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Do Low Carb Diets Work?

by Joseph Whittaker

Dr. Atkin’s Diet Revolution is the best selling diet book of all time, which helped popularise low carb dieting. Many people have had success on low carb diets; but are they better than regular diets? What are the long-term effects of them? And, surely eating all that fat is not good? I will answer all these questions and clear up the confusion around low carb diets.


What is a Low Carb Diet?

A low carb diet is typically 20% carb, 25% protein, 55% fat. However, diets under 35% carbohydrate can be considered low carb. If you are also cutting calories your diet may contain less fat, as you can cut the calories from the fat portion of the diet.

Very low carb diets, under 15%, put the body into a state of ketosis. This is when fatty acids are used in the liver to produce ketone bodies. Ketone bodies serve as a substitute fuel for glucose, which is unavailable due to the low level of dietary carbohydrates. In recent years, very low carb diets, named ketogenic diets, have become popular.

The main low carb foods are meat, fish, poultry, eggs, cheese, nuts, seeds, olives, avocados, and vegetables.

Low carb diets include:

  • Atkins Diet
  • South Beach Diet
  • Paleo Diet
  • Primal Diet (Mark Sisson)
  • Candida and Anti Fungal Diets
  • Bulletproof Diet (Dave Asprey)
  • Ketogenic Diets


Are Low Carb Diets Better Than Low Fat Diets?

The average protein consumption is very stable at 15-20%, whereas high protein diets are normally around 25%. Protein consumption cannot exceed 35% due to ‘rabbit starvation’ (explained here). So after protein intake is set, it is really a question of low carb, high fat or low fat, high carb?

A 2016 meta-analysis on low carb vs. low fat diets concluded:

‘Compared with partcipants on low fat diets, participants on low carb diets experienced a greater reduction in body weight and triglercides’ (1)

This meta-analysis included only randomized controlled trials: the highest quality evidence and studies of at least 6 months duration. The subjects’ diets followed the Atkin’s protocol, which starts off very low carb at 20-30g per day and increases to 60g per day, still very low carb (under 20%). Overall, the low carb dieters lost an average of 7.2kg, which was 2.2kg more than the low fat dieters. However, not all subjects strictly adhered to the diets, in trials where they did adhere subjects lost 12.2kg over 6 months on low carb (2).


Why Do Low Carb Diets Work?

Low carb diets work for one reason: people eat fewer calories. In the meta-analysis, low carb dieters reduced their calories by up to 30% more than low fat dieters. This is due to 3 related reasons:

  1. Low carb diets give you stable blood sugar. In contrast, on a high carb diet, you get a surge in blood sugar after each meal, followed by a dip, which results in you feeling hungry (your body’s signal that you need to boost your blood sugar). After you eat again, the process repeats itself and you go up and down all day. On a low carb diet, you don’t get these peaks and troughs. Your blood sugar remains stable and as a result, your hunger is curbed.
  2. Low carb diets promote satiety. Low carb foods such as meat, fish, and vegetables are some of the most filling foods on the planet. Compare this to low-fat products, aka high sugar foods, which are light and moorish. Taking the fat out of a food reduces its palatability, so manufacturers put in sugar to compensate. This makes them less filling and more flavourful per calorie, resulting in us eating more calories to feel satisfied. Furthermore, most of our favorite calorie dense junk foods (cookies, cakes, sweets) are carb heavy and therefore banned on low carb diets.
  3. Low carb meals keep you fuller for longer. Fat and protein are digested much slower than sugar and carbohydrates. Fat is well known to reduce gastric emptying, which is the rate your stomach releases food into the small intestine (3). Moreover, some satiety hormones such as CCK respond more strongly to fat and protein based meals (4). Both of these points explain why steak and veggies fill you up more than a bowl of cereal (even if the calories are equal).
Low Carb Diets Increase Satiety

Low Carb Diets Increase Satiety


So, Its All About Calories!?

Ultimately yes. However, as the 3 reasons above explain sustaining a reduced calorie intake on a low carb diet, is far easier than on a low fat diet. This is why the majority of studies show low carb diets to be superior. Subjects are largely left alone throughout these studies and have to maintain their diets without supervision. Therefore, the easiest diet to follow wins!

