Low Fat Versus Low Carb Diets


Whether to follow a low fat or a low carb diet is a polarising issue, but views are changing. Now that sugar is the new demon, evidence is swinging away from the traditional low fat diet towards one that is lower in carbohydrates and has a lower glycemic load. This reduces the blood sugar swings that trigger hunger, reduces release of insulin hormone which makes you store fat, and naturally increases your intake of protein which helps to fill you up so you eat less over all.

Low fat diets are not that effective 

If you need to lose weight, you’ve probably received advice to follow a conventional, low-fat diet which supplies 30% or less of your daily energy in the form of fat. You will be encouraged to eat more fruit and veg (great) and to concentrate on obtaining starchy foods such as potatoes, breads and bagels (not so great).

Yet this seems to make sense as a gram of fat provides 9 kcal energy per gram – more than twice the 4 kcals obtained from a gram of carbohydrate or protein. Cutting back on fats should be an easy way to cut calories and lose weight, right? Yet there is surprisingly little evidence to confirm the low-fat approach is effective.

A large analysis of data from 53 studies, involving over 68,000 people followed for a least one year, has now confirmed that low-fat diets are NOT more effective than higher-fat diets or low-carb approaches. In fact, those who followed a high-fat, low-carbohydrate diet achieved a statistically significant, greater weight loss of 3.8kg, compared with 2.7kg for those following a low-fat diet.

Although, let’s face it, neither result is impressive after at least one year of trying to lose weight!

A calorie is not just a calorie

In the laboratory, a single calorie of energy obtained from fat may have the same usable value as a single calorie obtained from protein or from carbohydrate, but this is not the case in the body. The way your cells process each energy source varies, as the food itself acts as a signal that releases hormones and switches on different genes and metabolic pathways.

When you eat protein, up to 25% to 30% of the energy it provides is lost as heat during processing, making protein a great fuel source when you want to lose weight. This is significantly greater than the energy used up to metabolise fats ( 2% to 3%) or carbohydrates (6% to 8%). When it comes to diets, a calorie is not just a calorie.

protein_diet_2If you ate a hypothetical meal providing 2000 kcals of protein and fat with zero carbohydrates, you would ‘only’ obtain around 1700 usable kcals of energy as 300 kcals are used up and given out as heat during protein metabolism (as shown on the 0% carbohydrate end of the graph).

If those same 2000 kcals were eaten in the form of 50% carbohydrate and 50% protein/fat, you would obtain around 1830 usable kcals of energy (thick red lines on the graph) – you obtain an extra 130kcals.

This thermogenic effect gives higher protein, lower carb diets a significant metabolic advantage.

Protein also has a satiating effect so you feel more full, more quickly and tend to eat less overall. This may partly result from protein suppressing the effects of ghrelin, a hormone that stimulates hunger.  There’s also good evidence that eating protein may stimulate the formation of lean muscle at the expense of fat – at least in some people.

Why low-fat diets are ineffective

When cutting out fats you eat relatively more carbohydrates. These are promoted on diet sheets in the form of wholemeal bread, cereals, rice, pasta, couscous, bagels, muffins and baked potatoes. Carbs are also hidden in low-fat foods such as yogurt to improve consistency and mouth feel. Check labels of the low-fat products you buy and you may be surprised at how many carbs and how many calories they actually provide.

Carbohydrates cause blood glucose levels to rise – even healthy carbs such as wholegrain bread, although the effect is slower and less marked. A rise in blood glucose triggers the release of insulin hormone from the pancreas. Insulin acts as a signal that tells glucose receptors inside muscle and fat cells to come to the surface where they serve as gates to let glucose enter.

Insulin is the main fat-storing hormone in the body, and fights weight loss. Insulin both escorts excess glucose into fat cells for storage and blocks an enzyme (hormone-sensitive lipase) which breaks down stored fat. This effect makes it biochemically difficult to burn fat and lose weight when you are following a low-fat, high carb diet. You may be eating fewer calories, but your higher intake of carbohydrate promotes release of insulin which fights to keep fat stored.

It makes more metabolic sense to lose weight by reducing your intake of carbohydrate and following a high protein, higher fat diet – especially if you have type 2 diabetes and are, to all intents and purposes, carbohydrate intolerant.

