Low Carb diets for fat loss - Science says you (probably) don't need them.

A simple to understand article outlining the current scientific research on low-carb diets & fat loss.

I’d say that every person on the planet who has every ventured in to the world of weight loss has encountered the idea of a “low-carb diet”.

We have, mainly through crappy mainstream media, deduced that carbohydrates are the cause of fat gain and limiting them (carbs) is the recipe for fat loss.

I’m here to spoil the party and tell you that this isn’t the case.

If you place a whole lot of trust in me, you could leave this page now – but hopefully you can manage another 5 minutes whilst I try and convince you some more.


Take Home Points:

Energy Balance (calories in vs calories out) is the primary dictator of fat loss/gain.
In other words, regardless of carbohydrate intake – fat loss is dictated by the degree of caloric deficit.

A hugely important part of successful fat loss comes from increased protein intakes.
This is because protein promotes lean mass retention/gain, appetite suppression & has a higher thermic effect.

Low Carb diets tend to cause caloric reductions when people do not monitor their intakes.
This is largely due to habitual protein increases.

The caloric drop & increased appetite suppression associated with low carb diets cause successful fat loss – which people misinterpret as being directly caused by low carbohydrates.
In actual fact, this fat loss can be explained almost entirely by reduced calories and increased protein.

Adherence to a calorie deficit long-term is key. Thus, the individual should eat in a way that they can sustain & adhere to long-term. If this means low-carb, then by all means employ it.
However if you enjoy carbohydrates (most people do), they can/should still be consumed provided a protein dense, caloric deficit is achieved.

The “Carbohydrate-Insulin Hypothesis”.

“Erm, I thought this was going to be simple?”

Look, this is gonna be real easy, okay?

Carbohydrates, when consumed, raise levels of insulin in the body.

When insulin levels are elevated, fat oxidation (i.e. actually using fat for fuel) is reduced/halted.

This sounds kinda bad, right?

If we limit our consumption of carbohydrates then, we can keep levels of circulating insulin lower, and allow fat oxidation to occur. Sounds great to me.

The above is essentially the carbohydrate-insulin hypothesis, or CIH for short (except I’ve over-simplified a little, which may offend some of its’ proponents – I’m sorry, kinda).

But it is not this black and white, so pick up the piece of toast you just put down and keep on enjoying it (because I sure don’t want to see it go to waste).

The elephant in the room – CICO.

CICO – Short for: “Calories in, calories out”.

You see, this is the fundemental principle of weight loss/gain. It’s more formally referred to as energy balance (energy = calories).

Weight maintenance: Energy Balance = Calories in = Calories out. 

Weight gain: Energy (calorie) Surplus = Calories in > Calories out.

Weight loss: Energy (calorie) Deficit = Calories < Calories out.

If the CIH (Carbohydrate Insulin Hypothesis) were true, then we would be able to lose weight in a calorie surplus, providing that carbohydrates were low (to keep insulin levels low).

We would also be able to gain weight in a calorie deficit if carbohydrates were too high (due to insulin secretion).

This is not the case.

Let’s have a look at our first paper of the day (woohoo!).

The paper, titled “Obesity Energetics: Body Weight Regulation and the Effects of Diet Composition” by Hall KD & Guo J was published in 2017, designed to look at the relevant available literature to compare different diets (i.e. low fat, low carb etc) and how they bodyweight manipulation [1].

Below is a nice image showing the pooled effect of these studies (if you don’t have a clue what you’re looking at, scroll down and I’ll interpret as simply as possible for you).

Also, before I go on (this is important, as you will later realise): this analysis only considered studies where protein & caloric intakes were matched/controlled for.

Here we have 20 studies showing that the difference between a low carb and low fat diet for fat loss is largely insiginificant, but with a slight favourability towards low fat diets (not low carb).

Remember this is not a single study, this is a meta analysis involving only calorie & protein-matched, controlled feeding studies – meaning we can remove a lot of uncertainty and ambiguity that are seen in other papers.

