Walnuts are a dietary paradox – or at least they appear that way. As you probably know, walnuts are very calorically dense, mainly because of their fat content. Just one ounce of walnuts – that is about 12-14 halves – contains 185 calories. If you’ve ever tracked your food intake, you know that sort of thing can add up very quickly, especially if you’re just grabbing handfuls and not measuring.
Energy density, in general, is a legitimate concern when it comes to weight regulation. A number of long-term studies have found that eating foods that are relatively high in volume – but low in energy density – promotes weight loss. Think spinach, as opposed to butter.
That’s kind of a no-brainer, but here’s a clinical trial that really illustrates how huge a difference this can make.
Over a five day period, subjects were provided all the food that they wanted, in unlimited quantities. They were separated into two different meal plans:
- Low energy density foods: fresh fruits, veggies, whole grains, beans
- High energy density foods: fatty meats, tasty desserts
At the end of the study period, the subjects in the low energy density group had consumed just over half as many calories as the high energy density group – a difference of 1570 calories per day versus 3000!
Based on findings like this, you would think that walnuts and nuts, in general, are a serious menace, from the standpoint of weight control. But if we take a look at the literature, we plainly see that people who eat nuts are actually less likely to gain weight, and long-term consumption of nuts is associated with reduced risk of obesity.
Even when walnuts are experimentally added to the diet without controlling for energy intake, body weight increases, but far less than what would be expected. One trial, for instance, found that subjects who added 35 grams of walnuts to their existing diet for six months only gained about one-tenth as much weight as would have been predicted.
So why are nuts such an outlier? I’ve seen a couple different ideas on why this might be the case.
Some have pointed to inefficient energy absorption. Metabolic studies have suggested that nuts are unusually resistant to digestion. Walnuts specifically have been shown to provide 21% fewer available calories than would be predicted by Atwater factors. This was attributed to incomplete breakdown of the plant cell walls, which limited accessibility of lipids in the walnuts. Interesting. We’ll come back to this.
But other studies of nuts suggest another mechanism. When subjects added 320 calories of almonds to their diet every day for six months, it was shown that most of the added energy from the almonds was being displaced by reduced intake of other foods. This resulted in minimal impact on body weight, despite the added calories. It is thought that this dietary compensation may account for 55-75% of the calories from nuts.
Might this finding apply to walnuts as well? Seems like it. Researchers affiliated with Beth Israel Deaconess Medical Center compared a walnut-based shake to a placebo shake and found that the walnut group reported greater satiety and sense of fullness after three days, despite the fact that both shakes were carefully matched for energy content and macronutrients.
So, what’s going on here? That same research team recently replicated that study but also has taken a look inside the brain to find out what is going on when people eat walnuts on a regular basis, and the findings are fairly revealing. Let’s take a look at what they did.
The researchers recruited ten adult subjects with obesity. Each subject was randomly assigned to one of the following:
- A smoothie made with 48 grams of walnuts
- A placebo smoothie, identical in macronutrient content but using safflower oil instead of walnuts, and using a walnut flavoring for masking effect
Obviously, this is a small study (in terms of number of subjects), but a very rigorous design. Participants stayed in the research center for the entire duration of each arm of the study (five days). They were given isocaloric diets that were carefully devised based on their gender and body mass, which they consumed during their stay in the facility. After five days, there was a one month washout period, and then subjects were readmitted to complete the opposite condition. So all subjects experienced both placebo and walnut smoothies.
On the last day of their stay, participants underwent neurocognitive testing and a fMRI while viewing a series of 150 images. The pictures included a mixture of super tasty food, less tasty food, and non-food items. Before and after the imaging, subjects also completed surveys to measure subjective hunger and appetite.
As hypothesized, participants reported feeling less hungry when eating the walnut smoothie as opposed to the safflower oil smoothie.
When reviewing the brain imaging, researchers observed greater activation of the insula in subjects consuming walnuts. The insula is a part of the brain that is associated with appetite regulation and reward processing. Furthermore, higher activation of the insula was correlated with less subjective hunger.
At first, this finding actually seems kind of counter-intuitive, since activation of the insula in response to food cues is often higher in people with obesity. But the brain is weird and complex, and different parts of the insula appear to be involved with different cognitive processes surrounding food.
If we zoom in a little closer, the walnut group experienced greater activation within the dorso-anterior insula in response to highly palatable food cues. This part of the insula has been shown to be related to inhibitory control and is apt to activate when resisting an impulse (like, for instance, looking at pictures of tasty fattening foods).
In this context, it looks like consuming walnuts increased cognitive control – which might be what helps people resist the enticing pull of doughnuts and cupcakes.
So, why walnuts specifically?
There is some research suggesting that walnuts can affect brain function (and hey, they do kind of look like brains). So it’s not totally crazy.
Feeding studies have shown that dietary supplementation with walnuts improves memory and cognition in animals. This is usually attributed to polyunsaturated fatty acids. Walnuts are a rich source of linoleic acid and alpha-linolenic acid – highest among nuts – and these essential fatty acids play a role in various cellular processes in the brain.
