Raw vs. cooked: What does science say about heating food?
Cooking is one of our oldest tricks. Without it, our ancestors couldn’t have extracted enough calories from food to fuel the evolution of our exceptionally large brains.
This is because cooked food is easier to digest and provides more energy than raw food. Cooking also reduces the risk of contamination and foodborne illnesses.
Why, then, do some people consider cooking unnatural and unhealthy — and is there any truth to this?
Here, we take a look at the benefits of cooking and what different methods actually do to your food.
But first, let’s debunk some diet myths.
To cook or not to cook
If you’re a fan of muesli for breakfast, you might have heard of the creator of the original recipe in the late 1800s, Maximilian Bircher-Benner.
He was a Swiss physician who prescribed a diet of raw fruit and vegetables because he believed that cooking sucked the nutrition and “vital substance” out of food.
The idea that cooking degrades the natural nutrients and enzymes in food lives on today, particularly in the raw food movement.
Its advocates reject any foods heated above 104°F (40°C) and stick to a diet of mainly raw fruits, vegetables, sprouted beans, nuts, and seeds.
But cooking is far from a destructive force. In fact, it enhances our ability to digest and absorb key nutrients like carbohydrates, proteins, and lipids.
Plus, studies have shown that cooking releases beneficial nutrients, like carotenoids. It does this by softening the food matrix — the structure of food and how it affects the availability of nutrients.
Our digestive enzymes can’t always access these nutrients when we eat foods raw.
Another misleading argument against heating food is that our metabolism hasn’t evolved to cope with a cooked diet since prehistoric times.
But genetic evidence suggests otherwise. It shows that humans adapted to a cooked diet at least 275,000 years ago.
Further research has found that not only is cooking inscribed in our DNA, it may also have shaped our gut microbiomes by changing the physical and chemical properties of food — more on that later.
Now that we’ve established that cooking is neither unnatural nor unhealthy, let’s take a closer look at what happens when we heat different foods.
There are many good reasons to cook your food. The simplest is that it makes staples like potatoes edible and tasty.
Most animal products have to undergo some sort of thermal processing to limit the risk of contamination with potentially life-threatening microbes.
The United States Centers for Disease Control and Prevention (CDC) advise against consuming unpasteurized milk and raw milk products, for example, to prevent infection with harmful bacteria, like E. coli, Listeria, and Salmonella.
You might want to cook sprouted beans and grains, too — the warm, humid conditions needed to grow sprouts are also ideal for cultivating bacteria that can cause food poisoning.
As well as limiting the risk of foodborne diseases, cooking allows us to digest a huge variety of foods and extract the energy and nutrients we need to stay healthy.
Let’s find out more by looking at some specific examples.
Our digestive system isn’t equipped to deal with starchy sources of carbohydrates, like rice, pasta, or potatoes, unless we cook them first.
The heat transforms hard-to-digest starch into a gelatinous goo and disperses the molecules in a way that allows access for our digestive enzymes.
Improved digestibility means we can extract more energy from carbs when they’re cooked.
One study found that mice weren’t able to obtain as much energy from raw sweet potatoes as from cooked sweet potatoes. This suggests that cooking can unlock more energy from food.
Next, the researchers analyzed the gut microbiomes of these mice.
They saw that the mice with raw food had lower bacterial diversity and abundance, and they noticed a shift toward Bacteroidetes — a group of microbes that break down starch.
What happens in rodents doesn’t necessarily mirror the effects in humans. But it seems that cooking not only improves the digestion of carbohydrates, it probably also helps shape the gut microbiome.
We’ve just seen that cooking increases the digestibility and energy gained from starchy carbohydrates.
To find out whether the same can be said of fats, scientists fed mice a diet of raw or cooked peanuts for 5 days. These nuts contain plenty of healthy fats, including omega-6 fatty acids.
The results suggest that cooking did enhance fat absorption by “pre-digesting” oil bodies.
There’s a simple explanation for this, which relates to the food matrix effect.
Peanuts and other whole nuts have sturdy cell walls and a layer of proteins that store fat and block access for our digestive enzymes.
When you cook nuts, these little capsules burst and release the fatty contents. The free-floating fat is then easily absorbed in the small intestine and less of it comes out the other end.
At ZOE, we know that your nutritional response to food is unique to you. That’s also true of the amount of fat you absorb from a portion of nuts.
