The Fast-Food Habit

Summarized by Robert W. Griffith, MD
February 25, 2005

Introduction

We've been urged for years to try to break the fast-food habit, but it seems to be so well established that nothing really gets done. There are about 250,000 fast-food restaurants in the USA, and there obviously wouldn't be so many if people were avoiding them.

We probably need more evidence than has been available so far, pointing out the real dangers of a lifestyle based on fast food. One such study has just been reported in the medical journal Lancet, and we summarize the findings here. They show that fast food, eaten constantly over several years, increases the likelihood of both obesity and diabetes. One of the problems is that fast foods have, in general, a high glycemic food index.¹

What was done

Four study centers in Alabama, Chicago, Minneapolis, and California supplied young volunteers for the Coronary Artery Risk Development in Young Adults (CARDIA) study. Equal numbers of white and African-American were enrolled in 1985 and 1986. They had 6 clinical exams and completed frequent lifestyle questionnaires over the next 15 years. At each exam the participants had extensive interviews on their dietary habits.

Blood was taken at each examination for fasting glucose and insulin determination, allowing calculation of insulin resistance.² Sophisticated statistical analyses were done to examine links between fast-food intake, weight gain and insulin resistance.

What was found

Over 5,000 people aged 18-30 entered the study, and ¾ of them completed the 15-year exam. After others were excluded because some test results were missing, there were roughly 3,000 subjects with full information for analysis.

The average reported frequency of fast-food restaurant visits per week was as follows:

	Enrollment (1985-1986)	15 Years Later (2000-2001)
Black men	2.4	2.3
Black women	1.8	2.0
White men	2.4	1.9
White women	1.6	1.3

It can be seen the frequency was relatively unchanged over time in black people, but fell in those who were white. Individuals with high fast-food frequency (and who were therefore more likely to consume food with a high glycemic index) were younger than those with low frequency.

After adjusting for different lifestyles (e.g. smoking, physical activity, hours of TV watching), in order to eliminate any bias from such factors, it was found that baseline fast-food frequency was linked to future changes in body weight in both black and white people of both sexes. A difference of 3 visits a week to fast-food restaurants was linked to a 1.7 times increase in body weight over the next 15 years. This was also accompanied by an increase in insulin resistance.

Increasing fast-food visits by 3 a week was linked to an average weight gain of 2.2 kg (4¾ lbs) in black people and 1.6 kg (3½ lbs) in white people.

Those participants who rarely visited fast-food restaurants (i.e. less then once a week at baseline and 15 years later) gained some weight over the 15-year period, but those who visited them more than twice a week gained 4.5 kg (almost 10 lbs) more. Moreover, increased insulin resistance in this latter group was double that in the infrequent users.

What this study shows

This study confirms the strong association between the frequency of eating at fast-food restaurants and an increase in weight gain and insulin resistance. As both obesity and diabetes - end results of these two changes - are reaching epidemic proportions in the USA, a reasonable step to improve public health would be to educate the public about the risks they run by unwise eating habits.

The study shows that white people have, indeed, reduced their fast-food visits over the last 15 years. African-Americans, however, did not demonstrate such an improvement.

The study doesn't indicate which aspect of fast food is responsible - portion size (which influences total energy intake) or the composition of the diet, or both. And doubtless the availability of giant soda drinks (Coca-Cola, Pepsi-Cola etc) at fast-food outlets must carry some responsibility. There's evidence from other studies that the composition of fast foods usually has a high glycemic food index¹, which is known to increase the risk for development of diabetes.

This study, along with others, leads naturally to the question: "Should fast foods be taxed?" After all, one of the attractions of fast foods is that they are comparatively cheap. However, a "sin tax", similar to that on cigarettes or liquor, is not the preferred way to encourage young people to eat more healthy diets. Health education, together with increased availability of good but cheap food, is the best approach. And you should study the glycemic food index for your favorite "fast" foods -they may surprise you!

Source

• Fast-food habits, weight gain, and insulin resistance (the CARDIA study): 15-year prospective analysis. MA. Pereira, AI. Kartashov, CB. Ebberling,  et al., Lancet, 2005, vol. 365, pp. 36--42
  
  

Footnotes
1. The glycemic index has been developed to determine which carbohydrate-containing food raise blood glucose levels more or less quickly after a meal. Substituting low- for high-glycemic index foods may even help prevent weight gain. One easy way to improve one's glycemic food index intake is simply to replace starches and sugars with whole grains and legumes (e.g., dried peas, beans, and lentils).
2. The homeostasis model (HOMA) was used for measuring insulin resistance; the calculation used was glucose (mmol/L) x insulin (mU/L)/22.5

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