Walnuts also stand out because they contain a wide array of polyphenols. In fact, walnuts comprise the seventh greatest source of total polyphenols based on serving size among common foods and drinks. One study found that a single serving of walnuts had the same amount of phenolics as 2.2 servings of red wine! This matters because these compounds have been shown to alleviate oxidative stress and inflammation in the brain. It is also thought that dietary polyphenols may influence neural signaling pathways that mediate food intake. Hmmmm.
Finally, oddly enough, walnuts are a natural source of melatonin! The circadian hormone readily crosses the blood-brain barrier and has been shown to exhibit various neuroprotective effects. It has been reported that walnut consumption specifically elevated melatonin concentrations in the blood, which in turn was associated with increased total antioxidant capacity.
So, we see that the components in walnuts could be affecting the brain in an array of different ways. And I have another idea of how walnuts might be affecting appetite regulation. I’m going to indulge in a little bit of educated speculation here.
Let’s backtrack to the discussion at the beginning of the article. You might remember I mentioned that energy from nuts is not as well absorbed as from most other foods. This inefficiency is due to incomplete breakdown of the cell walls of nuts, which shields portions from enzymatic degradation in the gut.
However, just because these pieces escape digestion does not mean that they are useless to the body.
These intact components of walnuts – including fiber, polyunsaturated fatty acids, and non-extractable polyphenols – travel to the colon, where they can fuel gut microbes through bacterial fermentation, which is the primary metabolic activity of the gut microbiota. Importantly, microbiota can respond to changes in nutrition remarkably quickly. Within just 24 hours of a dietary intervention, it is possible to detect meaningful changes in the makeup of the gut microbiota.
And a recent study does suggest that walnuts specifically can impact the composition of the gut microbiome. Supplementation with 42 grams of walnuts resulted in a shift toward more bacteria that generate butyrate, a beneficial metabolite that seems to play a role in regulating metabolism and energy balance. Something we probably want more of.
The reason why I bring all this up is because feeding the microbiota isn’t just important for the gut – it seems to be crucial to the function of almost every system in the body, including the brain. The brain and the gut are closely and reciprocally connected to one another, and interestingly the insula appears to be one area where they interface. Additionally, research suggests that butyrate, in particular, may reduce appetite and food intake, largely through its interactions via the gut-brain neural circuit.
Is it possible that consumption of walnuts influences neural control of appetite at least in part through the gut microbiota? There’s not really enough specific research to connect these dots yet, but I think it’s a plausible contributing factor.
Finally, why does any of this matter in the first place?
Nuts are extremely healthy foods. Nuts have been shown in large epidemiological studies to reduce the risk of cardiovascular disease and diabetes. For instance, the Nurses’ Health Study showed that consumption of one ounce or more of nuts, at least 5 times weekly, reduced the risk of cardiovascular disease by 35%. And a study of more than 118,000 subjects found that people who consumed an ounce of tree nuts per day had a 20% reduced risk of dying from any cause over a thirty year period. It would be unfortunate to omit something from the diet that seems to have such overwhelming positive effects. This, incidentally, is why we made a point to include both walnuts and almonds in our smoothie pack.
The one major drawback they carry is how many calories they’re packing, which has understandably made some people ambivalent about including them regularly in the diet. But it looks like the body responds to them in a way that is unlike perhaps any other energy-dense foods – so we probably shouldn’t avoid them outright.
Although the fMRI study looked at walnuts, it’s worth noting that this effect may not be wholly exclusive to walnuts, since other tree nuts seem to have similar effects. Almonds in particular seem to be pretty badass too.
And finally, a boring caveat: even with these compensatory mechanisms, the laws of thermodynamics still apply. Most experimental and epidemiological research looking at nuts use fairly reasonable portions, like around one ounce. At some point, the sheer volume of added calories is bound to overwhelm any satiety or absorption effects. Plus, we are all different, and I would imagine that the degree of dietary compensation varies a lot by individual. So go ahead and eat them, but don’t go crazy.
- Energy density is an important variable in weight regulation. Yet nuts, which are very calorically dense, have been shown to be inversely associated with weight gain.
- This is thought to be due in part to poor absorption, but also because of energy displacement. In other words, when people eat nuts, they often eat less of other foods.
- When people were given 48 grams of walnuts for five days, they reported being less hungry than counterparts in a placebo group. Additionally, there was greater activation in the dorso-anterior insula in response to pictures of highly palatable food. This part of the brain seems to light up when resisting an impulse.
- We’re not totally sure why walnuts have this satiating effect. Components of walnuts do appear to affect brain function, and we know that walnuts can shift the gut microbiota in ways that may affect neural regulation of appetite.
- Overall, research suggests that walnuts are a health-promoting food, and ~1 ounce is unlikely to result in weight gain. Your mileage may vary though.
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