the results found. The risk of deaths (rise in deaths with a 20 percentile rise in dietary scores) however rose with unhealthy low-carbohydrate diet (multivariable-adjusted hazard ratio score 1.07) and fell with a healthy low-carbohydrate-diet (score 0.91). Similarly the risk of death rose with unhealthy low fat diet (score 1.06) and fell with a healthy low fat diet (score 0.89). After adjusting for all the variables the results still proved to be robust, wrote the researchers.

The team wrote in conclusions, “overall low-carbohydrate-diet and low-fat-diet scores were not associated with total mortality.” They warned however that not all low fat or low carbohydrate diets are good enough to reduce the risk of deaths. They added, “Unhealthy low-carbohydrate-diet and low-fat-diet scores were associated with higher total mortality, whereas healthy low-carbohydrate-diet and low-fat-diet scores were associated with lower total mortality.”