Fasting + Weight Lifting: Expert Interview with Grant Tinsley, PhD
I recently interviewed Dr. Grant Tinsley, PhD, Associate Professor and Director of their Energy Balance and Body Composition lab at Texas Tech. Dr. Tinsley is leading the charge on almost everything we know about intermittent fasting, and more specifically time-restricted feeding (TRF*), used alongside resistance training (RT), which is a hot topic! His answers to my questions below are invaluable and very thoughtful so I am really excited to be posting this here. I am also a massive fan of Grant’s work, so it’s really cool that I was able to connect with him and share his knowledge with whoever stumbles upon this blog.
*TRF is a form of intermittent fasting that restricts all food intake to a condensed “eating window” of 12-hours or less, thus allowing a daily fast of at least 12-hours. The most popular way of following TRF is 16:8, which involves an 8-hour eating window, followed by a 16-hour daily fast. Grant’s research has shown that 16:8 TRF does not interfere with strength or muscle gains when combined with resistance training and that it may produce more favourable changes in body composition compared to an extended eating window without compromising performance.
Kristi: I believe in all of your studies of RT + TRF, participants train within their eating window. How important do you think this variable is, and do you think the results may have been different if they had been training fasted? Put another way, do you think that fasted exercise paired with a condensed eating window may compromise adaptations to training (e.g., MPS)? How would you recommend timing resistance training with TRF to maximize positive changes in body composition? Would these recommendations change if this question addressed endurance training?
Dr. Tinsley: Yes, that is correct. We essentially tried to incorporate traditional best practices for sports nutrition by requiring pre-exercise fuel and post-exercise nutrients to initiate recovery and adaptation. In that regard, the studies we have performed to date have not directly addressed the effects of time-restricted feeding with accompanying fasted training.
In terms of how different the results would have been if individuals were fasted during their training sessions, I think this would likely depend on the exercise training status of the participant, personal preferences about fasted training itself that could influence performance, and perhaps when the feeding window was initiated post-exercise. In general, the less exercise-trained someone is, the more you can “get away with” in terms of having some aspects of a program that wouldn’t be textbook ideal while still seeing meaningful progress. In our studies, participants have ranged from recreationally active to highly trained. My guess is that fasted training would have less of an influence on those with less training experience but could possibly have a more detrimental impact on those who are more trained (i.e., closer to their peak body composition and performance). With that said, personal perceptions are also important. If someone thinks they will perform worse in the gym when training fasted, they probably will. On the other hand, some feel better when training fasted and don’t have negative perceptions that could impair their performance. Of course, the type of exercise being performed, as well as the diet in the days leading up to the bout of exercise, influences this as well. Forms of exercise that are more glycogen-dependent could be impaired to a greater extent by fasted training, particularly if the fasted training is performed after a longer period of fasting or in combination with a low-carbohydrate diet. How soon after training the feeding window begins also matters. I think more detrimental effects would be observed if someone trained fasted and remained fasted for several hours after exercise. This essentially keeps you in a negative muscle protein balance and misses some critical periods for stimulating muscle protein synthesis through nutrient ingestion and initiating muscular adaptation. On the other hand, fasted training may be less of a concern if someone begins their feeding window as soon as possible after training.
For most of the population, I view fasted training as a matter of personal preference. For example, some people may train early in the morning and feel nauseous if they choke down breakfast minutes before a tough workout, or they find themselves losing an extra hour of sleep so they can eat early enough to feel fine during a workout. Some of these individuals prefer exercising fasted from a practical standpoint. For the general population who is just exercising to be healthy, feel better, and even look better, I think that fasted versus fed training should simply be a matter of what fits their schedule, makes them feel best during their workout, and supports their desired performance. For high-level athletes, the conversation is a bit different. I generally recommend that when considering deviations from sports nutrition best practices, more caution is used in higher level athletes. These individuals have smaller margins for error and more to lose than the general population.
In terms of minimizing detrimental effects to exercise adaptations, the diet consumed during feeding periods should contain enough protein and other nutrients to support recovery. Even in weight loss scenarios, sticking to a more moderate energy deficit – rather than an extreme one – may also help support exercise adaptations.
In the end, if your number one goal is performance, I would recommend exercising in the middle of your feeding window if possible. Similar to what we did in our studies, this would allow pre-exercise fuel and post-exercise nutrients in line with best practices based on decades of sports nutrition research. For body composition – particularly weight loss and fat loss – I think there is more flexibility. With that said, it is possible – but not fully established – that muscle could be better gained or maintained with exercising in a fed state. I think this is still an open research question in the context of modern intermittent fasting programs though. Again, the duration of fasting before fasted exercise and how soon after exercise the fast is broken would likely be meaningful here, alongside the type of exercise and individual’s training status.
