When it comes to storing dietary fat into fat tissue, are all fatty acids created equal?
Recently, I've been trying to find out more information on how different types of fats are absorbed and stored as adipose tissue. In particular, I would like to see direct comparisons between saturated, monounsaturated and polyunsaturated fat and how they affect weight and fat gain in humans when energy intake is kept constant. So far, I haven't had much luck finding such studies.
What I did find is an old paper that looked into how different dietary fatty acids relate to fatty acids in adipose tissue (link). In other words, the authors studied whether the ratio of saturated and unsaturated fats in fat tissue is similar to the ratio of fats obtained from diet. To answer this question, they first starved rabbits (rabbit starvation, anyone?) for 3-4 weeks to deplete their fat stores and then put them on diets with varying fatty acids to see how it affected fatty acid distributions in adipose tissue.
Fatty acid composition of the diets
Five different diets were used, each one containing 20% by weight of either palm oil, cocoa butter, safflower oil, linseed oil (also known as flax oil) or rapeseed oil. These fats were chosen because each one is rich in a particular fatty acid: palm oil in palmitic acid, cocoa butter in stearic acid, safflower oil in linoleic acid, linseed oil in linolenic acid, and rapeseed oil in erucic acid. In addition, a low-fat control diet containing only 2.2% fat was used.
All rabbits were fed ad libitum. Though rabbits presumably consume little fat in the wild, there appear to be no special problems with feeding fat to rabbits (link, link). Rabbits eating a diet high in fat do gain more weight, however.
The figure above shows the percentages of saturated (SA), monounsaturated (MUFA) and polyunsaturated (PUFA) fatty acids in each diet. As you can see, palm oil and cocoa butter are mostly saturated fat, safflower and linseed oil are mostly polyunsaturated fat, and rapeseed oil is mostly monounsaturated fat.
Saturated and unsaturated fatty acids in adipose tissue
When you feed animals various kinds of fat, you expect to see those same kinds of fat being deposited into their fat tissue, right? Saturated fat being stored as saturated fat, monounsaturated as monounsaturated and polyunsaturated as polyunsaturated. That seems logical enough but is apparently not the case.
The rabbits on diets high in saturated fat resisted the deposition of large amounts of saturated fatty acids in their fat tissue. In fact, the rabbits eating diets high in palm oil and cocoa butter had similar levels of saturated fatty acids in their adipose tissue as rabbits eating the low-fat control diet. This despite the fact that the palm oil and cocoa butter diets not only contained much more fat as a whole, but also more than twice the percentage of saturated fat relative to total fat intake than in the control diet.
The graph above shows the percentages of fatty acids in adipose tissue after each diet. As you can see, feeding the rabbits palm oil, cocoa butter or rapeseed oil resulted in a distribution that is similar to the result seen from eating the control diet. Feeding them safflower oil or linseed oil, on the other hand, resulted in a significantly different distribution, one that is high in PUFAs but low in MUFAs and SAs.
The plateau of dietary saturated fat deposition into adipose tissue does not seem to be a temporary phenomenon. To find out how long-term administration of saturated fat affected fat deposition, the authors fed two additional rabbits a diet high in cocoa butter for a year. These rabbits showed similar levels of saturated fatty acids in their fat tissue as the rabbits that consumed the same diet for only a month. In other words, saturated fat in their adipose tissue did not increase further even with prolonged intake of cocoa butter.
Individual fatty acids in adipose tissue
So the distribution of unsaturated and saturated fats in adipose tissue depends on the fat composition of the diet. As we've seen, the relationship is stronger for polyunsaturates than it is for monounsaturates and especially saturates.
What about individual fatty acids? The same discrepancy between dietary intake and adipose tissue deposition is apparent here. The palm oil diet was 164% higher in palmitic acid than the control diet, but the concentration of palmitic acid in adipose tissue was only 30% higher in the palm oil group. Similarly, the cocoa butter diet contained eight times as much stearic acid as the control diet, but stearic acid levels in the cocoa butter group were only 52% higher.
Again, things were a little different in the polyunsaturated department. The percentage of linoleic acid in the adipose tissue more than doubled on the safflower oil diet compared to the control diet, and the linolenic acid concentration in adipose tissue was 31.1% on the linseed oil diet compared to 3.9% on the control diet.
It seems that out of the six individual fatty acids studied, oleic acid (primarily found in cocoa butter and palm oil) and linoleic acid (primarily found in safflower oil) were deposited most effectively. Oleic acid (a monounsaturated fat) in adipose tissue constantly exceeded the amount of oleic acid present in the diets. Erucic acid (mainly in rapeseed oil) and stearic acid (mainly in cocoa butter) showed the least deposition.
Conclusion and discussion
Adipose tissue takes up many different fatty acids from the diet. The degree to which these fatty acids are absorbed varies, however. Specifically, a diet high in saturated fat raises saturated fat in rabbit adipose tissue only modestly, whereas a diet high in polyunsaturates increases PUFA levels in adipose tissue dramatically.
Several reasons may explain these differences in dietary and adipose tissue fat composition. First, the authors note that since the control rabbits did have saturated fat in their adipose tissue despite consuming a diet very low in fat, they presumably synthesized saturated fat from carbohydrates in the diet. Therefore, when the intake of certain fatty acids is low, the body adapts by producing them through other means, using carbohydrates or other fatty acids.
Second, when the intake of certain fatty acids exceeds certain limits, the body is able to convert dietary fats from one form into another. According to the authors, retroconversion (chain-shortening) and desaturation of dietary fats may be a necessary function of the body to ensure optimum cell membrane fluidity. That is, when saturated fat intake is very high, the body converts some of the saturated fat into unsaturated fat to keep things working properly.
This mechanism does not appear to be in place in rabbits for polyunsaturated fats, however. The fatty acid ratios in adipose tissue appear to reflect those in the diet very strongly when the diet is high in PUFAs. This makes me wonder whether a similar effect happens in other mammals, especially humans.
The authors do not report whether calorie intake varied between groups or how much actual adipose tissue the groups had. They only mention that during the 4-week experiment, all rabbits in the five experimental groups were healthy and gained weight.
For more information on fats and health, see these posts:
Blood Test Analysis: The Cholesterol and Saturated Fat Issue Revisited
Green Tea Extract Increases Insulin Sensitivity & Fat Burning during Exercise
Low-Carb vs. Low-Fat: Effects on Weight Loss and Cholesterol in Overweight Men
A Typical Paleolithic High-Fat, Low-Carb Meal of an Intermittent Faster