To conclude a little more concretely, directly from the paper:

“These results are in the opposite direction to the predictions of the carbohydrate-insulin model, but the effect sizes are so small as to be physiologically meaningless.

In other words, for all practical purposes “a calorie is a calorie” when it comes to body fat and energy expenditure differences between controlled isocaloric diets varying in the ratio of carbohydrate to fat.”

Basically, CICO is the predominant bodyweight regulator and not the CIH – fat loss occurs through a calorie deficit and not via limiting carbohydrates/insulin.

What am I saying, and what am I not saying?

What I am saying:

Calorie intake & calorie expenditure is the single most important factor(s) regarding bodyweight changes.

Whenever you’re deciding to change the way you look, so weight loss/gain – you should begin with ensuring you’re placing yourself in a calorie deficit/surplus.

What I am not saying:

  • Low fat diets are better than low carb and you should do them.
  • Nothing else matters except for calorie intake.
  • Low carb diets have no utility whatsoever.
  • All calories are created completely equal.

If you just wanted to get a really quick idea on whether or not low-carb diets were superior for fat loss, you could probably click away now.

Calories are king and carbohydrate intake is largely an afterthought within this. You can calculate the calories you need per day using my free calculator here.

However, I really haven’t painted the whole picture yet, so I do hope you can stick around a little longer. It may get slightly more technical, but I’m going to go step by step.

Below is an infographic from my instagram – be sure to follow me there for more informative content: @lukeknightspt.

“But low-carb causes more weight loss?” – What the researchers aren’t telling you.

Want a confession?

There’s quite a few studies out there that show better weight loss from a low-carb dietary approach compared to some other approach.

However, there’s some key things to note that are happening “behind the scenes” in a lot of these studies that researchers have pointed out.

Let’s talk through some of them that you should be aware of:

1. Appetite suppression (and calorie reduction)

Remember how calories in vs calories out is the key mechanism behind weight changes? Well low carb diets tend to cause people to feel more full & consume less calories.

So then a study may compare low-carb to some other diet, see more weight loss in the low-carb group, to which you then conclude that the low-carb diet caused more fat loss.

It didn’t truly cause this – it caused them to feel more full, which caused them to consume less calories, which caused them to lose more weight.

I hope you appreciate that there’s a big difference here, in terms of actual physiology.

Shall we have a look at what I’m talking about?

“Yes please I beg you!”

Thank you.

Below shows a forest plot from a paper titled Do ketogenic diets really suppress appetite? A systematic review and meta-analysis.” from Gibson et al. (2014) [2].

What you’re seeing here is essentially a compilation of 2-3 studies which looked at the impact of ketogenic diets (very low carb, high fat) diets on fullness, hunger & desire to eat.

Fullness increased.

Hunger decreased.

Desire to eat decreased.

All extremely favourable when dieting – all very likely to lead to a caloric reduction.

Do you now understand how a low carbohydrate diet may induce weight loss? Not magic – just less calories.

This is why, when trying to see if there is an advantage of low-carbohydrates, we need to match calories between groups. Otherwise, the lower-carbohydrate group simply eat less calories and therefore lose more weight (via their increased calorie deficit).

2. Increased protein intake.

There’s a few things we know about protein.

  • It helps build & retain lean mass (aka: gainz).
  • It has a higher thermic effect than both fats & carbohydrates (it takes more energy to metabolise).
  • It is the most filling/satiating macronutrient.

What’s the point of talking about this, you ask?

Most of the time, those on a low carbohydrate diet will habitually eat more protein, simply due to the food sources they now have available (and unavailable) to them.

So therefore, separating the effects of a lower carbohydrate approach and a higher protein approach can sometimes be difficult.

Basically: if we know for sure protein does these things, how can we be sure that it’s not the protein aiding fat loss, rather than the lower carbohydrates?

Just to carry on trying to science you a little (so you know I’m not just lying to you):


Protein is the most satiating macronutrient, meaning it makes you feel the most full (we will not discuss the mechanisms behind this, but the result is well established) [3].

