How Long Does It Take Your Body to Start Burning Fat on a Low-Carb Diet?

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Low carb diets can help you shed fat.
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Diets banishing bread, pasta and other starchy foods are popular with good reason — low-carb weight loss often happens faster than weight loss on other slim-down regimens, at least at first. With so much buzz around low carb you'll be hoping for rapid results, but it's best to stay realistic.

Tip

How quickly you start shedding body fat isn’t really affected by the carb content of your diet so much as its overall calorie content. Ultimately, if you create a significant energy deficit by eating fewer calories than your body uses up, you'll start losing weight, usually within a few days.

Water Weight Loss

One reason that you can may see fairly dramatic low-carb weight loss when you initially embark on the likes of a keto or Atkins diet is that you lose a bunch more water weight.

The reason? When you drastically cut carbs, you deplete your body's glycogen supplies (the glucose stored in muscles and liver) very quickly. As explained in this April 2018 review in Nutrition Reviews, this glycogen loss also causes a significant water loss (you go to the bathroom more often), because for each gram of glycogen, there's another 3 grams of water that is associated with it.

Given that the weight of the glycogen in our muscles is typically about 500 grams, this means you could lose 1,500 grams (3.3 pounds) of water along with the depleted glycogen.

Effectively this can give you something like a 48-hour weight loss jump start, which can be a big motivation when you're beginning a new diet regimen. Just be aware that you'll put those fluid pounds straight back on if you subsequently don't stick to your plan and eat high-carbohydrate meals.

Read more: How to Tell if Weight Is Water Weight or Fat Weight

Slow and Steady Wins

What matters most when you're slimming down is that those losses you're seeing on the scale are mostly fat. To ensure that this the case — and that you're not losing excess muscle — gradual weight loss is a good plan, according to the Mayo Clinic. A weight loss of 1 to 2 pounds a week is ideal, and though this may seem frustratingly slow, it's more likely to help you maintain your weight loss in the long term.

One small but informative piece of research is worth a mention, even though it's now a few years old. Published in the April 2011 edition of the International Journal of Sport Nutrition and Exercise Metabolism, this study investigated athletes who were losing weight, one group at the rate of at 0.7 percent body weight per week — equivalent to 1.4 pounds per week for a 200-pound person — and another at twice this rate — equal to 2.8 pounds per week for a 200-pound person.

Both groups kept up a program of resistance exercise, which you'd expect to help preserve muscle mass. The results were very much in support of the slow but steady approach. While both groups lost similar amounts of weight overall, in the the slower weight loss group, this was made up of significant fat loss and some lean muscle gain. In the faster loss group, the loss was made up of both fat and some lean tissue.

Per the Mayo Clinic, if you begin creating a daily 500-calorie deficit today — either by burning 500 calories extra in exercise, by cutting 500 calories from your diet or a combination of the two — you should be about a pound lighter in a week. However, the theory that cutting calories by 3,500 a week will result in a 1-pound weight loss reliably week after week isn't true, especially as time progresses. You need to be prepared for ups, downs and plateaus on your weight loss journey.

Read more: How to Lose Body Fat: The Science-Backed Ways That Actually Work

Low-Carb Fat Loss

The question as to whether low-carb fat loss is more effective than slimming down on a low-fat diet rages back and forth. Despite low-carb diets being a huge craze, it's not clear cut that they are always the best at blitzing fat.

In fact, a September 2015 study in the journal Cell Metabolism, conducted at the National Institute of Diabetes and Digestive and Kidney Diseases, found that cutting dietary fat reduced body fat more than cutting carbs.

In the study, 19 non-diabetic men and women with obesity spent two separate extended visits in a metabolic unit, allowing their food and activity levels to be strictly controlled. On one visit they were fed 30 percent fewer carb calories and on the other they were fed 30 percent fewer fat calories (protein levels stayed the same).

Extrapolating their results into the future, the researchers surmised that when people reduce their calorie intake by the same amount, they will lose quite similar amounts of fat over time — regardless of the ratio of fat or carb in their diet.

Tip

Do as Harvard Health Publishing suggests and add resistance training like weights, press ups and planks to your fitness regimen. If you’d like to lose fat faster, the good news is that this can help, particularly with shedding belly (visceral) fat, which surrounds your internal organs and is particularly unhealthy.

