Contrary to popular belief, carbohydrates are not readily converted to body fat. The big problem with excess carbs is the suppression of fat oxidation.
Bodybuilders have been drilled, seemingly since the Big Bang began the Universe, with the theory that carbohydrates will make us fat. Indeed, almost overnight, we all became anti-carbohydrate. But, guess what, even after jettisoning a lot of carbs, most of you are probably still fatter than you’d like to be – you know, a jiggle here, a roll there. The truth of the matter is that your low-carbohydrate diet probably isn’t working, not as well as it could be anyway. You know that you are just not peeling fat off your body as quickly as you can. I say that you can fix your diet and begin melting away body fat and revealing the lean, hard body underneath today!
THE GLUCOSE ECONOMY
I have written about this extensively before. Your body has an economy based on fuel called glucose (a.k.a. ‘blood sugar’). Strict management of the Glucose Economy (your body’s total supply of glucose) is metabolic priority #1, because running out of this preferred energy source could have dire consequences. As much as we blame glucose and other carbohydrate for our body fat woes, glucose is high-man on the totem pole of fuels ‘burned’, or oxidized, by your body. The more glucose you consume (normally in the form of sugars and starches), the more glucose your body burns. However, the caveat is — the more glucose in, the less fat your body burns!
Conversely, as your Glucose Economy shrinks (e.g., fasting between meals, overnight, during exercise), the fuel mixture shifts: you burn progressively less glucose and more fat.
Compared to all that fat you carry, your glucose economy is small5. Normally only about 4 grams of glucose, less than a teaspoonful, float around in your blood. Much more is stored in the form of glycogen in your liver and skeletal muscles. Your glycogen storage capacity is limited, unlike your virtually infinite capacity to store fat.
Rarely will your blood glucose level go up by more than another 1.5 – 2 grams (1/2 a teaspoon), and even then, only briefly. Within an hour of eating, it is generally back to normal. This tight control becomes even more evident when you realize that a typical meal dumps 50 grams to 150 grams or more of glucose into your body.
INSULIN-THE KING OF ENERGY STORAGE.
Management of your glucose economy is achieved in part by the hormone known as insulin, the Caesar of energy storage. Insulin is released from your pancreas in response to nutrients (most notably, glucose) and increases glucose transport into muscle cells. Insulin also stimulates the oxidation of glucose and its storage as glycogen.
Whereas blood glucose levels normally come down fairly quickly after a meal, insulin is a little more sluggish. Even after eating a small portion of carbohydrate, with blood glucose levels increasing only slightly, insulin levels can rise dramatically. They can take more than 2 hours to fall back to normal. The more carbohydrate ingested, the greater tends to be this effect.
In a 1993 study, when healthy people ate a meal providing just under 300 grams of carbohydrate, insulin levels were still 300% above normal 7 hours later13. Just small increases in blood glucose can cause dramatic and long lasting increases in insulin levels.
CARBS CONVERTED TO FAT?
There is an overly simplistic perception that sugars and starches (the form of carbohydrate most commonly eaten) are all bad. But this isn’t entirely true. Practically all of the carbohydrate you eat ends up going down two roads: (1) the glucose it provides is burned, or oxidized, as fuel and (2) the glucose is stored for later as glycogen1.
Did you know that even when healthy people eat an unusually large serving of carbohydrate – nearly 500 grams – most of it gets stored as glycogen and the rest is burned as fuel1,6,13. In fact, the more carbohydrate you eat, the greater tends to be the rate at which you oxidize glucose and store it as glycogen6.
Contrary to popular belief, dietary carbohydrate tends not to be converted into fat1,6. It’s not surprising that your body resists carrying out this conversion. For one thing, it’s an energy-expensive process6, for another, it’s a waste of your body’s most important fuel glucose. You see, once glucose is converted into fat, humans cannot reverse the process.
True, if you consume very large quantities of carbohydrate day after day, and fail to burn it up with exercise, you’ll push your glycogen storage capacities to their limit. The more muscle you carry, and the more you exercise, the greater your glycogen storage capacity. Some carbohydrates will end up as body fat. Compared to your virtually infinite capacity to store fat, your glycogen storage capacity is very limited5,6. However, this isn’t the problem.
