Low Carb or Low Fat: Which Is Best for Weight Loss?
If you want to lose weight, you could argue there’s never been a better time than now. Why? Because these days there are so many different diets to choose from and so much information available about which diets could help you lose weight the quickest and most effectively.
But too much choice isn’t always a good thing. According to research from the British Nutrition Foundation, 43 per cent of adults say they’re confused about healthy eating, thanks to the fact that much of the advice available is conflicting (i).
One of the big diet debates that has already caused its fair share of confusion is that of low-carb versus low-fat – which of these two types of diet is best for weight loss? Indeed, the media and experts alike seem to change their minds about this issue more often than usual.
Low-fat: the lowdown
The low-fat approach became popular in the 1980s not just for weight loss but also for heart health. By the 1990s, supermarket shelves were bursting with a growing variety of low-fat foods, and Rosemary Conley’s Hip and Thigh Diet – one of the most popular low-fat plans of its day – was a bestseller.
Today low-fat diets are still recommended to people who want to lose weight, as well as those who want to lower their cholesterol. That’s because saturated fats – found mainly in animal products – are widely thought to raise cholesterol, which in turn raises the risk of heart disease (unsaturated fats, on the other hand, aren’t associated with raising blood cholesterol).
Meanwhile the reason low-fat diets are recommended for weight loss is that fat of any type is high in calories, since a gram of fat provides nine calories compared to just four calories provided in a gram of protein or carbohydrate. Cutting back on fat has undoubtedly helped many people lose weight over the years, with studies suggesting lower fat intake leads to small but significant weight reduction in people of all ages (ii).
A low-fat diet that’s rich in nutritious, unprocessed foods can be very healthy. But one of the main arguments against the low-fat approach is that many foods manufactured to be low or reduced in fat are actually high in calories because, while they may not have much fat, they may be highly processed and contain lots of sugar. There’s also the idea that low-fat food products fool people into thinking they can eat more of them – who, after all, hasn’t gobbled down one too many biscuits simply because the label said they were fat free?
On the other hand a certain amount of fat is essential for a healthy, balanced diet, as it provides the body with essential fatty acids and helps with the absorption of fat-soluble nutrients such as vitamins A, D and E. It could also be argued that having some fat in your diet may even help you lose weight, since fat helps you feel fuller for longer by delaying stomach emptying and slowing down the release of sugar from carbohydrate foods in your digestive system (iii).
How much is enough?
According to the British Dietetic Association (BDA) adults should eat 70g fat a day, including up to a maximum 20g saturated fat, as part of a healthy balanced diet (iv). The British Nutrition Foundation claims the average UK person’s total fat intake is close to what it should be, though in general we’re eating more saturated fat than the recommended amount (v). The NHS, meanwhile, says current UK government guidelines advise cutting down on all types of fat, while replacing saturated fat with some unsaturated fat (vi).
If you want to cut back on fat to lose weight, however, you could be looking at a total fat intake of just 20g per day, depending on the diet plan you’re following.
Low-carb: what you should know
The low-carb diet is a much more recent phenomenon than its low-fat rival, and owes its popularity to carb-avoiding weight-loss plans such as Atkins, Dukan, South Beach and, more recently, ketogenic (or keto) diets.
Low-carb diets in general limit carbohydrate foods in favour of foods that are high in protein and fat. So you may, for instance, find yourself eating a lot of meat, poultry, fish, eggs and some non-starchy vegetables, but little to no grains (including bread, pasta, rice and oats), legumes, fruit and starchy vegetables (some low-carb diet plans also limit nuts and seeds).
Many low-carb diets boast fast, effective weight loss and are often associated with other health benefits, such as reducing the risk of type 2 diabetes and metabolic syndrome. Supporters believe the low-carb approach works because eating fewer carbs means you produce smaller amounts of the hormone insulin, which helps the glucose your body absorbs from carbs to enter your body’s cells. And since carbs are the body’s main fuel source, limiting them makes your body burn more stored fat to generate energy, which leads to weight loss.
Cutting carbs may well make your body use fat for energy, but it will use protein for fuel too, which can have a negative effect on muscle mass as well as immune function (vi). There’s also a good chance you’ll miss out on many important nutrients found in carbohydrate foods, including calcium, iron, fibre and B vitamins.
In fact the NHS says significantly reducing carbs from your diet in the long term could potentially lead to health issues. Even in the short term you could experience problems such as headaches, weakness, nausea, dizziness and irritability, thanks to the build-up of substances in your blood called ketones – this process is called ketosis, and it happens when your body breaks down stored fat to produce energy (vii).
Meanwhile by cutting down on carbs you may eat more fats and higher-fat sources of protein, which means you may be eating too much saturated fat and increasing your risk for higher cholesterol and heart disease.
How much is enough?
According to the NHS the government’s healthy eating advice recommends that just over a third of your diet should be made up of starchy carbs (potatoes, bread, rice, pasta etc), with another third made of fruit and vegetables (viii).
