Eating only raw foods has emerged as a popular dietary trend. Proclaiming an emotive health message, it is enough to make you think twice before next adding heat to your food. But fear not because on the scales of health, there is little to tip the balance in either direction. In this podcast, I look at the claims made about raw food diets and put these into context for the many health benefits that cooking food offers.
Cooking food is something we have done since the first caveman threw a mammoth steak on the fire. While it is true that cooking does cause chemical changes in food and a loss of some nutrients, it is a clear case of being alert and not alarmed.
Raw food advocates claim cooking food destroys the natural enzymes and nutrients that would otherwise give us optimal health and control body weight. Following a raw food diet means having almost entirely plant-based foods including fruits, vegetables, nuts, seeds, grains, vegetable oils and juices in their natural uncooked state.
On the pro side, there is a lot of merit to a raw foods diet. If you are currently eating a lot of highly processed food, then switching to raw foods will be a clear nutritional win. Where the science gets murky are around claims that raw foods are better because cooking destroys enzymes in plants. Now, it is 100 percent correct that cooking will do this. But so too does digestion. Few enzymes survive their trip through the hydrochloric acid spa bath in the stomach.
Where there is a small kernel of truth to raw food claims is the nutrient loses from cooking. Yet this is a clear case of being alert, not alarmed. Every time we cook food, there is some degree of nutrient loss. Exposing food to heat, oxygen or light alters the nutrients found in food. It is not all bad news though because cooking food also has its advantages. Cooking food can increase its digestibility, meaning you will get more nutrients out of it.
So, the big question: just how much of the nutrients in food do we lose when we cook it? Fortunately, the good people at The United States Department of Agriculture have compiled a detailed table of nutrient losses for 16 vitamins and eight minerals from 290 foods using a variety of cooking methods. I’ll link to this database in the show notes and for anyone overtly fixated on just what proportion of nutrients they are losing when they cook food. This is the best resource you will find. http://www.ars.usda.gov/main/docs.htm?docid=9448
Here’s the summary of all that information. Most minerals are unaffected by cooking and it is only vitamin C, folate and thiamin that stand out as being the main nutrients lost, but even then, levels rarely drop by 50%.
Losing vitamin C, thiamin and folate from cooking vegetables is easily offset by eating foods that are high in these nutrients to start with and are not normally cooked further.
Citrus fruits are an excellent source of vitamin C. Leafy green vegetables and avocados are high sources of folate, as is bread (which is fortified with folic acid). The Australian staple of Vegemite is packed with thiamin and because of fortification, so too is bread.
Putting nutrient losses from cooking into context, just storing fruits and vegetables for several days means a gradual loss of vitamin C. Fresh will always be best, but frozen vegetables are certainly an option to consider as they are blanched and frozen very soon after picking, effectively “locking in” their nutrients for months.
Food though is much more than just the essential vitamins and minerals. There are hundreds of bioactive compounds found in plant foods that have favourable health benefits. They can serve as antioxidant and anti-inflammatory agents and have anti-cancer and antibacterial roles as well.
Cooking losses of these bioactive nutrients are considered small; in many cases, cooking can actually increase the amount of some of these substances. Heating food breaks down plant cell walls so they can release more of their nutrients.
A good example is from the many studies that have shown that levels of the beneficial antioxidant, lycopene (which gives foods like tomatoes their red colour) is much higher in cooked tomatoes than in uncooked tomatoes. The reason why is that the heat breaks down the plants’ thick cell walls, making more of the nutrients available to absorb. One study from The British Journal of Nutrition found that people who followed a raw food diet had normal levels of vitamin A and relatively high levels of beta-carotene, but lower levels of lycopene.
Then there was the report from Journal of Agriculture and Food Chemistry that found that boiling and steaming better preserves antioxidants in carrots, zucchini and broccoli compared to frying them. In all cases though, antioxidant levels increased because of the cooking method compared to the raw state of the food. And I’ll link to this study in the show notes https://pubs.acs.org/doi/full/10.1021/jf072304b
Though it is not all good news about cooking as some research shows that some vegetables such as broccoli could have better health benefits raw rather than cooked. In this case, heat damages the enzyme myrosinase, which breaks down glucosinates found in broccoli into a compound known as sulforaphane. Sulforaphane is an anti-cancer compound found in cruciferous vegetables such as broccoli. It appears to have general, but potent antioxidant and possible anti-inflammatory actions similar to curcumin.