But, we know that carbohydrate tends to increase insulin levels more than fat or protein. As sugars and starches in the food you eat get digested in your gut and absorbed into the body as glucose, blood glucose and insulin levels rise. The insulin level rises quite a bit higher than glucose, and takes longer to return to normal.
We also know that eating carbohydrate stimulates the oxidation of glucose and its storage as glycogen1. This is due to glucose per se, as well as insulin, the level of which increases markedly as you eat more carbohydrate. Said another way, glucose and insulin shift your body’s fuel mixture in favor of the oxidation of glucose6 at the same time that they suppress the oxidation of fat9.
Insulin’s suppressive effect on your fat-burning rate is powerful. Insulin levels well below that found following most meals reduce the use of fat as fuel by 50%4,7,12.
Concerning carbohydrate calories, arguably the most important factor determining your ability to lose body fat is the quantity of carbohydrate you eat. The more carbohydrate you eat, the faster you oxidize glucose, and the slower you oxidize fat. If glucose is lower, your body works down the totem pole of fuel oxidation. The body burns progressively more fat in order to spare the small amount of glucose that is available.
In essence, this is why eating carbohydrates can make you fat.
The problem is not that carbs are converted into fat. Rather, it is that eating carbs ‘revs’ your glucose-burning engines, at the same time that it applies the ‘brakes’ on fat burning. This does increase the risk that even a small amount of fat eaten along side carbohydrate will be stored as body fat.
Interestingly, fat only very weakly leverages its own use as fuel. That is, eating more fat does not substantially increase fat burning but causes more to be stored6.
MAKE A LOW CARB DIET BETTER? HOW?
A zero-carbohydrate diet, (one in which no sugars or starches are eaten) is the most extreme dietary approach to threatening your glucose economy. Yes – this would stimulate your body to burn more fat which your muscle and other tissues can rely heavily on for fuel. But then, where do we get glucose? From the building blocks of protein, amino acids!
Certain amino acids in the protein you eat, and in the protein that makes up your tissues (e.g., your hard-earned muscle), can be converted into glucose, as well as ketones8. Ketones are another type of fuel source. The body can also manufacture ketones from fat. This process of forming ‘new’ glucose from non-carbohydrate sources is termed gluconeogenesis.
Fat and ketones serve as alternative fuel sources, thereby sparing glucose. In fact, the oxidation of fat and ketones provides the energy required to drive gluconeogenesis8,10. So, the burning of fat and the use of amino acids as an expensive source of glucose (via gluconeogenesis) go hand-in-hand.
A) Simply stated, to lose body fat faster, you’ve got to reduce the overall supply of glucose in your body – from glycogen stores (in your liver and muscle tissues) and from blood glucose. You can reduce your intake of carbohydrates and/or perform exercise. Intense workouts (e.g., hard running, step-ups, spinning classes, and pumping iron) generally burn the most glycogen. By keeping your muscles less than fully loaded with sugars, and by exercising, you increase your fat-burning rate.
B) As you eat less carbohydrate, more amino acids (derived from the breakdown of tissue and food protein) turn into glucose. You require more protein to avoid losing your hard-earned muscle. To protect your muscle, eat protein that can suppress the breakdown of your own tissue protein. The milk protein, casein, has been found to be superior to whey for reducing protein breakdown, and increasing protein gain3. Whey, though, stimulates protein synthesis best.
C) Extra protein will help you burn fat. When Forslund et al.  fed healthy people high-protein diets that were lower in carbohydrate, they burned more fat at rest and during exercise than did a group eating protein, but more carbohydrate.
D) How much fat you burn over the course of a day is a function of many factors beyond carbohydrate intake. These include: how many calories you burn each day, the type and amount of exercise you perform and your body composition (% fat vs. % muscle). When it comes to carbohydrate and body fat, it’s mostly quantity and not the type that matters most.
The size of your Glucose Economy from one moment to the next is what determines the speed of your fat-burning engines.