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There has been a lot of interest recently into Intermittent fasting as an approach not only for weight loss, but also to improve metabolic health such as increased insulin sensitivity and glucose tolerance. This can be performed either as Intermittent Energy Restriction (IER), which is generally eating significantly less on certain days for example the 5:2 diet or alternate day fasting or Time Restricted Feeding (TRF) where food intake is restricted to a narrow window in the day.

It should be no surprise to us that this form of dieting results in weight loss in the short term. If you don’t eat, you are in negative energy balance (i.e. you are burning more calories than you are consuming) and you will lose weight. What concerns me is whether, by interfering with patterns of eating you may be altering your metabolic rate, i.e. does your body go into shut down mode and preserve the reserves you have during fasting? Can you get all the nutrients you need to function when fasting? And is this pattern of eating sustainable for achieving long term weight loss?

I tuned into the Nutrition Society webinar on this a few weeks ago, which looked in depth at the evidence around these different approaches and strategies to fasting and their impact on glycaemic response and weight loss. Dr Antoni from the University of Surrey summarised the evidence highlighting that many of the existing studies were carried out in animals, which makes it difficult to interpret the potential impact on humans and human studies vary hugely in approaches tested, and study design is often limited by small sample sizes, so it is often difficult to compare outcomes. Nevertheless, she presented the findings and these were my seven take home points:

1. Intermittent fasting could be a useful tool if you have reached a weight loss plateau, you are pre type 2 diabetic or have raised triglycerides.

Studies looking at these types of fasting diets have seen improvements in markers in the blood for fats, glucose tolerance and insulin sensitivity. However, there is also some evidence of long-term negative consequences of alternate day fasting on reduced cardiac function and glucose tolerance.

2. Eat earlier in the day, not later:

When I have spoken to people about their approaches to TRF they tend to prefer to miss breakfast and even lunch and eat all their food as an evening meal. However glycaemic control/insulin sensitivity is greatest earlier in the day, and this affects our responses to food. Some studies have even shown negative effects when food intake is limited to the evening due to poor glycaemic control.

3. No easier or more effective for weight loss than other methods

Evidence suggests that intermittent fasting is no easier and no more effective in terms of weight loss than other calorie restricted diets.

4. The weight loss may be a short term fix

This type of dieting may result in greater weight gain following the weight loss phase compared to conventional daily calorie restriction diets. This could be because subjects may have focused more on when they are eating and not made a change to what they are eating, so it may be more tempting to return to usual eating habits.

5. There is no clear evidence for signs of reduced aging.

Although there seems to be some interest around this, evidence is generally limited to animal studies and some suggest intermittent fasting may even increase aging, so we can’t reach any conclusions on this yet.

6. Need to consider the individual preferences to determine which fasting approach is more effective.

In terms of choosing whether to adopt Intermittent Energy Restriction (IER)  or Time Restricted Feeding (TRF), it seems to be that whichever approach fits best for you and your routine will result in the most weight loss. In other words, which diet are you most likely to stick to?

If you do try a fasting approach, Dr Antoni advises it is important to limit strenuous activity, ensure adequate hydration and include a multivitamin and mineral on fasting days, and not to overeat on a non-fast day.

7. We need more high quality research

We need more high quality research in humans, particularly into any potential negative effects of fasting on metabolic effects and aging.

My conclusions

We live in an environment where it is incredibly easy to consume more calories than our body needs. I have spoken to a number of people who are taking the intermittent fasting approach to weight loss, weight control or for some it sounds like a lifestyle choice. They are singing its praises, saying it has given them more energy and it is the only approach that has made them lose the weight. If it’s working for them, then great. The big question for me however is WHAT are people eating? If they continue to eat what they were eating before, they may be losing weight, but they may not be feeding their body all the essential nutrients it needs. As a first step for weight loss I would still always advise turning to a lower calorie diet filled with high fibre foods, lean proteins and plenty of grains, fruit and vegetables, that sustains you for longer without needing to be hungry. However, for some people this approach has not worked and they are seeking an alternative.

I will wait in anticipation as more evidence comes to light as to the long-term safety of these diets. In the meantime, I do see the benefit in this obesogenic environment of setting ourselves guidelines such as only having three meals a day starting off with a healthy, filling breakfast; trying to not eat between meals, and not eating late into the night (for example not eating after 8pm). This way we should provide enough opportunity to get adequate nutrition without excessive calories.

References from Dr Antoni’s presentation

Ahmet I., Wan R., Mattson MP., Lakatta EG., Talan MI. Chronic alternate-day fasting results in reduced diastolic compliance and diminished systolic reserve in rats. J Card Fail. 2010 Oct;16(10):843-53. doi: 10.1016/j.cardfail.2010.05.007. Epub 2010 Jul 1.

Antoni, R., Johnston, K., Collins, A., & Robertson, M. (2017). Effects of intermittent fasting on glucose and lipid metabolism. Proceedings of the Nutrition Society, 76(3), 361-368

Antoni, R., Johnston, K., Collins, A., & Robertson, M. (2018). Intermittent v. continuous energy restriction: Differential effects on postprandial glucose and lipid metabolism following matched weight loss in overweight/obese participants. British Journal of Nutrition, 119(5), 507-516.

Carlson O, Martin B, Stote KS, Golden E, Maudsley S, Najjar SS, Ferrucci L, Ingram DK, Longo DL, Rumpler WV, Baer DJ, Egan J, Mattson MP. Impact of reduced meal frequency without caloric restriction on glucose regulation in healthy, normal-weight middle-aged men and women.

Metabolism. 2007 Dec;56(12):1729-34.

Cerqueira F.M., da Cunha F.M., Caldeira da Silva C.C., Chausse B., Romano R.L., Garcia C.C., Garcia C.C., Colepicolo P., Medeiros M.H., Kowaltowski A.J. Long-term intermittent feeding, but not caloric restriction, leads to redox imbalance, insulin receptor nitration, and glucose intolerance. Free Radic. Biol. Med. 2011;51:1454–1460. doi: 10.1016/j.freeradbiomed.2011.07.006.

Harris et al. (2018). Intermittent fasting interventions for treatment of overweight and obesity in adults: a systematic review and meta-analysis. JBI Database of Systematic Reviews and Implementation Reports. 16(2):507–547

Jamshed H, Beyl RA, Della Manna DL, Yang ES, Ravussin E, Peterson CM.

Early Time-Restricted Feeding Improves 24-Hour Glucose Levels and Affects Markers of the Circadian Clock, Aging, and Autophagy in Humans. Nutrients. 2019 May 30;11(6). pii: E1234. doi: 10.3390/nu11061234.

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