At a time when junk food is omnipresent, where pesticides and chemical additives are everywhere, many of us are rediscovering the joys of cooking, the delight of the taste buds and the interest of the principles of dietetics.

Perhaps we forget the essential: the consequences of poor food preparation.

What good is it to select quality products if it is to empty them of their added value when cooking or, worse, to load them with toxic compounds during cooking?

"This aspect of nutrition has long been neglected, recognizes Dr. Jean-Michel Lecerf, head of the nutrition department at the Pasteur Institute of Lille, while it is a key to understand the impact of our diet."

Cooking modes that degrade vitamins and micronutrients

The conquest of fire, which researchers now date back nearly a million years *, was an undeniable progress for humanity. By taming the cooking, our prehistoric ancestors not only made some food edible (try eating a raw potato!), But also favored the intestinal absorption of some nutrients and made their digestion easier.

All these processes have allowed the jaw to regress in favor of the cranial box. Not to mention the hygienic benefits: the heat destroys most parasites and pathogenic bacteria, which avoids many serious infections due to salmonella, trichina and other worms possibly present in food.

But any medal has its reverse. Starting with the risks of loss of minerals, vitamins and trace elements that our body needs to keep fit.

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Vitamin C vegetables, in particular, is extremely fragile. It is already degraded quickly at room temperature, and even more so as the thermometer climbs.

Vitamin B1 (thiamine), which is found in cereals (pasta, rice ...), is also affected by heat.

The vitamin degradation of vegetables however varies according to the type of cooking used: with steam, they lose only 3% of their initial vitamin C content and 20% of vitamin B1 content. And the more they are consumed al dente, the better their vitamin content is preserved.

Stuffing is even more favorable , especially for vegetables rich in water, provided that they are not impregnated with pesticides (because the food bathes at the bottom of the casserole in their juice). Under these conditions, the loss of vitamin C is limited to 25% and that of vitamin B1 to 10%.

But in boiling water, 50% of vitamin C and 35% of thiamine fade , besides the polyphenols, which are water soluble, also tend to flee. For example, 75% of the main anti-cancer agents contained in cabbage and broccoli (glucosinates) escape into boiling water, while they remain almost completely within the food after a transition to cooked Steamer or in a jumper.

It is therefore better to avoid dipping vegetables in too much water and unnecessarily prolonging their cooking time. "Do not salt either, advises Dr. Catherine Serfaty-Lacrosnière **, because salt promotes the loss of water, so the escape of trace elements." And do not cut them into small pieces   : their surface of contact with the air, then the boiling water will be increased, which necessarily amplifies the desertion of micronutrients.

If the vegetables are organic and washed well beforehand, some can be recovered by drinking the cooking water.

However, not all plant antioxidants are deteriorated by heat. It is even the opposite for some, especially pigments of the carotenoid family , which play a big role in the prevention of cardiovascular diseases.

" For example, it has been proven that tomato lycopene is better absorbed after cooking," says Dr. Lecerf. It would be the same for carotene carrots and lutein contained in spinach and cabbages.

Cooking food: chemistry (toxic) on the plate

The cascade of chain reactions that takes place during cooking is of such complexity that we are just beginning to understand the contours.

"The most worrying is the genesis of compounds that did not exist in the original food," says Dr. Lecerf. Their number is certainly impressive: about eight hundred , most of which remain unknown. Some are derived from the oxidation of lipids to heat.

When a food is fried, the oil is colored and lathered if the temperature is excessive. These changes are a sign of an alteration of its fatty acids, which induces the appearance of toxic such as acrolein, suspected of being carcinogenic .

All oils do not have the same sensitivity. Some can be heated without worry, while others are quickly damaged. This is the case of oils rich in polyunsaturated fatty acids (oily fish, flax, soybean ...), whose cold consumption is recommended for health. But, at high temperatures, they distort themselves illico.

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Wok and soybean oil, a carcinogenic duo

The very high rate of lung cancer that affects women in Hong Kong or Singapore would be due to the way they cook: wok with soybean oil . Unlike a conventional stove (in which the temperature is distributed over the entire flat surface), it concentrates heat in the middle.

It can rise to 240 ° C on the bottom, a temperature at which soybean oil is transformed and releases powerful carcinogens. Fries and fritters therefore prefer to use only more resistant oils (sunflower or, better still, olive).

It is important to filter the frying bath after each use to eliminate burnt waste. Renewing it completely after five or six times is also a wise precaution, which, unfortunately, hardly applies to restaurants.