An example of this is a study by Weigle et al. (2005) where participants who were placed on an ad-libitum (not calorie restricted, eat as desired), 30% protein diet found their average caloric intake dropped by 441 calories per day [4].

This is important because if low-carb dieters are habitually eating more protein, this is going to contribute to the appetite reductions we spoke about earlier.

Increased thermic effect of feeding (TEF):

Think of the TEF simply as the amount of calories you burn per day, just by digesting & processing the nutrients (food & drink) you consume.

“In general, dietary protein requires 20–30% of its usable energy to be expended for metabolism and/or storage, whereas carbohydrates require 5–10% and dietary fats require 0–3%” – Taken directly from a paper published in the American Journal of Clinical Nutrition titled; “The Role of Protein in Weight Loss and Maintenance” [5].

Protein quite literally requires more energy (calories) just to be metabolised.

So if you’re increasing your protein intake in place of either fats or carbs (in this case, carbs) – you’ll see some kind of “metabolic advantage”.

For those confused – think of it this way:
If you consume 100 calories of protein, the “net” calories (after the TEF) would be around 70-80, whereas 100 calories of carbs or fats would end up around 90-95 or 97-100 calories, respectively.

A win for protein.

Again, not truly a low-carb benefit (if protein was controlled for).

More muscle, less fat:

Something that most people are aware of & do not dispute – that an increased protein intake is useful for both building muscle & retaining muscle when calorie restricted.

Your muscles are essentially just proteins (actin & myosin) – so it seems logical that a diet plentiful in protein will be useful for anybody with muscle/strength related goals.

This study from Layman et al. (2009) took 2 groups of people (obese adults), both consuming the same amount of calories.

One group (group A) consumed 0.8g of protein per/kg (the RDA), whilst group B consumed 1.6g per/kg. 

At 4 months, group B (higher protein) had lost 22% more fat mass, but weight loss was the same (as they had lost less lean mass than group A/gained some lean mass) [6].

A similar study in adult women from Laymen et al. (2005) showed similar, favourable results [7].

A randomised trial from Longland et al. (2016) found that after a 4 week isocaloric (calories matched between groups) intervention, participants consuming 2.4g per/kg of protein saw increased fat loss & lean mass gain compared to the group consuming 1.2g per/kg [8].

Basically, another win for higher protein (and not the low-carbs) because those that increase their protein see more fat loss and less muscle loss (maybe even muscle gain).

3. Weight & fat are not the same.

Weight loss isn’t fat loss.

Fat loss is a component of weight loss – but you can lose muscle, water, glycogen, heck you can even cut your foot off and lose weight. None of those are desirable though.

What we’re all interested in, is fat loss.


If you keep your calories completely identical right now, but slash your carbs to zero (and eat more fat) – you may not lose any fat, but you’ll definitely lose some water.

This is quite a known effect of carbohydrate reduction, and can be a contributing factor to why people/studies show solid short-term weight loss compared to other dietary interventions.

They haven’t lost extra fat, just water.

Carbohydrate reduction also will lower levels of stored glycogen, over time.

Another loss of weight that isn’t actually fat.

I was going to dig in to some literature here, but I don’t think it’s necessary. Nobody is really contending this idea anyway, I just think it’s important for people to note that Person X going on a low-carb diet and losing 6lbs in week 1 did not lose 6lbs of fat.

So low-carb diets are a complete waste of time then?

Whilst I am not an active proponent of low-carbohydrate diets, I do accept their utility (if properly understood).

Here’s what I don’t like (“myths”):

  1. You can eat as much fat as you want on a very low carbohydrate diet, without getting fat.  
  2. You are now burning fat as fuel, rather than carbohydrate – therefore you’ll essentially be a fat burning machine.  
  3. Insulin is the key fat storage hormone, so keeping insulin levels low(er) will mean less fat gain/more fat loss.  
  4. Low carbohydrate diets offer a metabolic advantage compared to conventional diets.

Commonly touted benefits which aren’t true in the real world.

