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  • Writer's pictureSue Wharton

How do low-fat and low-carb diets match up to our 5 weight loss criteria?

Losing weight is relatively easy but keeping it off is extremely hard! So, does it matter which diet method you use to lose weight? The traditional and most common way to lose weight is to restrict the number of calories eaten per day, usually by removing as much fat from the diet as possible. An alternative, and increasingly popular approach to weight loss, is to restrict the amount of carbohydrate eaten rather than fat, a so called low-carb diet. Is one better, easier, tastier, more nutritious, more sustainable or better for long term weight loss than the other?

What should a good weight loss diet achieve?

1. It should control hunger. There is no point in staying on a diet that leaves you feeling hungry all the time – you will just be setting yourself up to fail. Hunger is a very strong emotion controlled by several appetite regulating hormones such as leptin, insulin, glucagon cholecystokinin and many others that are secreted in your intestinal tract as food passes down.

Most of these hormones are appetite suppressants, but one, ghrelin (produced in the stomach), is an appetite stimulant. If the food you eat doesn’t activate the appetite suppressant hormones adequately, you will be left feeling hungry, and hunger is a very strong emotion – however strong your willpower you won’t be able to resist hunger for ever, one day you’ll crack. A diet that does not enable you to control hunger in the long term is doomed to fail.

Low-calorie/low-fat diets are notoriously bad at helping you to control appetite because they do very little to stimulate those appetite suppressant hormones. The trick here would be to make sure you eat a very high fibre diet. Fibre may suppress appetite through fermentation by gut microbes to produce acetate which has a direct effect on the brain (1). Flaxseed in particular appears to have appetite suppressing effects (2). So if you choose to loose weight using a calorie restricted diet make sure you eat plenty of fibre with every meal.

A low-carb diet is very good at helping you suppress hunger because the higher protein (3) and fat content of the diet are very strong stimulators of appetite suppressing hormones (4) Once established on a low-carb diet most people have no need to snack and may even change to only eating twice a day.

2. It should enable you to lose fat rather than muscle.

When we talk about weight loss, what we really mean is fat loss, particularly the fat that is stored around your middle. This is often referred to as ‘visceral fat’ because is surrounds the organs (or viscera) inside your abdomen. This is the unhealthy, inflammatory fat that can lead to chronic illnesses such as diabetes, heart disease, fatty liver disease and even cancer.

The fat stored on your limbs, bottom or breasts is called subcutaneous fat and is not inflammatory and therefore not a threat to health.

A good weight loss diet should allow you to lose visceral fat (you’ll probably lose some subcutaneous fat as well – consider this a bonus) whilst preserving muscle mass.

Muscle is your friend, it is very metabolically active meaning that you burn lots of calories from it, even when you are at rest, so it helps you with weight loss as well as helping you to maintain strength and support your bones.

Low calorie diets are often lacking in good quality protein because they require you to reduce your meat, egg and dairy intake (due to high levels of fat). Protein is essential for maintaining and building muscle, and studies have shown that low-calorie diets tend to lead to more muscle loss than low-carb diets which tend to be higher in protein (5). The reduction in muscle mass seen in calorie restricted diets can be offset to some extent by doing regular resistance training (lifting weights) (6)

3. It should help you to control your blood sugar levels and prevent you going on the blood sugar ‘roller coaster ride’.

If the foods you eat on your weight loss diet cause your blood sugar levels to rise quickly and then fall just as quickly several times during the day, this is what we call the blood sugar roller coaster. Each time it rises you may experience a burst of energy, but when it falls you will experience extreme fatigue, and worse still, hunger, it may also affect your mood making you feel ‘hangry’. You will crave sugar, such as a biscuit or piece of cake. Not only that, when your blood sugar rises so does your blood insulin levels. Since you can’t use all that sugar for energy, insulin will instruct your liver to turn it into fat and store it around your belly, and now your diet is busted!

Blood sugar control can be more of a problem with low-calorie, low-fat diets that don’t restrict starchy carbohydrates or actually encourage you to eat them for energy. If you follow a low-fat diet then make sure you eat lots of fibre with every meal to slow down sugar absorption and reduce those insulin spikes (7). Also try to avoid shop bought ‘low-fat’ processed food products such as low-fat yoghurts – often the fat has been replaced with sugar. You can also reduce the blood sugar spikes by going for a walk after each meal (8)

Low-carb diets, by their nature, restrict starches of all kinds and so blood sugar control is easier to achieve, and thus low-carb diets are generally better for people with type 2 diabetes, pre-diabetes or any degree of insulin resistance (an inability to metabolise sugar properly)(9). A low carb breakfast is particularly useful for improving blood sugar balance throughout the day (10)

4. It should provide you with all the nutrients you need for health.

The problem with all weight loss diets is that they require you to restrict some kinds of food or the quantity of food. Low calorie diets require you to eat a lot less food per day, mainly in the form of fat restriction (because fat contains more than twice the calories per gram than proteins or carbs), but if you are eating smaller portions then you may also eat less protein, less carbohydrates and less vitamins and minerals (11).

