Adding Carbs to the Diet Increases Saturated Fat in Blood

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The idea of a low-fat diet is healthy for your heart has faded in recent years, but the "fact" that saturated fats are bad for us remains strong. A recent study sheds some light on the details of what happens in our bodies when we vary the number of saturated fats and carbohydrates we eat.

The idea that eating foods high in saturated fats is bad for us is 50 years old at this point, and much research has gone into "proving it". If the theory was true, you'd think the mounting evidence would have grown stronger and stronger over the years, and yet (to the surprise of many) this has not happened. The evidence against saturated fats in the diet is neither strong nor consistent, and several attempts in recent years to scrutinize the results of the accumulated evidence have reached the conclusion that saturated fat in the diet does not seem to be related to heart disease.

And yet, there is evidence that saturated fats in the blood, particularly certain types of fats (such as palmitic and palmitoleic acids), are being found to be associated with heart disease and diabetes.

So what gives? How could the fats get there if we didn't eat them? Answer: we make them.

What do we make the fats out of? Mainly excess carbohydrate. This has been known for a long time, but the question has been "to what degree does this happen, and under what circumstances?" There has been a fair amount of observational research showing the relationship between carbohydrate consumption and certain fats in our bodies, but until now it hasn't been carefully studied. However, a recent very carefully controlled study published in PLOS One did just that.

Here's the idea. The researchers took a group of 16 people and brought them through a series of six diets with varying amounts of carbohydrate and saturated fat. They gave them all their food and had them bring back the used containers so that they could track how much they ate. By the end, all the participants had been on each diet for three weeks. Most of them started at the low-carb end of the spectrum and gradually added a carb and subtracted saturated fat, but a third of them did it in the other direction.

And it turned out that eating the carbohydrate, not the fat, made the "bad fats" in the blood go up.

The Details

The men and women in the study were all overweight or obese, with an average BMI of 37. They all had metabolic syndrome (and so were insulin resistant) but did not have diabetes or any other metabolic disorder.

The diets were composed of whole foods, with carbohydrates coming from relatively low-glycemic sources such as whole grains. Fatty cuts of meat, whole dairy products, etc. were provided during the lower-carb phases, and leaner cuts of meat, low-fat dairy products, etc. during the higher-carb/low-fat phases.

The diets were constructed to facilitate weight loss. Each person's metabolic rate was measured, and the daily calories computed to be 300 less than it would take to maintain weight. The average calories consumed across the participants was 2500 calories per day, but each person ate the exact same amount of calories and protein every day, no matter which diet they were eating. During the lowest-carb phase the participants ate an average of 47 grams of carbohydrate per day (7% of calories) and 84 grams of saturated fat, and during the highest-carb phase, they consumed 346 grams of carbohydrate per day (55% of calories) and 32 grams of saturated fat.

There was also a low-carb "run-in diet" for three weeks before the study began so that each person's body would be adapted to low-carb eating (which, as we know, can take a while).

The Results

In every case, there was more of the palmitic and palmitoleic acids in the blood the more carbohydrate they ate, despite cutting the amount of saturated fat in the diet more than in half. For most (but not all) of the participants, there was also total saturated fat in the blood the less saturated fat and more carb they ate! Here are some more interesting things the researchers found:

  • There was more variation in the amount of "bad fats in the blood" at the high-carb end than the low-carb end. In other words, different people responded differently to the high-carb diet, even though there were similarities in that they all had metabolic syndrome, etc. At the lower end, the results were much more similar.
  • On average, people had lower blood glucose, insulin, blood pressure, and triglycerides in the lowest-carb/highest fat phase, but lost those advantages when adding carb (this was true whether they started the study on the low-carb or the high-carb end).
  • Serum ketones were increased about 5-fold in the lowest-carb phase, 3-fold after the second-lowest carb phase, 2-fold after the 4th phase, and then went to baseline.Participants lost an average of 22 lbs.

My Thoughts

To my knowledge, this is one of the few studies that has really looked at what happens in the same individuals at several different levels of dietary carbohydrate and fat, and it's the first one of this kind to look at saturated fats in the blood. This helps zero in on some of the conditions where we tend to make fat from the carb, and in what quantities.

I like that they looked at overweight and obese people with metabolic syndrome because we know that these are the ones that are most likely to respond well to a low-carb diet. Often people with metabolic syndrome are excluded from studies such as this.

It makes intuitive sense to me that there was more variation in the results of people on the high-carb diet than the low-carb diet. It gives more reinforcement to the idea that different people have more or less "trouble" processing carbohydrates. It also means that as people on low-carb diets such as Atkins begin to add carbohydrate back in they should remain alert to possible effects such as changes in blood glucose, blood pressure, etc.

They may have more trouble or less trouble with carbs than their friend.

I'm somewhat disappointed that the researchers elected to have the participants lose weight -- I think this muddies the waters, as some of the effects could have been due to weight loss. However, one good reason for this could be that they wanted to make sure that people weren't eating excess calories. We know that when people eat a whole lot more carb than they need for energy, they start massively converting that carb to fat. But that argument cannot be made if people are actually consuming fewer calories than they need to maintain a stable weight.​

The Bottom Line

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