Robb Wolf: On Palmitic Acid, Carbs, LDL and CVD
Palmitic acid is 16 carbons long, fully saturated, and commonly found in palm oil and animal products, including beef, eggs, milk, poultry, and seafood. Palmitic acid has long been implicated in CVD, as it tends to raise LDL cholesterol. Among the saturated fats, it would appear palmitic acid does pose the greatest likelihood of increasing LDL cholesterol. However, palmitic acid has also recently been shown to be vital both to forming new memories and accessing long-held memories. As we shall see when we investigate how our diet has changed, a Paleo diet supplies an adequate amount of palmitic acid for optimum cognitive function while limiting the intake to levels that are not harmful to the cardiovascular system. It is also important to note that excessive carbohydrate intake leads to palmitic acid production. If you recall from the insulin chapter, when liver glycogen is full, additional carbohydrate is converted to palmitic acid. This process appears to blunt our sensitivity to leptin, which then inhibits our satiety to a normal meal. This is the beginning of insulin resistance and is at the heart of the mechanism of how we cease to respond to food by feeling "full."
- Palmitic acid content of various animal products varies widely. See my Fatty Acid Contents posts.
- A citation for the role of palmitic acid in memory would be nice here. I personally have observed that cognitive function of those from cultures historically consuming low fat primarily seafood based diets does not seem to be impaired. And no that's not a stereotype, it's first hand knowledge, given my majors/fields, having studied and worked with many Asian students.
- What is the evidence that "a Paleolithic diet" supplies just the right amount of palmitic acid? Given the wide variety of "paleo" diets, even those that could be constructed within the guidelines Robb sets out in the book.
- Do we even know what this proper balance is? Is there any research on minimum levels for cognitive function and/or maximum levels above which cardiovascular function and leptin sensing are impaired?
- If palmitic acid is required for good cognitive function, why would de novo lipogenesis of "excess" carbs be necessarily a bad thing? I've somewhat jokingly addressed this before: There's no dietary need for saturated fat ... . One point made in that post would seem to go against the notion of a palmitic acid requirement for proper cognitive function, that being that DNL is only upregulated in times of excess, not scarcity. One might make the case that since we evolved to consume large amounts in the diet no endogenous source is required, but then why wouldn't we have evolved to manufacture EPA and DHA from excesses?
- Even in gross excesses, palmitic acid produced by the liver is on the order of a few grams per day (see here and here). If palmitic acid blunts leptin sensitivity, then dietary intake would far exceed DNL and blunt leptin sensitivity long before we get to excess carb contributing its piddly amount. So ... if palmitic acid manufactured from excess carbs are detrimental, why aren't larger quantities obtained directly from the diet even moreso?
- Leptin is not a postprandial satiety hormone. If it were, it would indeed need to "spike" after meals, counter to claims made by some. Then impaired leptin sensitivity would be a plausible route to reduced satiety. But if anything leptin is depressed slightly after a "normal" mixed meal.
- The last statement, "This is the beginning of
insulin resistance and is at the heart of the mechanism of how we cease
to respond to food by feeling 'full'." is a huge stretch. Stated as definitively yet vaguely as it was, I consider irresponsible. Robb was careful to use the qualifier "appears to" with regards to leptin sensitivity. At the very least a "may be" in plase of "is" is in order in that statement. But going from leptin sensitivity to insulin resistance and impaired satiety signalling in basically one sentence? This is the sort of thing readers pick up on, perhaps hear someone else say similar, and before long it becomes "fact" and "common knowledge", repeated more and more and nobody questions where the concept came from.