An avocado a day

I don’t know about you but I love avocados.  In the last year or so, my diet has evolved to incorporate a great deal of healthy fats in the form of avocados, nuts, seeds, and eggs. The jury is still out as to whether fat is good or bad for you.  The Dietary Guidelines Advisory Committee has recently modified their recommendations about fat and cholesterol, calling instead for a reduction in sugar consumption and a move towards healthy fats like in the Mediterranean diet.  This seems to me like a move in the right direction, but there is still some push-back from people (and big players in the food industry) who think fat is bad.

I was pleased to come across this recent paper which reports a significant beneficial effect of eating an avocado per day.  In particular, switching from the “average American diet” to a moderate fat diet that incorporates one avocado per day results in a decrease in LDL cholesterol, the “bad cholesterol”.  The most piquing result is that the decrease in LDL cholesterol is greater in the moderate fat, avocado group than in the moderate fat, no avocado group – presumably due to something unique to avocados. The study looks pretty solid to me:

  • They used a randomized crossover design – each subject gets all three diets, so we get the benefit of bigger treatment groups to estimate the causal effect of each dietary intervention. Randomizing the order in which a person gets the three treatments controls for the possible effect of a particular sequence of treatments.
  • The statistical analyses look acceptable.  They measured the “causal effect” of each diet on baseline characteristics using linear mixed models and non-parametric ANOVA, correcting for multiple tests with Tukey’s method.  If the randomization worked, then all confounding variables are balanced between the groups receiving a particular sequence of diets, making them alike in every way except for the treatment received.  This means the coefficient for the diet in the model is actually estimating what it is supposed to.
  • It’s hard to assess compliance for a dietary intervention. Subjects have to eat the prescribed foods in the right amount.  Without constant supervision it’s hard to know what really happened.  The authors tried to assign subjects meal plans with just enough calories to maintain their weight.  Then they assessed compliance by weighing subjects every day.  This is probably as accurate as they can get without constantly watching the subjects.  Perhaps they could do urine/stool samples to check the amounts of certain nutrients, but that seems like overkill.
  • The sample size is small, n = 40. However, they did a power analysis based on a similar study of a pistachio diet and in an idealized situation, a sample size of 37 would give them 95% power to detect a 10% decrease in LDL-C due to the avocado diet versus the low fat diet.

All diets had a beneficial effect on various subtypes of LDL and HDL cholesterol, but the avocado diet surpassed the other two.  The next steps are more studies to look at the individual micronutrients in avocados that are driving this beneficial effect.  In the mean time, I plan to continue eating my avocado a day.

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