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Hardly a day goes by without one hearing some piece of diet advice, be it sound, questionable or just plain quackery. We’re surrounded by nutrition conversations – on social media, in magazines and books, on the news, at the gym, even at the hair salon.

With so many trends, fads, opinions and "experts" out there, it's easy to get confused – and become frustrated. Sometimes it's hard to know just what to eat. (To be fair, even legitimate nutrition experts differ in opinion, often the result of evolving, back-and-forth science.)

This week, I'm taking a science-based look at six nutrition beliefs that keep bouncing around. Chances are, you've heard a few of them and, perhaps, even based your diet around one or two.

Coconut oil is more nutritious than other cooking oils

Often dubbed the "world's healthiest oil," coconut oil is touted to boost brain health, speed weight loss, improve complexion, prevent heart attack and a whole lot more. Yet there's scant, if any, evidence that coconut oil does any of these things.

When it comes to nutritional value, 82 per cent of the fat in coconut oil is saturated, the type the raises LDL (bad) blood cholesterol. However, studies suggest it doesn't increase blood cholesterol to the same extent that butter does. And it may even boost HDL (good) blood cholesterol.

Coconut oil also contains medium chain triglycerides, fats that are metabolized rapidly and not stored in fat cells as are other fats.

Even so, coconut oil is not more nutritious than unsaturated cooking oils. It lacks alpha linolenic acid, an omega-3 fatty acid found in canola, walnut and flax oils.

It's also void of vitamin E, a nutrient that protects brain cells and boosts the immune system. Excellent sources of vitamin E include sunflower, safflower, grapeseed, wheat germ and almond oils.

Cook with virgin (cold-pressed) coconut oil if you like (I do for some dishes). But don't use it at the exclusion of other oils just because you've heard it's more nutritious.

Raw foods improve digestion by preserving enzymes

Advocates of a raw food diet claim that cooking kills natural enzymes in foods. That's a bad thing, they say, since these natural enzymes boost digestion and guard against disease.

A raw food diet is entirely or predominantly plant-based, including fruits, vegetables, nuts, seeds and grains in their uncooked forms. Healthy, nutritious foods, to be sure.

Yes, it's true that cooking foods destroys plant enzymes. But so does the human digestive process. Enzymes in raw plant foods are denatured and deactivated by hydrochloric acid in the stomach.

Such enzymes are present for plants to survive, not humans. They're needed for photosynthesis, the process by which plants convert sunlight into energy for growth.

Raw foods are good for you, but not because of their enzymes. Raw broccoli and cauliflower, for instance, contain more anti-cancer glucosinolates (phytochemicals) than when they're cooked.

Bottom line: You don't need to think twice about heating your food.

Pitch foods as soon as best-before dates have expired

Not so fast. Best-before dates refer to quality, not safety. They indicate how long a product will retain its optimum freshness, flavour and texture. (Once you open a food package, best-before dates no longer apply.)

Many foods, though, are perfectly safe to eat soon after their best-before dates have passed, provided they've been stored properly.

That's true for milk, yogurt, eggs, many condiments and pantry stables. Milk, for example, is safe to drink five to seven days after the best-before date.

A food just past its best-before date, though, may not taste as fresh.

Microwaving veggies zaps their vitamins

Not true, unless you cook vegetables on high power in lots of water, and/or for too long.

Water, temperature and time (e.g. prolonged cooking) are nutrients' enemies. That's true whether you microwave, boil, braise, steam or grill your veggies.

Nutrients especially sensitive to heat and water loss are thiamin (B1), folate and vitamin C.

Since microwaving cooks vegetables quickly, and uses very little water, it does a good job of retaining nutrients.

So, if you're short on time, the microwave is a convenient alternative to the steamer basket.

Drinking milk causes mucus

Don't drink milk if you've caught a cold or have asthma is often-heard advice. The reason: Dairy increases phlegm production.

Whether it does, though, remains to be proven.

One study, conducted among 60 cold sufferers, found no link between dairy intake and nasal secretions or congestion. A number of studies have also turned up no convincing association between milk intake and worsening of asthma symptoms.

Some scientists contend that a certain protein, beta-casomorphin-7, derived from the breakdown of milk in the gut, may increase mucus production in sensitive people.

The amount of beta-casomorphin-7 in milk varies depending on factors such as breed of the cow and stage of lactation. No studies have examined if milk with different protein compositions affect mucus production and asthma.

Still, there's a reason why some people might think dairy causes mucus. Higher-fat milk, often consumed by young children, can cause existing phlegm to thicken, making it more irritating.

My advice: If avoiding dairy products helps you feel better, that's great. But be sure to get your calcium from other sources (especially important for growing kids).

Drinking water makes bloating worse

Not necessarily. Drinking more water can actually help ease bloating. First, though, let's define bloating.

Stomach bloating, or distention, happens when your belly feels inflated. Overeating, drinking too much water on a full stomach, and certain medical conditions that delay stomach emptying can cause, sometimes painful, stomach distention.

In these cases, it is prudent to drink water slowly, or to drink water between, rather than with, meals.

In my private practice, however, most clients complain of uncomfortable bloating in their lower intestine or colon, not their stomach. This bloating is usually a side effect of constipation or poorly absorbed carbohydrates called FODMAPs. (FODMAP sensitivity is not affected by drinking water.)

Drinking enough water is key to preventing constipation. Fibre in foods absorbs water and, in so doing, increases stool bulk and increases gut motility.

Women are advised to drink nine cups (2.2 litres) of water a day; 12 cups (3 litres) for men.

Leslie Beck, a registered dietitian, is based at the Medisys clinic in Toronto.

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