Calories and Carbohydrate: a Natural Experiment

In the lab, we work hard to design experiments that help us understand the natural world.  But sometimes, nature sets up experiments for us, and all we have to do is collect the data.  These are called "natural experiments", and they have led to profound insights in every field of science.  For example, Alzheimer's disease is usually not considered a genetic disorder.  However, researchers have identified rare cases where AD is inherited in a simple genetic manner.  By identifying the genes involved, and what they do, we were able to increase our understanding of the molecular mechanisms of the disease.

The natural experiment I'll be discussing today began in 1989 with the onset of a major economic crisis in Cuba. This coincided with the loss of the Soviet Union as a trading partner, resulting in a massive economic collapse over the next six years, which gradually recovered by 2000. 

Unlike most industrialized nations, Cuba keeps systematic health records of its citizens, including the incidence of major diseases and deaths.  It has also performed surveys of body weight and height in several areas and at multiple timepoints.  This allows unbiased tracking of these factors over time in the population.  

The economic collapse resulted in major changes in diet, which Melissa McEwen recently brought to my attention.  The first was a decrease in energy intake from about 3,100 to 2,300 calories per person per day, according to FAO data (1, 2)  These numbers are probably not particularly accurate, but they serve to illustrate the trend.  The second change was a decrease in dietary fat and protein consumption, and increased reliance on inexpensive refined carbohydrate and sugar (2): 

The percentage of dietary fat in the energy intake decreased, while the contribution of carbohydrates (polished rice and refined cereals) increased from 64% in 1990 to 79.4% in 1993. Availability of essential dietary amino acids and fatty acids declined as a consequence of a reduced availability of animal protein and edible oils and fat. Sugar cane, a traditional source of energy in the Cuban diet, rose to 28% of total energy intake, almost three times that of the fat contribution.

In addition, a shortage of gasoline meant that people began walking and riding bicycles more, such that total physical activity increased (1, 2). 

OK, so we have 10 million people who are eating about one-quarter fewer calories overall, and exercising more, but the diet is heavily focused on refined carbohydrate and sugar (and monotonous).  Plus we have body weight and health statistics.  This is a great opportunity to see which factor is more important for obesity and chronic disease at the population level: refined carbohydrate/sugar, or calorie intake and exercise.  Make your guess!

There are at least two different data sets, and they show similar but not identical trends-- I'll try to synthesize them (1, 2).  During the economic crisis, the prevalence of obesity declined by about half, and overweight declined or stayed the same depending on which data you believe.  As calorie intake went back up during the economic recovery, obesity rebounded.  The prevalence of underweight increased, suggesting that a small fraction of people weren't getting enough calories to maintain weight.  Total mortality, diabetes, and coronary heart disease rates declined during the crisis. 

We're fortunate that something good came of the hard times in Cuba-- a natural experiment demonstrating the effects of a major nationwide change in diet and exercise.  Taken together, the data suggest that energy intake and physical activity are dominant factors in obesity, and major factors in the diseases of affluence, trumping the influence of refined carbohydrate and sugar at the population level.  The overconsumption of energy from food is at the core of obesity and strongly contributes to cardiometabolic disease.  Some of the harmful consequences of eating refined carbohydrate and sugar do not materialize when there is no calorie excess.  This natural experiment is consistent with a large body of evidence from controlled feeding trials in humans and animals.

That being said, refined carbohydrate and sugar are generally not healthy, and they do promote obesity and derail fat loss efforts.  Say what?  Although refined carbohydrate is secondary to energy intake when it comes to obesity, at any given level of energy intake, unrefined carbohydrate is generally superior to refined carbohydrate for overall health*.  Also, foods made with refined carbohydrate and sugar promote overeating**, so they contribute to the energy excess that drives obesity and chronic disease.


* Modest quantities of certain refined carbohydrates like white rice are probably fine in the context of a nutritious varied diet, but there are better foods to place at the center of the diet.

* Due to their high energy density, lack of fiber, and higher palatability and reward value.  The high palatability/reward value is partially due to the fact that refined carbohydrates are an ideal "blank slate" for creating energy-dense hyperpalatable foods like brownies, corn chips, cookies, deep-fried foods, and pastries.