What Makes a Diet Low Carb or Keto is NOT How Much Fat it Contains

Many people assume that a “low carb” or ketogenic (“keto”) diet for weight management and blood sugar reduction requires them to eat a diet high fat — piles of bacon,  bulletproof coffee, fathead pizza and ‘fat bombs’, but this is not true. Not only is eating that way not ideal for many people seeking weight loss — but some actually end up gaining weight doing so.

In fact, what makes a diet “low carb” or “ketogenic” is how much carbohydrate it has, not how much fat it contains.

Defining Low Carb and Keto Diets

Different people define “low carb” or “keto” level of carbohydrates in different ways, but Feinman et al [1] defined three categories of reduced-carbohydrate diets as follows;

(a) very low carbohydrate ketogenic*: carbs limited to 20–50 g per day or < 10% of total energy intake.

Note: What makes a keto diet ketogenic is that that low carbohydrate intake puts people into a state of ketosis, which is a normal physiological state that people go into after an overnight fast. It is where the body burns stored fat for energy, releasing ketones which can be used by the brain and other tissues.

(b) low carbohydrate: carbs limited to < 130 g per day or < 26% of total energy intake.

(c) moderate carbohydrate: carbs limited to 130–225 g per day or 26–45% of total energy intake.

In my clinical practice and on other articles on this web page, I define “low carb” and “ketogenic” as Feinman et al did; with a low carb diet being one that is < 130 g per day and a ketogenic diet as one that is 20–50 g per day.

Protein, Fat and Carbohydrate — are all essential?

Many people assume that people need to eat all three macronutrients; protein fat and carbohydrate but from a dietary point of view, only protein (as specific amino acids) and fat are considered “essential” in the diet. For a nutrient to be “essential”,  it is required to be eaten in food because the body can’t manufacture it.

There are two fats that are considered “essential”; linoleic acid (an omega 6 fat) and alpha-linolenic acid (an omega 3 fat). We need to eat meat and eggs and/or nuts and seeds for the major dietary sources of linoleic acid, as well as nuts such as walnuts and seeds such as flax to get rich sources of alpha-linolenic acid, although many people obtain these from eating chicken, cheese and whole milk [2].

Amino acids are the building blocks of protein, and there are nine amino acids that are considered “essential”; histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine.

Below are the best low carb foods for obtaining each of these[3]  are listed below.

    • The best sources of histidine include pork, beef, lamb, chicken, turkey, fish, dairy such as milk and cheese, nuts, seeds, eggs.
    • Best sources of isoleucine are eggs, turkey, chicken, lamb, cheese and fish and for vegetarians, soy protein and seaweed.
    • Rich sources of leucine include chicken, beef, pork, tuna, milk, cheese, and egg and for vegetarians tofu, legumes and pumpkin or squash seeds.
    • The best sources of lysine include red meat such as beef and lamb, pork, poultry,  some cheeses such as Parmesan, certain fish such a cod and sardine, egg and whey protein isolate, and for vegetarians there is tofu, isolated soy protein and spirulina.
    • Foods high in methionine include , beef, lamb, cheese, turkey, pork, fish, shellfish, eggs, dairy and nuts, and for vegetarians, soy and legumes.
    • High phenylalanine foods include cheese, nuts and seeds, beef, lamb, chicken, pork, fish, eggs and dairy, and for vegetarians soybeans and legumes.
    • Foods rich in threonine foods include lean cuts of beef, pork, chicken, liver, cheese, shellfish, nuts, seeds, and for vegetarians, soy, legumes and lentils. 
    • While many people associate turkey as being high in tryptophan, there are other foods as well. Tryptophan is required for the body to make the neurotransmitter serotonin, so getting enough tryptophan is…pardon the pun, essential. Good sources of tryptophan include salmon, all kinds of poultry (including turkey), egg, milk, nuts and seeds and for vegetarians, soy products.
    • High valine foods include cheese, beef, lamb, chicken, pork, nuts and seeds, and fish, and for vegetarians, soybeans, legumes and mushrooms.

