Q: Can I really cut out carbs completely and still exercise at a high level, as some proponents of low-carb and paleo diets suggest?
A: Yes, you could cut out carbs and rely on fats alone for fuel—and it is completely safe. Certain nutrients in your diet are absolutely essential, including a couple different fats, a handful of amino acids, and lots of vitamins and minerals. No sugars or carbohydrates make the “must-eat” list.
In order to function without carbs, your body does a very good job either making the sugars it needs or finding alternate energy sources. For example, when you drastically reduce or eliminate carbs from your diet, your body is able to make sugar to store as glycogen.
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Your brain is notorious for being a sugar glutton, as it requires a lot of energy and sugar is its preferred source. But despite your brain’s love affair with carbohydrates, it is more in love with survival. As a result it adapts and thrives, fueling itself with ketones (a byproduct of excessive fat breakdown) when carbs are not around. In fact, your brain may have switched to this alternate fuel source without you even knowing it if you have ever eaten a very low-carb or ketogenic diet, where you consume 60 to 70 percent of your calories from fat and only 20 to 30 grams (g) of carbs per day (eventually upwards of 50g a day). These diets are very effective for fat loss, reducing certain risk factors for heart disease, and treating diabetes and epilepsy.
So yes, if you wanted to, you could completely cut out carbs, power your body with fats, improve your health, and exercise at a high level. But the question becomes: Do you really need to? From an application standpoint, a very low-carb diet is restrictive when it comes to food choices—20, 30, or even 50g of carbohydrates is not much, and you can only eat so many mushrooms, asparagus, and spinach.
Here's an alternative, more customized approach to carb cutting that will progressively have your body rely more on fats, and then, if need be, almost exclusively on them. I created this “hierarchy of carbohydrates” to provide a user-friendly guide for consuming and restricting carbs based on individual needs.
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This simple hierarchy is based on the fact that since not all carbs are created equal, there is a spectrum in which you can restrict them. Foods at the top of the list are more carb- and calorie-dense while containing fewer nutrients. As you move down the list, foods become less carb- and calorie-dense while containing more nutrients—these are the foods you want to pile on your plate. In other words, consume more spinach (at the bottom in the green vegetable category) than soda (at the top in the added sugar category).
1. Foods containing added sugars
2. Refined grains
3. Whole grains/starches
6. Green vegetables
Try to reduce and/or eliminate foods and drinks from the top two positions, and if you need to further lower your carb (or calorie) intake to elicit greater fat loss and better control blood sugar, then work to reduce and/or eliminate foods in the next group on the list. Adopting this approach to carb restriction will help you focus on more nutrient dense carbs while also getting you to restrict the level of carbs that is appropriate for you and your daily needs.