Thanks to Catarina Soares, medical student at the University of Nottingham, for putting this piece together. Read her previous piece about this misunderstanding around whether fruit is really ‘good or bad’ here.
Finding a diet that works for you can be incredibly tricky: for every person swearing by the latest diet craze, there seems to be another saying they tried it to no avail. This becomes more confusing with all the conflicting information online, and it’s so easy to find article after article promising to hold the key to shedding the pounds, often deceivingly or unhealthily.
So how can we figure out which diets help, and which don’t? Evidence-based medicine prompts us to look at studies to understand whether certain diets work, just as we would do with medications. About one in four adults in the UK is obese, according to the NHS (1), and as obesity rises, it becomes increasingly important to find lifestyle interventions that are known to work effectively and safely, and steer clear of fad diets that only make matters worse.
The DIETFITS study (2) is a recently published trial that tried to find out what kind of diet works best. They studied just over 600 people who were told to follow either a healthy low- fat or a healthy low-carb diet, which they maintained for a year, and measured their weight loss, cholesterol and blood sugars in response to the diet. The paper asked a few questions:
- is a low-carb diet or a low-fat diet more effective for weight loss?
- can genes or insulin levels (a hormone associated with controlling our blood sugars) determine which one works better for each individual?
What kind of study was it?
The study was a randomised clinical trial (RCT), the gold standard for clinical trials: researchers compare two groups: in this case, a group with a low-carb diet (LC) and a group with a low-fat diet (LF). Ensuring that the participants in each group are very similar in everything except the interventions they’re comparing is key. This allows scientists to assume that any differences between the two groups are down to the interventions studied. Around 20% of the participants dropped out by the end of the study, in roughly equal numbers between the groups, which is quite normal.
What did they do?
They started by monitoring the participants over a month, during which they were told to keep their normal habits. After this, the LC group were instructed to have only 20g of carbs per day, and the LF group 20g of fat per day, with specific guidance about what foods they would do best avoiding. After the first 2 months, they were allowed to slowly increase the amount of fat or carbs they ate until they reached the lowest amount they believed they could maintain long-term.
Throughout the year, they had 22 sessions led by registered dietitian’s. Both groups were encouraged to maintain a healthy lifestyle: “maximise vegetable intake; (…) minimise intake of added sugars, refined flours, and trans fats; (…) and focus on foods that were minimally processed, (…) and prepared at home whenever possible”; and exercise regularly. They were also given support to help keep up their healthy new habits. The researchers had them fill out questionnaires about their food sporadically, to assess how closely they were following their diet.
Why did they do this?
The low-carb vs low-fat war has been waged on- and offline for a while now. Low-carb diets rose in popularity from the late 90’s (3), and have come in and out of fashion since. This category includes the Atkins, paleo and keto diets (4), which may differ in some respects but have the low carb content in common. The principle behind them is that when we cut down on the amount of carbs, our body’s main fuel, we re-programme to run on ketones instead. Ketones are the back-up fuel we get from burning fat, and, in theory, this process results in weight loss (3,4). Although this idea makes sense on paper, it’s been quite difficult to prove – a low-carb diet does seem to lead to weight loss, but the mechanism is still not very well understood (3,4).
On the other hand, low-fat diets have been around for the past century. They were initially based on the fact that fat is more calorie dense than carbs or protein: it has 9 calories per gram, whereas the latter both have 4 calories per gram (5). The idea is that by reducing the amount of fat in our diet, we can reduce the number of calories we consume more easily than by reducing carbs or protein (5). In fact, when comparing the three macronutrients, fat makes you feel the least satisfied, so we eat more of it without realising (6,7). There is some evidence for these claims, bit it has been difficult to establish if this diet is more effective than a low-carb diet.
What were the results?
In the end, the study shed some light on this debate:
- there was no difference between a healthy low-fat and a healthy low-carb diet: both resulted in an average weight loss of 5kg, or 11 pounds (although some lost no weight, and others even gained weight!)
- neither the genes nor insulin have any effect on the response to each of the diets. The only difference between groups was cholesterol: the LC group had higher levels of LDL (the “bad” cholesterol). On the other hand, they also had higher HDL levels (the “good” cholesterol) and lower triglycerides, another kind of fat in the blood.
Key points from the DIETFITS study
It’s important to focus on the fact that the people in both the LF and LC groups had many sessions with registered dietitian’s who helped them make the best decisions about their food, and they were all encouraged to go for high-quality foods no matter which diet they were following. Fizzy drinks are technically low-fat, but that doesn’t make them healthy or any good for weight loss. They were also told to decrease how much they ate by as much as they could, without feeling hungry and without counting calories. As the year went on, they ended up reducing their daily intake by about 500 calories, which is important for weight loss.
Although the study didn’t find that one diet is better than the other, it did show us that education and support alongside realistic goals make for a great lifestyle intervention for obesity. These results are important both to the general public, because we’re better informed; and to organisations like GP surgeries, and even the NHS, because it tells them
how they can best help their patients.
For me, the key message from the DIETFITS study is that the specifics of a diet don’t actually matter that much for weight loss in the long run, as long as it’s a healthy and realistic reduction in calorie intake.
1. NHS Choices. Obesity [Internet]. 2016 [cited 2018 May 27]. Available from: https://www.nhs.uk/conditions/obesity/
2. Gardner CD, Trepanowski JF, Del Gobbo LC, Hauser ME, Rigdon J, Ioannidis JPA, 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. JAMA [Internet]. 2018 Feb 20 [cited 2018 May 22];319(7):667. Available from: http://jama.jamanetwork.com/article.aspx?doi=10.1001/jama.2018.0245
3. Astrup A, Larsen TM, Harper A. Atkins and other low-carbohydrate diets: hoax or an effective tool for weight loss? Lancet [Internet]. 2004 [cited 2018 May 28];364:897–9. Available from: https://ac.els-cdn.com/S0140673604169869/1-s2.0-S0140673604169869-main.pdf?_tid=36ac9770-549a-4480-a654-
4. Noakes TD, Windt J. Evidence that supports the prescription of low-carbohydrate high-fat diets: a narrative review. Br J Sports Med [Internet]. 2017 Jan 1 [cited 2018 May 29];51(2):133–9. Available from: http://www.ncbi.nlm.nih.gov/pubmed/28053201
5. La Berge AF. How the Ideology of Low Fat Conquered America. J Hist Med Allied Sci [Internet]. 2007 Aug 30 [cited 2018 Jun 5];63(2):139–77. Available from: https://academic.oup.com/jhmas/article-lookup/doi/10.1093/jhmas/jrn001
6. Jéquier E, Bray GA. Low-fat diets are preferred. Am J Med [Internet]. 2002 Dec 30 [cited 2018 Jun 5];113(9):41–6. Available from: https://www.sciencedirect.com/science/article/pii/S0002934301009913
7. Freedman MR, King J, Kennedy E. Popular Diets: A Scientific Review. Obes Res [Internet]. 2001 Mar [cited 2018 Jun 5];9(S3):1S–5S. Available from: http://doi.wiley.com/10.1038/oby.2001.113