What to do about the broccoli dilemma then: to cook or not to cook? The middle ground here is to lightly steam it so it’s cooked but still crunchy as this seems to minimise sulforaphane losses. With a crisp and tender texture and more flavour, you’re likely to eat more of it - and that is what matters the most.
So how do you ensure the highest retention of nutrients during cooking? The three keys are water, temperature and time. As temperature, cooking time, and water volume go up, so too do nutrient losses. If you can, steam rather than boil as this cuts down on nutrient leaching. Cook at lower temperatures where possible or use a higher temperature for a shorter time.
Keep the size of vegetable pieces as large as possible to minimise oxidation losses. And don’t overcook food: you want your broccoli firm and green, not wilted and white. Choosing an exclusively raw-food diet can potentially put a person at a small disadvantage. When you exclude all cooked foods, you also exclude many healthy foods, and the nutrient diversity of your diet narrows.
Take the nutrient powerhouses beans and lentils. When cooked, they are a cornerstone to super-healthy vegetarian, Japanese and Mediterranean diets. Legume-based diets have many health benefits including the prevention and management of obesity, heart disease and stroke, diabetes and metabolic syndrome.
And the elephant in the room when it comes to cooking is not what it adds, but what it takes away. Cooking food at 75°C or hotter will kill most bacteria that cause food poisoning.
Unquestionably, there are many benefits to eating plenty of fruits and vegetables. These foods are high in nutrients and fibre and low in kilojoules. But is raw superior to cooked? A review of 28 research studies found that eaters of both cooked and raw vegetables had a lower risk of cancer compared to people who did not eat many vegetables in the first place. And I’ll link to the study in the show notes http://cebp.aacrjournals.org/content/13/9/1422.long
Our biggest health problems in a country like Australia come from eating too much highly processed, nutrient-poor food. Worrying about losing some vitamin C every time you stir-fry or steam your vegetables fades into insignificance when you consider that most people are not eating enough of these foods in the first place. Less than 7 percent of us are eating the recommended number of servings of vegetables.
So the bottom line becomes to eat your fruits and veggies no matter how they are prepared. If cooking makes them tastier and more palatable then you are more likely to eat them. That is where the biggest health gains are to be had.
Research Wrap Up
Now on to my research wrap up segment where I profile a study that has grabbed my attention during the week. And this research is about how exposing children to a wide variety of healthy foods can help instil good eating habits later into their lives.
The palatable, energy-dense foods that make up our modern food environment can promote unhealthy eating habits. Developing a liking for sweet tastes can mean a rejection of more sour and bitter tastes – the sorts of tastes found in many healthy foods such as vegetables. Food tastes are not set in stone, so it is the early stage of life that represents the most fertile time to set these malleable food preferences on the right path.
How eating habits can be influenced during early life stages was the focus of a recent narrative review. The research team reviewed 40 studies looking at how infants and young children develop a taste for healthy foods, especially vegetables and fruits. And I’ll link to the study in the show notes https://onlinelibrary.wiley.com/doi/abs/10.1111/obr.12658
The key finding was that repeatedly exposing kids to a variety of healthy foods during pregnancy, infancy and early childhood predicted better adoption of such foods later on.
Even during pregnancy, healthy eating is important as the food flavours can make their way into the uterus exposing the foetus to the flavours. Breastfeeding also can pass on flavours of healthy foods to the infant. Taken all together, there is much that can influence an infant’s food tastes even before they take their first mouthful of solids.
After infancy, the simple process of repeated exposure of the child to healthy foods, even ones they reject initially, can help them develop a taste for those foods. For parents, if at first you don’t succeed, then try, try and try again!
Exposure to a variety of healthy foods from the start of a child’s life, including during the prenatal period, early milk feeding and the introduction to complementary foods and beverages, can support later acceptance of those foods.
So that’s it for today’s show. You can find the show notes either in the app you’re listening to this podcast on if it supports it, or else head over to my webpage www.thinkingnutrition.com.au and click on the podcast section to find this episode to read the show notes.
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I’m Tim Crowe and you’ve been listening to Thinking Nutrition.