If you are on a low-carb diet then you will be replacing the carbohydrate foods with more fat and protein. Protein and fat tend to suppress appetite by stimulating all those appetite suppressing hormones in the gut, so you may find that you want to eat less often. This can also lead to some vitamin and mineral deficiencies if the type of food eaten is not properly planned, or proprietary ‘keto’ foods are used as substitutes for real, whole foods (12).

People on very low-carb diets (ketogenic diets) often need to take ‘electrolytes’ in the form of a drink to replace some minerals such as potassium, sodium and magnesium, or by increasing consumption of vegetables (13).

It is important therefore that whichever dietary pattern is chosen for weight loss, the food consumed is nutrient dense, i.e., it contains a lot of vitamins and minerals per gram. The most nutrient dense foods available are animal foods e.g., meat, offal, fish, dairy and eggs. They are particularly good for quality protein (needed to preserve that all important muscle mass), all the fat soluble vitamins (A,D, E and K), iron, zinc, calcium, folate and vitamin B12. Oily fish also contains high levels of the essential omega-3 fats.

Animal foods tend to be higher in fats so are restricted on a low-calorie/low-fat diet, but allowed and encouraged on a low-carb diet. Low-carb diets therefore tend to be more nutrient dense than low-fat diets. However, a low-fat diet that contains wholegrains, legumes and lots of vegetables will contain moderate amounts of water-soluble vitamins such as B vitamins (apart from B12), vitamin C, some vitamin K, as well as the minerals magnesium, potassium, sodium and copper.

The best way to get all the nutrients required is to eat real whole food, cooked from scratch rather than relying on shop bought ‘weight loss’ foods which tend to be very processed and often contain sugar and ‘microbiome destroying’ additives.

5. It should be sustainable for life.

When you embark on a weight loss diet you have to be thinking long term. Any diet adopted needs to be seen as a ‘lifestyle’ choice and a permanent change to eating habits. If you stop your diet after losing weight and go back to your old habits then you will regain the weight, and possibly more. So, the way you have chosen to diet needs to be sustainable for life.

Once your body has got back to a normal healthy weight, weight loss will cease (your body knows how heavy it should be) but you will have to continue eating the same diet to maintain this. If you decide that to start eating more food or unhealthy processed foods again then, hey presto, you will start putting the weight back on! Severely calorie restricted diets, though effective in the short term, can lead to long term problems with a low metabolic rate (metabolic adaptation) which can last for years, and will make you prone to putting the weight back on with only small increases in calories (14)

So, what kind of eating pattern can you sustain for life? Some people can sustain eating a low-calorie, low-fat way of eating for life. Longevity research suggests that calorie restriction for life can increase longevity, at least in animals but possibly in humans too (15). However, it is a struggle for many people to maintain this way of eating and nutrient deficiencies can accumulate if the diet is not planned well and a lot of variety added.

Many people find low-carb diets easier to maintain in the long term because they enable you to eat tasty, fattier foods that are more flavoursome and satisfying. Hunger is also better controlled and muscle mass easier to maintain due to the higher protein intake. However, some people find the lack of starchy staples on their plate hard to maintain and miss the ease of making a sandwich for lunch.

So which type of diet is better for weight loss?

It depends.

Some studies show that people lose similar amounts of weight with both low calorie/low-fat and low-carb diets. Other studies suggest that low-carb diets are better for weight loss. The Public Health Collaboration collated 67 Randomised Controlled Trials (RCT) comparing the effectiveness of low carb diets vs low fat diets for weight loss. They found that in 57 of the studies low-carb diets had better weight loss, 7 studies found low-fat diets produced better weight loss and 2 studies showed no difference (16).

For many people losing weight is not an end in itself, improving health may actually be the primary aim. Many people with obesity also have poor metabolic health which pre-disposes them to diseases such as diabetes, heart disease and fatty liver disease. Though both types of diets can lead to improvements in metabolic health, low-carb diets seem to do it better since they tackle the root cause of metabolic poor health, which is insulin resistance.

One study has found that both low-calorie and low-carb diets improve metabolic health, but this is only because people following a low-calorie diet also restrict carbohydrate consumption as well as fat in order to reduce portion sizes, so they are often on a low-fat and lower-carb diet at the same time (17). However, they do no better than people eating low-carb and high-fat diets (think about all those tasty foods you can eat). Restricting fat in itself does not seem to lead to improved weight loss or better health outcomes, however counterintuitive that may seem.

So, the bottom line is that it is up to you. The evidence suggests that both low-calorie/low-fat diets and low-carbohydrate diets can be effective for weight loss, at least in the short term, but if you have a metabolic health condition, which the vast majority of obese people will have, then a low-carbohydrate diet may be the best choice.


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