Time restricted feeding (TRF) is one of the things I get asked most about. If I were to give my opinion succinctly, it's the following:
The potential health benefits are so:
inconsistent AND/OR
tiny OR
non-existent
AND the potential impact on one's social life is so large (eg not eating anything after 3pm, lol)…
I wouldn't recommend it.
Now here’s the long version:
TRF is basically where you restrict the number of hours that you spend eating without intentionally changing what you eat. For example you might only eat within a window of about eight hours. The rest of the time you spend fasting, and usually only things like plain tea/coffee/water are allowed. People might only eat in the morning and early afternoon, for example from 6 am to 2 pm. We typically call this early TRF (eTRF). Conversely other people might skip breakfast, skip lunch and maybe only eat in the late afternoon and evening. We typically call this late TRF (lTRF).
So, what do studies tell us about the effect of time restricted feeding on different health outcomes? Let’s review:
Weight management
A number of studies have examined the effect of time restricted feeding, whether early or late, on weight loss. In general the results from these studies are pretty unexciting - in this study, people who limited the eating window to FOUR HOURS A DAY got a weight loss of THREE PERCENT after 8 weeks. Like wow, that’s A LOT of sacrifice for a REALLY limited weight loss. (For comparison a total meal replacement diet gets ~10% weight loss in 8 weeks). And there’s this one and this one where a TRF diet did not cause greater weight loss than eating throughout the day. And a meta-analysis which pooled studies using TRF of over 8 weeks found no superiority of TRF over a regular energy-restricted diet.
And as far as we can tell in studies where people do lose weight via TRF, it is simply due to people eating less. This isn't really a surprise, given that if we have less time to eat, we consume fewer calories.
Now, there will never be one perfect diet which helps everyone meet their weight loss goals - and in most dietary studies there are people who do succeed in losing lots of weight even if the average weight loss is unimpressive. So if people try TRF and find it helps them lose weight, great.
Blood Pressure
There does not seem to be any or much effect of time restricted feeding on blood pressure unless the diet also causes weight loss. (Note - read the article not the abstract - the abstract does not seem to reflect their text in the article or the tables). Here is an excellent review article that looks at the observational and controlled trial evidence for the effect of meal timing etc on cardiometabolic risk factors including blood pressure. And note studies which show worsening of blood pressure with lTRF.
So if blood pressure is your goal, there are much better ways to do it - eg weight loss via any diet, physical activity, a whole-plant-based diet, replacing refined carbohydrate with protein or healthy fat and of course the DASH diet.
Glucose Concentrations
We see the same fairly unexciting story with glucose - some studies show reductions in glucose or HbA1c (average glucose concentration over about 3 months) with TRF and some don’t, also here. If there is a weight independent effect, it seems to be with eTRF only. Interestingly, it might be the case that it’s mostly just fasting glucose which is reduced on eTRF: eg here, here and maybe here. In terms of the magnitude of effect - the reduction in night time glucose is ~7mg/dl (0.4mmol/L) on eTRF from 8am-2pm and about the same reduction (guessing from the graph) on eTRF from 10am to 5pm and eTRF 8am to 4pm. So not a meaningless reduction but pretty disappointing given the sacrifices you need to make not to eat after 5pm (or 2pm…..). And again, the more sociable lTRF doesn’t seem helpful, and might even be detrimental to glucose control.
Insulin sensitivity
This is a difficult one to summarise because insulin sensitivity can mean lots of things. Sometimes when people talk about insulin sensitivity they mean does a person have hyperinsulinaemia (ie too much insulin in the blood stream). Since the evidence for hyperinsulinaemia and many diseases is not really contested I’ll focus on “insulin concentration” as the outcome here.
One study found that eTRF reduces post-prandial insulin in prediabetes. In this study the effect was clinically relevant (eg reduction of peak insulin after a meal of about 50uU/ml here. For context, peak insulin response to an oral glucose tolerance test reaches about 130-160uU/ml in prediabetes, and goes down to about 40-50uU/ml in normal glucose tolerance. Conversely, eTRF did not have a clinically relevant effect in healthy individuals. (Hutchinson) or in people with obesity but normal fasting glucose. And in this study also in individuals with obesity but normal fasting glucose, eTRF significantly lowered insulin during the morning, but significantly increased it in the evening.
There does not seem to be an effect on fasting insulin though here, here and here
And there is a similar story with lTRF which does not appear to reduce insulin: neither fasting nor post prandial) . The 10am-5pm TRF protocol did not have a significant effect on insulin either.
So overall, nothing to get excited about…..again.
Lipids
eTRF does not appear to lower total cholesterol or LDL cholesterol (Sutton), or here and a few studies show small increases. And lTRF sometimes causes no change in cholesterol but has also been shown to increase both and here.
eTRF might also increase fasting triglycerides or have no effect eg here and here. Same thing with lTRF here. This study found a significant reduction in fasting triglycerides in eTRF compared to lTRF - but is eTRF better than regular meal times? The rest of the literature suggests not.
So again, to claim any improvements in classical lipid risk markers, you really need to clutch at straws.
Bottom Line
Right now, there’s nothing exciting to see here in terms of clinical utility. It’s actually kind of amazing how utterly unimpressive the outcomes are from these studies, especially given the sacrifice (IMO) it would take to follow eTRF.
(I’m aware that some scientists are interested in the role of TRF for things like longevity and cancer. These are not areas I have any expertise in so I can’t provide any insight. However, I will emphasise that - hopefully you can see from the above - the data really do not match the hype you see about TRF and cardiometabolic outcomes. So remain cautious about what you read on TRF and other health matters too).