This means that more than half of your daily calorie intake should come from carb-rich foods. If you’re eating the recommended 2,000 (for women) or 2,500 (for men) calories a day to maintain your weight, that means around 1,000 or 1,250 of those calories should come from carbs. However, a typical low-carb diet may only provide 80 to 240 calories’ worth of carbs, which many critics of the low-carb approach say is unsustainable.
Low-fat vs low-carb: what’s the evidence?
Researchers have been debating whether low-carb diets are better for weight loss than low-fat ones for decades. However two studies in recent years found there’s actually not much difference between them.
The first, published in the journal Cell Metabolism in 2015, is a small-scale study involving just 19 obese adults (ix). The study, which was carried out by researchers from the US National Institutes of Health, saw volunteers follow an extreme low-carb or low-fat diet for six days.
The outcome was that the people on the low-carb diet lost more weight overall, but those on the low-fat diet lost more body fat. And despite the fact that the researchers suggested fat loss is more important than weight loss in the treatment of obesity, you couldn’t really argue that the study’s results were a win for either the low-carb or the low-fat camp.
Published in the Journal of the American Medical Association in 2018, this was an investigation by scientists at Stanford University’s School of Medicine (x). In the study 609 obese adult men and women were followed for a year after having been randomly assigned to eat either a low-carb or low-fat diet. The volunteers didn’t follow a set diet plan or count calories, but were just asked to cut back on foods high in fat or carbs.
During the first month the participants were asked to eat just 20g fat or 20g carbs per day – which was very low compared to their normal intake. Then after the first month they were allowed to start adding more fats or carbs to their diet until they reached the lowest level of fat and carbs they felt they could stick to in the long term (by the third month those in the low-fat group were eating an average of 42g fat per day, while the low-carb group was having an average of 96.6g carbs per day).
Volunteers in both groups also had sessions with a dietitian during which they learned about healthy eating. The dietitians instructed the study’s participants to eat plenty of vegetables and other nutrient-dense whole foods, with as few processed foods as possible and minimum amounts of added sugars, refined flours and trans fats. They were also asked to prepare their meals themselves at home whenever possible.
Altogether, 481 participants finished the year-long trial (a certain number of drop-outs are expected during long-term diet studies). On average, the low-fat group lost 5.3kg (11.7lb) and the low-carb group lost 6kg (13.2lb). And while the low-carb group did lose slightly more weight, over a year the amount in question wasn’t thought to be relevant.
No clear winner
In other words, in the long term, neither diet proved to be more effective than the other. It did, however, suggest that both types of diets can work if they include healthy foods with limited amounts of sugar and processed ingredients such as refined flour. Indeed, the main message may be that the weight-loss diet that works best is one you can stick to, not just for a few weeks but for the foreseeable future. And whether you choose low-fat or low-carb may simply depend on which diet plan you find suits you best.
So it seems that, to date, there’s still no definitive answer to the low-fat vs low-carb debate – indeed there may never be one. Arming yourself with reliable and up-to-date information about nutrition and weight loss is arguably the best thing you can do: why not take a look at our other sports articles and visiting the weight management section of our pharmacy library?
Hooper. L., et al. Effect of reducing total fat intake on body weight: systematic review and meta-analysis of randomised controlled trials and cohort studies. BMJ. (2012 Dec 6). 345:e7666. Available online: https://www.ncbi.nlm.nih.gov/pubmed/23220130
Available online: https://www.gisymbol.com/what-affects-the-gi-value
Available online: https://www.bda.uk.com/foodfacts/FatFacts.pdf
Available online: https://www.nhs.uk/live-well/eat-well/different-fats-nutrition
Available online: https://www.nhs.uk/live-well/healthy-weight/why-we-need-to-eat-carbs
Hall. K.D., et al. Calorie for Calorie, Dietary Fat Restriction Results in More Body Fat Loss than Carbohydrate Restriction in People with Obesity. Cell Metabolism. 22, 427–436. (September 1, 2015). Available online: https://www.cell.com/cell-metabolism/pdf/S1550-4131%2815%2900350-2.pdf
Gardner. C.D., et al. Effect of Low-Fat vs Low-Carbohydrate Diet on 12-Month Weight Loss in Overweight Adults and the Association With Genotype Pattern or Insulin Secretion. The DIETFITS Randomized Clinical Trial JAMA. (2018). 319(7):667-679. Available online: https://jamanetwork.com/journals/jama/article-abstract/2673150?redirect=true
Christine Morgan has been a freelance health and wellbeing journalist for almost 20 years, having written for numerous publications including the Daily Mirror, S Magazine, Top Sante, Healthy, Woman & Home, Zest, Allergy, Healthy Times and Pregnancy & Birth; she has also edited several titles such as Women’ Health, Shine’s Real Health & Beauty and All About Health.