What is a conscientious dieter to do? Is there a correct mix of carbohydrates, protein, and fats that are nutritious and healthful? The literature used to blame fats for our problems; perhaps it’s carbohydrates turn. But like the report from the Lancet on salt, a new report on carbohydrates, again from the Lancet is going to raise some academic and clinical eyebrows. Spoiler alert – like salt, carbohydrates effects on health look more like Nike’s logo than a straight line.
Researchers made use of the Atherosclerosis Risk in Communities (ARIC) dataset – a prospective, observational, study of mid-life adults (45 to 64) in four representative US communities; recruited 30 years ago and seen approximately every five years. At those visits, participants completed food frequency questionnaires which were subsequently converted into calories ingested, as well as the percentages of carbohydrate, proteins and fats and their sources, animal or plant.  Extremes of caloric intake were excluded leaving about 15,500 participants followed for a median of 25 years. The outcome of interest was all-cause mortality.
The mean carbohydrate intake was approximately 49% with the data “binned” into five groupings or quintiles ranging from 37% to 61% carbohydrate intake. At every carbohydrate levels, more animal-based proteins and fats were eaten than plant-based sources. There were no significant differences in weight gain. The only significant difference - participants in the lowest carbohydrate quintile ate a greater average of protein and fats from animal sources than the other cohorts. Here is what they found:
“…the relationship between carbohydrate consumption and risk of mortality was significantly non-linear (p<0.001), resulting in a U-shaped association, with the lowest observed risk associated with carbohydrate consumption of 50-55%.”
The graph showing the hazard ratio of all-cause mortality plotted against energy intake from carbohydrates is to the left. Like the previously discussed study of dietary salt, the consumption of carbohydrates follows that J-shaped or in this instance, U-shaped curve – the pattern you see when nutrients are essential. Too little carbohydrate and you risk deficiency; too much, and you risk problems. The accompanying editorial used the term toxicity, but I think that may be connotatively too extreme. In any instance, there is a sweet spot, in the middle, where moderation lives.
In a sub-group analysis, the researchers noted that when animal-derived fats and proteins were coupled with a low carbohydrate diet, mortality was even higher; when plant-based fats and proteins complimented a low carbohydrate diet, mortality was reduced but still greater than a mid-range intake of carbohydrates. 
An additional component of the study was a deconstruction and reconstruction of a previous meta-analysis of a low-carbohydrate diet’s all-cause mortality. In reconstructing that analysis, researchers using the same criteria and methodology incorporated two additional studies published after the 2012 meta-analysis was completed along with the findings of this study. The reconstructed meta-analysis considered eight papers involving 432,000 patients. Again, higher mortality at higher carbohydrate percentages persisted, but the prior studies did not have data at the lower end of carbohydrate intake, reflecting only mid and higher range levels. Bottom line, the meta-analysis fit well with the current data.
The study has limitations. Besides the accuracy of a food frequency survey, the survey was only done at two intervals, six years apart and dietary habits change. The findings with respect to animal-based sources of proteins and fats may also be culturally limited as Asians, who contributed significant numbers to the meta-analysis eat more fish than the ARIC communities. Most importantly, the effect demonstrated by these dietary considerations was moderate; we have yet to identify the diet guaranteeing you a long and disease free life. And yes, you still have to exercise.
Is there one best diet? We still do not know. But what is increasingly clear is that everything in moderation, carbohydrates, proteins, and fats, is the best general advice we can offer. And plant-sourced protein and fats seem to have a protective effect although as I have said before, this may be an indirect reflection of other lifestyle choices.
 Let us declare right away that food questionnaires have problems in accurately reflecting what people eat.
 That the source of our proteins and fats makes a difference goes against some of what I have said in the past. So while proteins and fats from these sources can be chemically identical, there may be other factors at play; in any case, I was wrong in some of my earlier thinking.
Source: Dietary carbohydrate intake and mortality: a prospective cohort study and meta-analysis The Lancet DOI: 10.1016/s2468-2667(18)30135-x