A lower carb diet reduces insulin secretion and allows hormone-sensitive lipase to mobilise the stored around your hips and waist, so you slim down naturally. At the same time, your increased intake of protein curbs your appetite and helps you burn off more energy as heat.

What Low Carb, High Fat, High Protein Actually Means

Cutting back on carbs doesn’t mean cutting out the whole food group. Even the lowest carb phase of the lowest carb diet around – the Atkins Diet – provides around 8% of energy as carbohydrates, as these are naturally present in the non-starchy vegetables, salad leaves and berries that you eat alongside your high protein, higher fat foods.

Following a lower carb diet basically means cutting out the simple carbs (sugars and processed starches like white flour, white rice and pasta) which cause significant swings in blood glucose levels and insulin secretion.

Eating less carbohydrate means you automatically increase your relative intakes of both protein and fat. This doesn’t necessarily mean that you eat more of them – in many cases you actually eat less because the satiating effect of protein curbs your appetite.

The easiest way to visualise this is to imagine an experimental meal that provides equal amounts of energy from carbohydrate, protein and fat. For example, a nice, juicy rib-eye steak with oven-baked chips, asparagus and tomatoes sautéed in olive oil could be concocted to provide:

  • 33% of energy in the form of fat
  • 33% of energy as protein
  • 33% of energy as carbohydrate.

If all you did was to scoop the oven-baked chips off the plate and ate the steak and vegetables, the meal now provides a different relative balance of macronutrients, with 50% of the energy in the form of fat – even though you’ve consumed LESS food and calories over all by ditching the oven-baked fries.

Eating a meal that provides almost 50% fat may sound scarily unhealthy, but it’s just the relative quantity that’s increased – the meal is still a healthy option. And the evidence that fat – especially saturated fat – is bad for your health is not as clear-cut as some would like us to believe.

Cutting back on carbs can help you lose weight

As well as helping you lose weight, cutting back on carbohydrates can lower your blood pressure, improve insulin sensitivity and glucose tolerance as well as having beneficial effects on blood cholesterol and triglyceride levels.

I’m not advocating very low carbohydrate diets which don’t suit everyone, but following a lower glycemic diet, such as the Mediterranean diet (without excessive amounts of pasta and focaccia bread), or a more Japanese style diet that focuses on fish and vegetables is well worth adopting.

As well as cutting back (ideally avoiding) cakes, biscuits, pastries, jam and other foods with added sugar, also cut back on white potatoes (sweet potatoes, paradoxically are OK) and foods that might appear on the healthy side of eating guidelines: starchy foods – even those made with wholegrains. you might find this helps you lose weight.

In addition:

  • Put less on your plate – use a smaller plate so the portion looks more reasonable.
  • Eat slowly and chew each mouthful thoroughly so your brain has time to receive the messages that tell you when you’re full.
  • Stop eating when you get those ‘full’ messages – ignore the ingrained rule of having to clear your plate.
  • Drink a glass of water before and after your meal.
  • Avoid alcohol as much as possible – its 7kcals per gram can fuel rockets let alone weight gain.

You may want to consider protein-based meal replacements, green coffee bean extracts or raspberry ketones, too.

What diets and diet tips have worked for you?

Image credits: pixabay

About Dr Sarah Brewer

QUORA EXPERT - TOP WRITER 2018 Dr Sarah Brewer MSc (Nutr Med), MA (Cantab), MB, BChir, RNutr, MBANT, CNHC Cert IoD qualified from Cambridge University with degrees in Natural Sciences, Medicine and Surgery. After working in general practice, she gained a master's degree in nutritional medicine from the University of Surrey. Sarah is a registered Medical Doctor, a registered Nutritionist and a registered Nutritional Therapist. She is an award winning author of over 70 popular self-help books and a columnist for Prima magazine.

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4 thoughts on “Low Fat Versus Low Carb Diets

  • Scott G.

    I have always found this type of information very confusing. I have read many different opinions regarding this subject but this makes perfect sense! I need to read up on it more, I am doing well with my latest plan to lose weight. I say latest because like most people I have started and failed a few times.

    • admin

      HI Scott, lower carb works best for most people – but you don’t have to go too low. Low GI tends to be better than Atkins, for example. Keep a daily track of your goal or its easy to stray off the diet and back to old habits! Make this the time it really works for you.