In terms of endurance training, I will briefly mention a couple of things. Some athletes intentionally use fasted training or other low-carbohydrate availability training for select training sessions as a form of “training low.” The idea here is that training with low carbohydrate availability can promote some impressive cellular adaptations relevant to endurance performance. With that said, these athletes are essentially willingly compromising their performance on select sessions – or just performing lighter intensity sessions – in order to promote these cellular adaptations. When maximal performance is needed, such as during the most intense training sessions and competitions, fasting should not be considered in my opinion. Those are scenarios when the pre-exercise fueling component is critical for optimal endurance performance.
Kristi: Besides timing which we touched on above, are there any special considerations for those trying to maximize muscle gains while also getting the benefits of TRF? For example, because TRF can lead to a spontaneous reduction in caloric intake, do you think that those with goals to improve strength, performance, and muscle gains have to be very cognizant of what and how much they eat during their eating window?
Dr. Tinsley: I do think there are scenarios in which those who want to improve strength, performance, and muscular gains should be cognizant of what and how much they eat during their eating window. As usual, there are some important contextual factors here. One is the intensity of the fasting program. If you take someone who typically eats over a 10-hour period each day and put them on a time-restricted feeding program with an 8-hour window, it isn’t likely that this will cause a massive reduction in energy intake unless there are other dietary modifications. In this sense, we likely wouldn’t be concerned that the individual isn’t receiving enough caloric support for building muscle and improving performance. With more intense programs, this would be a greater challenge. As a more extreme example, someone following a one-meal-a-day (OMAD) program would likely have more difficulty consuming the right amount of energy, protein, and other nutrients to optimally support their goals.
To dive into this a bit more, here are some things to think about if you’re in this situation. First, as described above, different “intensities” of time-restricted feeding (or other forms of intermittent fasting) will lead to different degrees of reduction in spontaneous energy intake. Importantly, there is also quite a bit of variability in how much an individual will reduce their energy intake. Some individuals don’t end up reducing energy intake at all but rather use TRF as a weight maintenance strategy. Second, there are several important items that can be addressed within the feeding window to promote optimal adaptations. Not having too large of a calorie deficit is one. While the numbers can vary, limiting the deficit to no more than ~300 to 500 calories per day, relative to your weight maintenance intake, would be a decent place to start. A major caveat here is that some people use TRF because they don’t want to count calories. In that case, monitoring body mass and ensuring that you aren’t losing weight too rapidly could be another way to look at this. Protein intakes of about 1.6 to 2.0 grams per kilogram of body weight have been shown to optimally support muscular adaptations and promote recovery. The ratio of carbohydrates and fats can be fairly flexible in most of the general exercising population. For some higher-level athletes or those who engage in specific forms of activity (e.g., long distance runners), the appropriate carbohydrate intake may become more critical. There are also a small number of dietary supplements with strong research support for improving muscular adaptations and performance. For those who struggle to achieve their target protein intake with food alone, high-quality protein supplements can be a useful tool. Creatine monohydrate is one of the best supported sports supplements, with decades of research support for its ability to improve strength and power performance and support lean mass increases over time. Some other substances, such as caffeine, can help improve exercise performance. This can lead to better exercise adaptations over time. Of course, it is also critical to remember that the exercise training stimulus itself is the largest factor determining exercise adaptations. In addition to addressing the nutritional support for these adaptations, a progressive, well-designed exercise program is essential.
From a practical perspective, I think someone using a TRF program should simply “take stock” and see if they are making their desired progress in terms of performance and body composition. If so, it may be that nothing needs to be changed. If not, some of the items mentioned above could be addressed.
Kristi: Have you found any advantages to TRF in active individuals that are inherent to TRF itself?