When Low Carb is Best

That said, reduced carb diets do appear to offer benefits to specific groups — if you have blood sugar control issues, it could definitely be the way to go to maintain a healthy weight.

In a November 2018 article in the British Medical Journal, people who had already lost about 12 percent of their starting weight (averaging 25 pounds) were randomly assigned to one of these diet groups:

  • High (60 percent) carbohydrate and low (20 percent) fat
  • Moderate (40 percent) carbohydrate and (moderate) 40 percent) fat
  • Low (20 percent) carbohydrate and high (60 percent) fat diet

What the researchers found was that people could eat slightly more calories and still maintain their weight when the proportion of carbohydrate in their diet was lower. But this was only really marked in individuals who secreted high levels of insulin.

The researchers said this showed a low-carb diet is probably the most effective way to lose or maintain weight if you have insulin resistance or diabetes.

However, a November 2018 editorial in Science suggests that for most people, focusing on diet quality — replacing saturated fats with unsaturated fats and replacing refined carbohydrates with whole grains and non-starchy vegetables — allows effective weight management within a broad range of fat-to-carbohydrate ratios.

Keto Regimens and Fat Loss

A ketogenic diet is about as extreme as you can go with carbohydrate restriction. The premise of the "keto" variety of low-carb weight loss is that if you deprive the body of glucose, an alternative fuel called ketones are produced from stored fat.

As described by the Harvard T.H. Chan School of Public Health, when carbohydrates are slashed on the keto diet — often to as low as 20 grams a day — the body first pulls stored glucose from the stored glycogen in liver and muscles and then, when glucose stores are fully depleted, begins using fat as its primary fuel, while drastically dialing down production of insulin.

When you are in "ketosis," ketones produced from fat in the liver are used to fuel the body and brain instead of glucose. Harvard T.H. Chan School of Public Health lists the following purported theories as to how maintaining ketosis may promote fat loss — though they point out these have not been consistently shown by research.

  • A satiating effect with decreased food cravings due to the high-fat content of the diet.
  • A decrease in appetite-stimulating hormones, such as insulin and ghrelin, when eating restricted amounts of carbohydrate.
  • A direct hunger-reducing role of ketones.
  • Promotion of fat loss over lean body mass, partly due to decreased insulin levels.

The keto diet could potentially work if you have had no luck at all losing weight with other methods, but advice from a health professional such as a dietitian or doctor is recommended. You'll need to take care to avoid a nutritional deficiency, and ideally to keep an eye on your blood work in case of any unfavorable biochemical changes.

Read more: Is Keto the Right Diet for You? Here's What You Should Know

Better to Focus on Health

It's possible to obsess too much about which regimen is going to burn fat fast, when what's more important is the overall healthiness of the diet you choose. Unfortunately, the nutritional value of most low-carb diets, the keto diet in particular, comes up rather lacking, according to the Academy of Nutrition and Dietetics.

Nutrient-rich vegetables and fruits tend to be cut out, which means intakes of vitamin A, C, K and folate usually are low. And removing high-fiber whole grains increases the risk for constipation — keto dieters may even end up requiring fiber supplements to help stay regular, which is not healthy.

The risk that keto and other low-carb/high-fat dieters might be taking with regards to their long-term cardiovascular health is also not fully understood. The USDA suggests limiting saturated fats to less than 10 percent of calories per day — and replacing saturated fats with unsaturated fats — but keto diets, especially those which include a lot of animal fats, could significantly exceed this limit.

Choose What Works for You

It's also very important to find a diet that you can stick to and enjoy. Low-carb weight loss may be where it's at right now, but it's no good for you if you'll be totally miserable without bread or pasta.

The Mayo Clinic says you should avoid gimmicks and evaluate diets carefully to find one that works for you. Need a guide? Here are some of the features to look for:

  • Flexibility: A flexible plan doesn't forbid certain foods or food groups, but instead includes a variety of foods from all the major food groups. It should allow for occasional, reasonable indulgence too, so that you never feel deprived.
  • Balance: Your plan should include adequate nutrients and calories. Eliminating entire food groups (like carbs), can cause nutritional problems. Safe and healthy diets do not require excessive vitamins or supplements.
  • Likeability: A diet should include foods you like, that you would enjoy eating for life — not ones you can tolerate over the course of the plan.
  • Activity: Your regimen should include physical activity, too.

Read more: How to Find the Best Weigh-Loss Diet for You

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