Studies that offer more support and behaviour management programs reduce the differences between diets. For instance, one study included in the meta-analysis provided group treatment sessions once a week for the first 20 weeks and every 2nd week for the next 20 weeks. The low carb group had lost 12.2kg at 6 months, and the low fat group lost 11.3kg, not much of a difference there. This conveys that if you can maintain a low fat diet, it works almost as well. However, left to your own devices a low carb diet is probably better.


Are Low Carb Diets Dangerous? – High Cholesterol!

Ok, so low carb diets help you lose weight, but won’t all that fat clog up my arteries and give me a heart attack!? Well, the meta-analysis discussed earlier commented:

‘participants on low carb diets experienced… a greater increase in HDL-cholesterol and LDL-cholesterol… [and]  that the beneficial changes of low carb diets must be weighed against the possible detrimental effects of increased LDL-cholesterol’

It is true that low carb diets raise LDL cholesterol i.e. bad cholesterol, and low fat diets decrease it. However, that is only one risk factor. Low carb diets also increase HDL cholesterol (good cholesterol), reduce triglycerides, blood pressure and VLDL cholesterol much more than low-fat diets (2). All these changes decrease our risk for heart disease. Moreover, VLDL cholesterol has a stronger association with heart disease than LDL and therefore may be a better predictor (5). Losing weight also decreases your risk for heart disease and many other chronic diseases such as cancer (6). Thus, taking a more holistic view, low carb diets may be better than low fat diets for heart disease risk.

In addition, new evidence has found that in those over 60 years old, high LDL cholesterol has an inverse association with mortality (7). Meaning, the higher your cholesterol, the longer you will live. This makes the rise in LDL cholesterol on low carb diets seem beneficial! However currently, there is little research on the long-term effects of low carb diets, thus staying on a low carb diet for longer than 2 years may be detrimental.

Low Carb Diets Are Healthy!

Low Carb Diets Are Healthy!


Is The ‘Carbs Make You Fat’ Theory Dead?

There is a theory out there, that since carbs elevate insulin they are solely responsible for weight gain. This was popularized by Gary Taubes’ book Good Calories, Bad Calories.

The Theory:

When you eat carbohydrates, your blood sugar rises, this signals the pancreas to release the hormone: insulin.  Insulin signals the liver, muscle and fat cells to uptake glucose from the blood, to reduce the blood sugar. The muscles and liver only have a limited storage capacity for glucose, so once that is met, all the glucose goes into the fat cells, making us gain fat.

When we have low blood sugar levels another hormone glucagon signals the liver to release its stored sugar into the blood and also signals the fat cells to release their lipids for energy.

So when insulin is high, the fat cells fill up with sugar. When insulin is low the fat cells empty for energy to burn.

The main dietary influence on insulin is carbs. When we eat carbs, it prompts the release of insulin, which sends the sugars (the broken down carbs) into our fat cells, making us gain weight.

The Problem:

All of the above is true. However, the problem is: if we eat replace carb calories with fat calories, what happens to those?  Well, they go through a different route of transportation in the body, but any excess is still stored in the fat cells. So it essentially the same, excess fat or carb calories are stored in the fat cells.

Tightly controlled experiments show that the ratio of carbs to fat does not matter in weight loss, only the calories (8).  However after weight loss, a person’s metabolic rate tends to slow down, and the person may regain some of the weight. Low carb diets keep the metabolic rate higher than low fat diets and therefore, makes sustaining weight loss easier (9).

Furthermore, research indicates that low carb diets are better than low fat diets for people who are insulin insensitive (10).  This means they are insensitive to insulin’s effects and therefore cannot store carbohydrates effectively. Insulin insensitivity is associated with weight gain, particularly around the waist.


Key Points

  • Low carb diets are 20-30% carbs, 15-25% protein, 50- 60% fat.
  • Low carb foods include meat, fish, poultry, eggs, cheese, nuts, seeds, and vegetables.
  • Research consistently shows low carb diets are better than low fat diets for weight loss.
  • This is because, low carb foods and meals keep you fuller for longer, therefore you end up eating fewer calories overall.
  • Also, you have to avoid most junk foods, as they are mostly high in sugar.
  • Low fat diets can work but are generally harder to stick to.
  • Low carb diets are safe and do not cause heart disease, however effects over 2 years are relatively unknown.
  • Carbs do not directly cause weight gain. However, some people may ineffectively store carbs, making them gain weight quickly on a high carb diet.


Photo Credit: Full Food,  CC BY-SA 4.0

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