But broadly speaking, the more you break down the food matrix by cooking or roasting whole nuts, the more energy you’ll get from them.
Phytochemicals are natural chemicals in plants, and many have antioxidant, anti-inflammatory, and antimicrobial properties that benefit our health.
Vegetables are packed with phytochemicals like carotenoids, glucosinolates, and polyphenols — but some cooking methods can reduce their concentration.
Boiling the life out of your greens does just that. A review of more than 100 studies found that the longer you cook vegetables like broccoli and Brussels sprouts in water, the more glucosinolates seep out.
Time is of the essence — boiling broccoli for 10 minutes has a minimal effect on glucosinolate loss, whereas boiling for 40 minutes results in a major loss of glucosinolates (and very mushy broccoli).
Another key factor is whether your vegetables swim in or hover above the water.
Because boiling puts food in direct contact with the cooking material, this leads to greater degradation of the food matrix and the release of more soluble compounds into the water.
The best way to preserve the phytochemicals and reap the health benefits of your vegetables is to steam them. This relatively gentle method doesn’t put food in direct contact with boiling water or sizzling oil.
Surprisingly, though, carotenoids followed a slightly different trend. These phytochemicals are abundant in yellow-orange fruit and vegetables, and in dark leafy greens.
Cooking may increase the amount of extractable carotenoids in cabbage, cauliflower, pumpkin, and artichoke.
Meanwhile, phytochemicals like polyphenols exist in a wide range of foods, like cereals and beans, some of which require thermal processing.
Rice, for example, is a starchy carbohydrate, and as we’ve already discussed, those need cooking. But phenolic compounds in rice are very sensitive to heat, and you lose up to 80% of them through boiling. The same goes for beans.
The good news is that you can make up for these losses by eating a varied diet that includes plenty of cooked, raw, and fermented foods.
What are the effects on the gut microbiome?
As we hinted at above, the trillions of microbes in our guts shift in response to a raw or cooked diet.
Research indicates that the adoption of habitual cooking profoundly affected us and our gut microbiomes.
But how exactly do our microbes respond to cooked food?
A study simulating the human gut microbiome response to five different foods suggests that your gut bacteria respond both to cooking and the intensity of the cooking technique.
For red peppers and chicken, the scientists found that intense cooking methods like roasting and grilling promoted beneficial Bifidobacteria and Ruminococcus bacteria more than boiling.
For chickpeas, boiling them was the best way to enhance certain populations of gut bacteria. But grilling chickpeas may trigger a rise in the production of butyrate, a short-chain fatty acid that helps maintain the gut barrier and offers protection against harmful microbes.
On the other hand, raw bread led to higher levels of butyrate than toasted bread.
Finally, the scientists found that you can’t go wrong with raw bananas, but roasting or frying them can boost butyrate production — and make a delicious and healthy dessert.
The bottom line
Cooking has helped make us human, both from an evolutionary perspective and because it shapes our culture — big celebrations often revolve around cooked meals that bring families and friends together.
Contrary to some claims that cooking destroys the nutritional content of food, heat actually makes it easier for us to digest our food and extract enough energy from it.
Cooking also makes some potentially dangerous foods, like raw meat, fish, and dairy, safe by eliminating pathogenic microbes.
While heat can reduce the amount of certain nutrients, the key is to eat a varied diet that incorporates different approaches to cooking.
Food should be enjoyed, so while you might want to steam rather than boil your vegetables, it’s important to factor in taste, too.
Cooking increases net energy gain from a lipid-rich food. American Journal of Physical Anthropology. (2014). https://onlinelibrary.wiley.com/doi/10.1002/ajpa.22622
Cooking shapes the structure and function of the gut microbiome. Nature. (2019). https://www.nature.com/articles/s41564-019-0569-4
Effect of food thermal processing on the composition of the gut microbiota. Journal of Agricultural and Food Chemistry. (2018). https://pubs.acs.org/doi/10.1021/acs.jafc.8b04077
Foods that can cause food poisoning. (n.d.). https://www.cdc.gov/foodsafety/foods-linked-illness.html
Genetic evidence of human adaptation to a cooked diet. Genome Biology and Evolution. (2016). https://pubmed.ncbi.nlm.nih.gov/26979798/
The effect of cooking on the phytochemical content of vegetables. Journal of the Science of Food and Agriculture. (2014). https://pubmed.ncbi.nlm.nih.gov/24227349/