Carbohydrate is Not Essential in the Diet

The Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein and Amino Acids [4] which sets the standard for macronutrient consumption in both the US and Canada states;

The lower limit of dietary carbohydrate compatible with life apparently is zero, provided that adequate amounts of protein and fat are consumed. 

The lower limit of dietary carbohydrate

In short, there is no essential need for dietary carbohydrate provided that “adequate amounts of protein and fat are consumed”The reason carbohydrate is not essential is because the body can make what it needs from dietary protein and fat!

The Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein and Amino Acids (2005) goes on to explain the process;

“In the absence of dietary carbohydrate, de novo synthesis of glucose requires amino acids derived from the hydrolysis of endogenous or dietary protein or glycerol derived from fat. Therefore, the marginal amount of carbohydrate required in the diet in an energy-balanced state is conditional and dependent upon the remaining composition of the diet.”

What this means is that the body will synthesize the little bit of glucose needed by the brain and red blood cells, etc from the protein taken in through the diet —provided it is in adequate amounts, or from glycerol which is formed when fat is broken down. 

I encourage people to target protein first, add low carb veggies and fruit (such as lime, lemon, and a few berries), then add a bit of fat to make things taste good and to provide a source of essential fatty acids. Protein and fat both provide satiety (not feeling hungry) but protein has significantly less calories than fat. If one is aiming for weight loss, adding lots of extra dietary fat (in addition to what comes naturally in protein foods) is counter-productive. That doesn’t mean avoid fat, either. Eat the egg with the yolk and marbling in steak is fine and so is using some fat for cooking food or putting on veggies, but for weight loss, it’s best to avoid extra fat that doesn’t come a good source of protein.

Final Thoughts…

What makes a diet low carb or ketogenic is how much carbohydrate it has, not how much fat it contains; with a low carb diet being one that is < 130 g carbohydrate per day and a ketogenic diet as one that is 20–50 g carbohydrate per day. The amount of fat in the diet does not make it low carb or keto!

While a low carb diet is often called a “LCHF diet” i.e. “low carb high fat”, it is really only “high fat” relative to the recommended American or Canadian diet which is supposed to be < 30% fat.  A low carb diet that targets protein may be as high ~50-55% fat or significantly lower if lower fat protein is chosen — but is no where near the 75% fat of a classic ketogenic diet (KD diet) used for those with epilepsy*, or some fat-based variations low carb diets. It is my experience that many people, especially peri-and post menopausal women do much better on the higher protein version. A Low carb or keto diet is not about eating lots of bacon, avocado, heavy whipping cream or ‘fat bombs’ and “bulletproof” coffee. It is about eating less carbs.

*Addendum (Nov 1, 2019 @ 2:35 pm) It was pointed out to me that the Charlie Foundation uses a Low Glycemic Index Treatment Diet for the management of epilepsy that is closer to 60% fat, which allows for less fat than a strict KD diet.

Yes, we need some essential fat in the form of linoleic acid and alpha-linolenic acid which can easily be obtained by eating meat and eggs, and for vegetarians to eat nuts including walnuts and seeds such as flax seed, but for the most part, eating the foods listed above that are rich in the nine essential amino acids will provide all the essential fat we need.

Update (Nov 3, 2019): The following article outlines which protein foods are best for weight loss on a low carb or ketogenic diet.

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  1. Feinman RD, Pogozelski WK, Astrup A, Bernstein RK, Fine EJ,Westman EC, et al. Dietary Carbohydrate Restriction as the First Approach in Diabetes Management: critical review and evidence base. Nutrition. 2015;31(1):1–13.
  2. National Cancer Institute, Food sources of alpha-linolenic acid (PFA 18:3), Epidemiology and Genomics Research Program, Table 6, https://epi.grants.cancer.gov/diet/foodsources/fatty_acids/table6.html
  3. USDA Food Composition Databases, https://fdc.nal.usda.gov/
  4. National Academies of Sciences, Engineering and Medicine, Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein and Amino Acids, 2005, https://www.nap.edu/catalog/10490/dietary-reference-intakes-for-energy-carbohydrate-fiber-fat-fatty-acids-cholesterol-protein-and-amino-acids

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