Haro on the overcooked meat

Even more dangerous are heterocyclic amines, "which are mainly produced during the cooking of meat and fish," says Dr. Lecerf, who, in the heat, transform their proteins and give rise to dozens of undesirable compounds, including carcinogenic potential. has been proven in animals " . Epidemiological studies in humans clearly suggest that the longer a meat is broiled, the greater the risk of cancer of the colon, rectum, lung, bladder, breast and prostate.

And the increase in risk is far from negligible. "It rises from 200% to 300% depending on the type of tumors, says Dr. Lecerf.This is probably the reason why a significant consumption of meat is carcinogenic. In terms of health, a very rare steak is better than steak.

But the ideal is boiled meat (the famous stew) rather than grilling or frying.

Barbecuing is even worse, as it induces both heterocyclic amines and aromatic hydrocarbons (eg benzopyrene) due to the release of smoke produced by burning wood or coal.

Without banning totally this mode of cooking, it is better to reserve its use on some festive occasions. But not more than six times a year and respecting certain safety rules.

Diabetes, obesity ... The advanced glycation byproducts involved?

The last major category of inopportune compounds generated by cooking: those resulting from the interaction between sugars and certain amino acids of proteins , glycation reaction known under the name of Maillard.

Acrylamide is one of them. Traditionally used for the manufacture of plastics, its presence has been reported since 2002 in a number of industrial foods made at high temperatures (chips, soluble coffee, biscuits, bread crumbs ...). Contrary to what was initially thought, this substance would be only a little carcinogenic. On the other hand, its accumulation in the body creates a stressoxidant, a chronic inflammatory state favorable to the development of diabetes , obesity, cardio-vascular disorders ...

On the skin, the effect of these "" advanced glycation by-products (AGE) is also catastrophic: by activating a battery of devastating enzymes, they "lead to the destruction of collagen, essential to support the dermis, says the Dr. Serfaty-Lacrosnière, nutritionist. This results in the appearance of fine lines that will soon become wrinkles, accompanying a weakening and aging of the skin.

Our body also makes small quantities of EFAs . And some foods contain naturally (butter, parmesan, avocado ...). But the amount of AGE generated by the cooking is much higher and proportional to the temperature.

A slowly boiled chicken breast thus delivers five times less AGE than the same roast piece in the oven, and nine times less than if it is fried. To swallow as little as possible, overdose of overheated industrial products should be avoided and only mild cooking should be used at 120 ° C maximum. This is enough to rehabilitate the microwave, which usually does not heat above 100 ° C ... and the good old-fashioned dishes simmered.

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Eat raw, good or bad idea?

Foods do not provide the same energy intake if eaten raw or cooked. By improving their digestibility, cooking allows the body to make less effort during digestion, thus burning fewer calories.

The demonstration was made in November 2011. American researchers fed mice with raw ground beef, and others with exactly the same amount of cooked meat.

After a few weeks, the animals in the second group were more plump than those in the first group. Their weight was about 30% higher, without this being explained by a difference in genetics or physical activity. But for all that, "it is better to reduce the rations than to eat raw," says Dr. Jean-Michel Lecerf, "we are really not raw".

Yes to cooking in foil, but without aluminum foil

This method of cooking is ideal for simmering foods in their juices and limit the loss of nutrients.

But beware of aluminum foil that contaminates food , especially in the presence of acidic ingredients. After having been prepared in this way, 100 g of tomatoes can contain up to 5 mg of alumina oxide. Under the effect of heat and acidity, fish cooked in foil with lemon is also riddled with aluminum, soluble in the body, metal involved in many diseases of the digestive tract and in the disease. Alzheimer.

To enjoy the benefits of the papillote without the disadvantages, it is better to use parchment paper.

Grill: how to limit the damage?

The barbecue is one of the pleasures of summer. To reduce its harmfulness, it should be used sparingly and follow a few rules.

• Ensure that food does not come into contact with flames and that grease falling on embers does not emit smoke towards them (vertical barbecues are therefore less harmful than horizontal models).

• Do not use treated wood (vines, old framework ...). Charcoal sold commercially is the least toxic.

• Marinate meat in olive oil with herbs and spices.

• Do not eat the charred parts .

* According to a recent study published in Proceedings of the National Academy of Sciences (April 3, 2012).
** Author of the Secrets of the Anti-inflammatory Diet (published by Albin Michel).