Many diets from the past two decades cast carbohydrates as hell-spawn sent to fatten you up and destroy you with diabetes. These diets demonised all carbs, from whole grains to fruit and urged you to exorcise them from your life. Some forms of this nutrient deserve that evil reputation, but many don’t. By avoiding bad carbs you can speed weight loss and dodge disease. But beware: shunning good carbs may hurt your health and fitness goals. Here’s how to slice it.

What Carbohydrates Really Are

Like proteins and fats, carbohydrates are macronutrients – in other words, substances that stoke your metabolism and keep you alert and active.

As an analogy, think of carbs as your body’s primary source of crude oil. Through digestion, carbs are transformed into glucose – kind of like high-octane unleaded gas. “Carbohydrates are the only nutrients that exist solely to fuel the body,” says Donald Layman, a professor of human nutrition. Without glucose, your blood oxygen levels suffer, your energy levels tank and your risk of a brain fart rises.You should consume 45 to 65% of your daily calories from carbohydrates, according to the USDA’s most recent, globally recognised recommendation.

So if you’re a moderately active guy consuming 10 760 kilojoules a day, that means 4 840 to 6 690 kJ of that total should come from carbs. And since carbohydrates – whether from sugar, starch or fibre – contain 16 kJ per gram, you should shoot for 295 to 425 grams a day.

This amount, says Layman, will help your brain, blood and nervous systems function at their best.If you keep your intake under 80 grams of carbs a day, as some diet plans suggest, your body will begin breaking down its fat stores to produce ketones, a poor man’s “glucose,” which can lead to that low-carb “cloudy” feeling. Excess dietary carbs become excess body fat. So losing weight and helping your heart is as simple as cutting back on the white bread, right? Read on.

Why You Should Strive for Complexity

The quality of your carbs is as important as the quantity, says Frank Sacks, a nutrition professor at the Harvard T.H. Chan School of Public Health. Complex carbs, found in starchy vegetables and whole grains, are linked to healthier weight and lower risks of type 2 diabetes and heart disease.“Complex carbohydrates are difficult for the body to break down and that’s a good thing,” says Gail Cresci, a researcher in gastroenterology and nutrition at Cleveland Clinic. These carbs digest slowly and the absorption of sugars into your bloodstream is also slower as a result.

The increases in your blood sugar and insulin levels are moderate enough that they don’t reach levels associated with body-fat storage, Cresci says. Plus, they make your good gut bacteria happy. “The gut microbiota preferncomplex carbs over any other food source,” says Cresci. After these buggers feast on the carbs, they send beneficial compounds into your bloodstream.

These compounds, called short-chain fatty acids, may help lower inflammation and strengthen your immune system, says Cresci.

Most foods that contain complex carbs are also high in fibre (another form of carb), which regulates blood sugar and helps you feel full. In a 2015 study, people who were asked to eat 30 grams of fibre a day on top of their normal diet lost about as much weight as folks on a strict diet.

How the Wrong Kinds of Carbs Can Hurt You

Refined carbs, such as those in white bread, ncookies and chips, have the opposite effect that complex, unrefined carbs have. After you eat, say, a doughnut, your blood sugar rises; in response, your insulin levels skyrocket. Plus, your gut bacteria spit out inflammatory compounds, says Cresci. So a daily doughnut sets you up for metabolic malfunction, elevated triglycerides, obesity and the chronic diseases that accompany them. Cutting out those breakfast doughnuts may help you erase the one encircling your waistline.

And yes, if you eliminate almost all carbs from your diet, you’ll drop even more weight – but not for the reason you might think. On a low-carb diet, your body churns through its stores of muscle glycogen. And for every bit of muscle glycogen you burn, your body releases twice as much H2O, Cresci says. So initially you’ll lose weight, but it will be more from water than from body fat.To effectively reverse the weight gain associated with the intake of simple carbs, you have to shop smart. Beware of products that market themselves as low-fat. When food producers remove fat from cookies, ice cream, yoghurt or salad dressings, they often replace the lost flavour with processed sugar (yes, sugar is a carb), which is more easily converted into body fat than unprocessed carbs, Cresci says.

And don’t let the “gluten free” trend hook you in: many gluten-free pastas, breads and baked goods contain more sugar and calories than their conventional counterparts. In fact, a 2014 review published in the Journal of Medicinal Food concluded that people who followed a gluten-free diet had a greater risk of obesity later in life than those who didn’t. So unless you’re among the very small minority of Americans who have celiac disease or known sensitivity, there’s probably no health reason for you to cut out gluten-containing whole grains like wheat, barley and rye.

And don’t forget: gluten is a form of protein, your muscles’ friend.A general rule: the fewer ingredients in the food you’re buying, the better. Choose apples, not low-fat, gluten-free, cinnamon-sugar apple chips. While fruit does contain simple carbs, those carbs come with plenty of fibre. In fact, a 2013 BMJ study found that fibre from fruit may reduce your risk of cardiovascular disease. “Anyone who cuts down on fruit to reduce their sugar intake is making a big mistake,” says Dr Sacks.

How Carbs Fuel Your Fitness

Carbohydrates are stored as glycogen in your muscles and liver and also serve as fuel for high-intensity and endurance exercise. If your fitness regimen is intense (let’s say you’re training for a marathon) you need 40 to 60 additional grams of carbs per hour of exercise to help your performance, says Stuart Galloway, who studies exercise metabolism at the University of Stirling, Scotland. Another way to think about this: one extra gram of carbs per minute you work out.

According to 2013 research in the journal Sports Medicine, carbs boost performance during endurance and high-intensity workouts. Better performances burn more calories.And while carb loading before a race is a must, no evidence suggests that switching between high- and low-carb days (or “carb cycling”) helps performance. Some experts say it may even hurt your health. Shifting between low and high insulin levels can cause low-grade inflammation, says Cresci.

After your workout, you need to consume just as much carbs as protein – ideally more. That’s because insulin may help with protein synthesis and muscle building, a 2015 study in the Journal of the International Society of Sports Nutrition suggests. Aim for a 1:1 or 2:1 carb-to-protein ratio after you hit the gym. Lift some chocolate milk, apple slices with almond butter, or pita and hummus.So here’s your prescription: eat a consistent amount of complex carbs every day (unless you’re running a marathon or doing some similar activity the next day) from the greatest variety of whole sources possible. Then enjoy all the disease-fighting, gut-filling benefits.