Here’s a link to an interesting 30 day dietary intervention study out of Australia where over 5000 people were self selected to participate in a standard program which advocates a low fat, plant based diet.

Here’s the study’s description of the nutrition intervention:

The CHIP intervention, previously described, encouraged and supported participants to move towards a low-fat, plant-based diet ad libitum, with emphasis on the whole-food consumption of grains, legumes, fruits and vegetables. Specifically, the program recommended less than 20% of calories be derived from fat. In addition, participants were encouraged to consume 2-2.5 litres of water daily and limit their daily intake of added sugar, sodium and cholesterol to 40 g, 2,000 mg, and 50 mg respectively. Furthermore, the program encouraged participants to engage in 30 minutes of daily moderate-intensity physical activity and practice stress management techniques.

All the biometrics measured were lowered including HDL. HDL dropped 8.7% on average and was worse if the individual started out at a higher level. Of note the LDL and Triglycerides dropped some too. They changed more than the HDL typically so the ratios of LDL:HDL and Total Chol:HDL improved.

Interestingly, 323 participants resolved their diagnosis of Metabolic Syndrome because of their changes but 257 participants acquired the diagnosis of Metabolic Syndrome typically because of their lower HDL numbers.

The study didn’t disclose percentages of fat, protein, and carbohydrates that the participants were eating nor do we know if they measured them. So it leaves the question of what exactly caused the changes. A diet with a lower fat intake, in my experience, is replaced with higher carbohydrate intake with a relatively stable level of protein. Think about yogurt. All the yogurt is low fat but plain yogurt is not very palatable so they add fruit and flavorings (ie, carbohydrates). However this is my speculation only.

From experience, I see HDLs routinely rise as patient’s carbohydrate intake drops. We know that a low HDL is one of the most risky signs of cardiovascular disease, too.

Remember that humans can’t eat a zero protein or zero fat diet and live healthy nor long. They can, and many do, eat zero carb diets and remain very healthy. That puts carbohydrates in a different category as proteins and fats.