Intermittent fasting is the diet everyone’s tried, or knows someone who has. Skip breakfast, eat between midday and 8pm, and the fat falls off. That’s the pitch.
Here’s what the best trials actually found, the catch that matters most if you’re over 40, and my honest take.
What the Trials Say
The biggest question is simple: does fasting work better than just eating less? Two good trials have tested exactly that.
In a year-long study published in the New England Journal of Medicine, researchers put 139 adults with obesity on a calorie-restricted diet. Half were also told to eat only between 8am and 4pm. The other half could eat whenever they liked. After 12 months, the fasting group had lost 8 kilos and the eat-anytime group 6.3 kilos. That gap was small enough that it could have been chance, and there was no difference in waist size, blood pressure or cholesterol. The authors’ conclusion: the eating window might help some people eat less, but it’s the eating less that does the work.
The second trial is the one most people haven’t heard of. In the TREAT study, 116 overweight adults did either 16:8 fasting or three structured meals a day for 12 weeks. The fasting group lost under a kilo, and there was no meaningful difference between the groups.
Australia’s Healthdirect sums it up the same way: the weight-loss benefits of fasting most likely come from eating fewer calories overall, not from the timing.
The Catch If You’re Over 40
Here’s the part of the TREAT study that should make anyone over 40 pause. Fifty of the fasting participants had body-composition scans. Of the weight they lost, about 65 per cent was lean mass, which is mostly muscle. In a normal diet you’d expect 20 to 30 per cent.
It was a small group, and the researchers themselves said it needs more study. But it lines up with common sense. If you cram a day’s food into eight hours, most people end up eating less protein, and less often. After 40, when you’re already fighting to hold on to muscle, that’s the wrong trade. Losing muscle makes you weaker, slows your metabolism, and makes the fat easier to regain.
The fix, if you do fast, is non-negotiable: hit your protein target inside the window, and keep lifting.
Who Shouldn’t Fast
Healthdirect is clear on this. Intermittent fasting isn’t suitable for children and teenagers, anyone pregnant or breastfeeding, people who are underweight or have a history of disordered eating, older adults, or anyone on medication that has to be taken with food. If you have diabetes or you’re going through cancer treatment, talk to your GP before you even think about it.
Side effects are common early on: hunger, headaches, feeling weak, poor concentration, dehydration. If that’s you in week three, it’s not a sign of toughness. It’s a sign the approach doesn’t suit you.
My Take
I don’t recommend fasting to my clients.
I’ve tried it myself, but not for weight loss. I did it for autophagy, the cell-cleanup process some people fast for, and that’s a different conversation. I’ve written about autophagy, fasting and keto separately. I also have a good friend who fasts all the time. He has an unusual autoimmune condition and he believes it helps him. That’s his experience, and I’m not going to argue with it. But it’s not a reason for you to skip breakfast to lose five kilos.
I’ve watched plenty of clients try the 16:8 approach over the years. The honest pattern is this: it takes a lot of work and a lot of organising, and you have to be the sort of person who, if you’re going to do it, does it properly. Done half-heartedly, people end up eating next to nothing. They have no energy in their sessions, they get no results, and they’re hungry all the time. Most find it very hard to stick to.
If eating less is the goal, there are easier ways to get there that don’t cost you muscle.
The Short Version
Fasting works about as well as any other way of eating less, and no better. If it makes eating less easier for you, fine. If it makes you skip protein, train tired and lose muscle, it’s costing you more than it gives. After 40, muscle is the thing you can least afford to lose.
If you want the simpler route, my post on how much protein you need after 40 is where I’d start. And if you’d like someone to look at what you’re eating and build a plan around it, connect with me today.
Related reading: how nutrition affects gut health.