A healthy diet is not as simple as low-carbohydrates or low-fat.  

If you ask ten people to define a healthy diet, you will get at least 8-9 different answers.  May will say that is some variant of a low-fat diet, but others will say it is low sugar or low carbohydrate.  The human diet is just too complex to simplify this concept to this degree and simplifying it does not take into the genetic diversity that is being a human being.  

Humans are truly a diverse group of beings.  Let avoid the obvious metabolic genetic variability and focus on what you see.  The diversity of hair color is an example.  You have color, texture, and thickness.  Each one is genetically different and runs in families.  

If you look at metabolic diseases, you will see similar diversity.  Some folks have an intolerance to sugars and carbohydrates and need to avoid them.  Others have similar issues with fats.  It is so complex that it is difficult to define what is truly meant by a “healthy diet”.   The problem is that some of the perceived “healthy diets” are not really healthy.

There is growing evidence that dietary effects are not so simple.   A study from January of 2020 looked at low-carbohydrate or low-fat diets and mortality.  The study was a cohort study that sought to investigate the associations of low-carbohydrate and low-fat diets with total and cause-specific mortality among US adults.  The study looked at the diets of over 40K subjects so it does have a diverse view.  

The study found that overall low-carbohydrate-diet and low-fat-diet scores were not associated with total mortality.  The researchers found that the lowering of risk was more complicated than going low-carbohydrate or low-fat.  

The quality of food matters.  Low-carbohydrate with low quality highly processed carbohydrates and sugars appears to be just as much of a risk as a high-carbohydrate diet.  The same concept is true with fat with the risk being lower the intake fo fat but retaining saturated fat.  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.

The bottom line:  Low-carbohydrate-diet and low-fat-diet scores intake does not appear to lower total mortality.  Lower-quality carbohydrates and fats obviously increase your risk of higher mortality.  I would recommend reducing your processed carbohydrates and saturated fats and avoid focusing on just the macronutrients themselves.  These findings suggest that the associations of low-carbohydrate and low-fat diets with mortality may depend on the quality and sources of macronutrients.