Here’s what I DO like (“reality”):

  1. Low-carbohydrate/ketogenic diets have appetite suppressive effects, making it easier to eat lower calories.
  2. Higher protein intakes play a large role in aiding satiety, fat loss & muscle gain/retention. 
  3. Lower carbohydrate diets remove hyper-palatable foods from the diet, potentially reducing the urge to binge.
  4. Low-carbohydrate diets are very successful for those eating ad-libitum (as they please), short-term.

They are definitely not a complete waste of time.

In fact, there will be some individuals out there whereby a low-carb diet is actually very well suited to them.

But this will almost certainly be a minority of people – which is why I don’t like low-carb being a one-size-fits-all prescription.

Fat loss is about long-term adherence to a calorie deficit.

Read that again, seriously.

If there is anything about your diet that you find a struggle each day (most people like carbs), you’re probably going to find yourself screwing up and/or not sticking it out long-term.

Low-carb works great short-term, calories are slashed, hunger is down.

But short-term isn’t real life…

Suddenly you’re invited out for dinner and you just want to eat like a normal person.

You want to go out for drinks with a few friends.

You want to simply eat a piece of cake for lunch at the weekend.

Restriction isn’t fun, and that’s successful fat loss endeavours are about finding the least restrictive way possible to employ a calorie deficit.

Don’t become part of the restrict-binge cycle. It’s not healthy. You can eat whatever you like.

A quick summary (before more research).

If you came here for some practical, take-home points about low-carb diets and fat loss, here’s what you need to know (and you may leave after).

  1. Fat loss is due to a calorie deficit.
  2. Carbohydrate consumption does not affect fat loss, if calories are controlled.
  3. You do not need to go low-carb just to be in a calorie deficit (but you may choose to).
  4. Higher protein intakes improve satiety, muscle retention & fat loss.
  5. Low-carb diets are good appetite suppressants (combined with high protein) in the short-term.
  6. The fat loss recipe: Calorie deficit + Increased protein.
    Whichever diet you enjoy the most that does this, is the best long-term diet. Fact.

Honestly, adherence is king and research 100% agrees with this. If you find a way of eating that you can stick to for the long-haul, that is the best “diet” for you.

As far as physiology goes, you simply need a deficit of calories and plenty of protein (whilst not feeling overly hungry). Not that complicated.

Keep it simple.

Should you go low-carb?

You don’t need to (just established), but if you want to eat ad libitum for a short period of time and (hopefully) drop weight – a low carbohydrate diet may help reduce your calories and increase your protein intake, whilst reducing hunger.

If you’re willing to monitor your energy/protein intake (even slightly), you can likely consume a more moderate carbohydrate diet and enjoy great success with better long term adherence.

Okay, let’s dig in to a little more research – for completeness.

Right, let’s do this.

Very-low-carbohydrate ketogenic diet v. low-fat diet for long-term weight loss: a meta-analysis of randomised controlled trials (Bueno et al., 2013) [9].

They concluded that the VLCKD (very low carbohydrate ketogenic diet) resulted in slightly greater weight loss compared to the low-fat group(s).

The authors noted that although the effect was statistically significant, the practical significance was low.

Below is the plot of results for bodyweight.

But what else should we point out here?

Protein has not been controlled for between these 2 groups.

That alone could cause the difference in weight loss, as we know.

In fact, the comparison between groups is not even isoenergetic (the inclusion criteria was based around the content of fats vs carbohydrates, no mention of calories that I can see).

So whilst this analysis is very good at showing that VLCKD are good for weight loss (which I’ve already agreed upon), it again appears that this mechanism is likely via increased protein and reduced calories due to improved satiety & TEF (thermic effect of food).

Okay, another paper.

Low carbohydrate versus isoenergetic balanced diets for reducing weight and cardiovascular risk: a systematic review and meta-analysis (Naude et al., 2014) [10].

Right off the bat, this paper is only examining isoenergetic diets (aka no caloric difference between groups).

This is a massively important variable because now we can isolate the actual impact of reduced carbohydrates with more ease.

Below are the data plots from 3-6 months post intervention.

Marginally more weight loss was seen in the low CHO (carbohydrate) group, however we are literally talking an average of 0.7kg difference over 3-6 months.

And when you consider that protein intake was higher in the low CHO groups (fact), and that the low CHO groups likely lost some water/glycogen – well that easily could explain that small weight difference (and then some).

Concluding remarks from this paper:

“Trials show weight loss in the short-term irrespective of whether the diet is low CHO or balanced in terms of its macronutrient composition.”

In fact I would like to highlight another paragraph from this paper:

“Participants lost weight in both groups, with similar before and after average loss after 3–6 months, and 1–2 years of follow-up. There was little or no difference in weight loss and change in BMI between the low CHO and balanced weight loss diets in the two follow-up periods. The similar reported mean energy intakes in the low CHO and balanced diet groups and the corresponding similar average weight loss in the diet groups supports the fundamental physiologic principle of energy balance, namely that a sustained energy deficit results in weight loss regardless of macronutrient composition of the diet.

CICO, mate.

What about the paper we analysed at the very beginning?

Obesity Energetics: Body Weight Regulation and the Effects of Diet Composition by Hall KD & Guo J, 2017 [1].

We saw in that paper that there was no favourability towards a low carbohydrate diet what-so-ever.

Remember the beauty of that analysis? They controlled for protein and calories. All studies were controlled feeding studies.

This really does help prove that there just is no real, apparent metabolic advantage to low carbohydrate diets – at least nothing meaningful anyway.

Metabolic advantages from diet alone likely come from increased protein intakes.

Summarising – how to create the perfect diet:

If you could create a diet that you could adhere to long-term, that placed you in a calorie deficit and provided you with an adequate amount of protein – that is the perfect diet.

This may be a ketogenic diet.

This may be a conventional diet.

This may be an intermittent fasting based conventional diet.

This may be an intermittent fasting based ketogenic diet.

This may be a lower fat diet.

Okay I’ll stop. It’s largely irrelevant.

Fat loss is a long-term game, and you like to hope whenever your fat loss endeavour is over, you’ve set yourself up for long-term success (and not a massive rebound).

This requires making lifestyle changes and enjoying the way you eat.

Don’t look for short-term fads, look at the bigger picture.

Want some assistance? Get in touch for some personalised help:

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[1] Obesity Energetics: Body Weight Regulation and the Effects of Diet Composition; Hall KD & Guo J, (2017).

[2] Do ketogenic diets really suppress appetite? A systematic review and meta-analysis; Gibson et al., (2014).

[3] A high-protein diet for reducing body fat: mechanisms and possible caveats; Dominik H & Varman T, Nutrition & Metabolism (2014). 

[4] A high-protein diet induces sustained reductions in appetite, ad libitum caloric intake, and body weight despite compensatory changes in diurnal plasma leptin and ghrelin concentrations; Weigle DS et al., Am J Clin Nutr. (2005).

[5] The role of protein in weight loss and maintenance; Leidy J et al., The American Journal of Clinical Nutrition, Volume 101, Issue 6, 1 June 2015.

[6] A moderate-protein diet produces sustained weight loss and long-term changes in body composition and blood lipids in obese adults; Layman DK et al., J Nutr. (2009).

[7] Dietary protein and exercise have additive effects on body composition during weight loss
in adult women; Layman DK et al., J Nutr. (2005).

[8] Higher compared with lower dietary protein during an energy deficit combined with intense
exercise promotes greater lean mass gain and fat mass loss: a randomized trial; Longland TM et al., Am J Clin Nutr. (2016)

[9] Very-low-carbohydrate ketogenic diet v. low-fat diet for long-term weight loss: a meta-analysis of randomised controlled trials; Bueno et al., (2013) 

[10] Low carbohydrate versus isoenergetic balanced diets for reducing weight and cardiovascular risk: a systematic review and meta-analysis; Naude et al., (2014).


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