Dr. Tinsley: When viewing our five studies to date (four in individuals performing resistance training and one in cyclists), we have not found strong enough evidence to indicate a clear, definitive advantage of TRF at the group level. A couple of our studies have shown potential body composition advantages, particularly fat loss with maintenance or an increase of lean mass. However, due to the challenges of precisely monitoring nutrition intake in free-living settings, we cannot be certain these improvements are due to TRF itself rather than simply the caloric deficit TRF often produces. In order to be certain of effects unique to TRF, independent of energy intake, we really need information from studies where participants are kept at a research facility for a prolonged period, with all foods provided and carefully monitored (i.e., metabolic ward studies). These studies are enormously expensive and difficult to conduct, and the likelihood of a funding organization supporting a study to look at TRF and exercise adaptations through a study like this is extremely small, in my opinion. So, we need to make the best judgments we can with the available data. All that to say, as of now, our research has indicated that TRF is a viable but not inherently superior option at the group level. However, there are certainly individuals who have either experienced great success or notable frustration with TRE programs. Some participants have reported that TRE is the easiest dietary program they have ever followed and that they plan to continue using it due to its simplicity and the results it provides them. Others feel hungry, irritable, and do not wish to restrict their eating period after the study concludes. So, I would say that TRF could have inherent benefits – or inherent detriments – for some individuals, largely due to personal preference and behavioral factors.
Kristi: In your research, have you observed any large variation in how participants respond to TRF, both objectively and subjectively? If so, are there any characteristics that make someone more likely to respond one way or another?
Dr. Tinsley: Yes, we have observed large variation in both objective and subjective responses to TRF. On the objective side, we have seen the full range from fantastic changes (i.e., loss of fat mass, gain of lean mass, improvement in muscle performance) to mediocre changes or no changes at all. On the subjective side, I somewhat answered this in my response to the previous question.
Determining which characteristics predispose someone to have success with TRF is a critical next research step based on the current data indicating that, at the group level, TRF is viable but may not be inherently superior to other methods of energy restriction. (As a side note, TRF can be employed without energy restriction.) While this hasn’t been fully explored, there have been a few observations that are relevant to this point. First, it has been demonstrated, by our lab and others, that individuals vary substantially in their energy intake after an acute period of fasting. We performed a study of 24-hour fasting in our laboratory, with the fast ending in the early evening. When participants were allowed unrestricted food access following the fast, some individuals only ate about 10% of their weight-maintenance needs. Others ate over 100% of their weight-maintenance needs, putting them in a caloric surplus for the day despite fasting for most of the day! On average, individuals ate about 60% of their weight-maintenance needs. So, one factor that could influence someone’s success with a fasting program, including TRF, is how much they tend to overeat or “compensate” following a period of fasting. Second, as mentioned previously, the degree to which a TRF or other fasting program deviates from an individual’s typical eating patterns is important to consider. The smaller the change from “normal,” the smaller changes in energy intake and body weight or composition should be expected. Third, it is essential to consider the feasibility of the program for a given individual. For any dietary intervention to be successful in the long term, it is imperative that an individual can actually adhere to the changes the intervention requires. For many individuals, shifting the timing of eating without explicitly changing what is eaten is a simple, sustainable strategy. This may not be the case for others. If you find that TRF is an easy way to reduce your energy intake – if that is indeed your goal – it may be a suitable strategy for you. Beyond these items, there are open research questions regarding whether there are more concrete, physiological factors may be involved in someone’s success with a TRF program.
Kristi: Given you have worked with both female and male subjects, are you aware of any sex differences in regard to TRF or are there any special considerations based on sex?
Dr. Tinsley: This is a great question. We have conducted studies in both females and males, but we have not yet included them both within the same study. So, for our own work, we can’t fully isolate the effect of sex since there have been some design differences between studies. From the big-picture perspective, we have clearly seen that TRF can be a viable strategy in groups of females and males. With that said, early research on some forms of fasting, particularly alternate day fasting, indicated that there could be some adverse effects on glucose tolerance in females but not males. However, this form of fasting involved recurring 36-hour fasts, which are longer than most people following TRF will perform. My guess is that similar adverse effects would not be seen with more moderate duration fasts (i.e., <20-24 hours). However, I would also recommend that females interested in performing TRE or other forms of fasting pay particular attention to how it affects their bodies, including menstrual function if applicable. There are also other special physiological states females may experience that would be viewed as a likely contraindication for fasting, such as pregnancy and breastfeeding. Overall, I think that certain individuals will find better success, either objectively or subjectively, with TRF; however, being female shouldn’t be viewed as an absolute contraindication to performing TRF unless there is a clear medical reason.
Kristi: Based on your research findings, do you think it is fair to say that overall energy and protein intake are more important than meal timing or frequency for driving changes in body composition and adaptations to strength training? If so, would this still apply if someone was only eating one meal a day (OMAD) and therefore fasting for up to ~23-hours per day? I.e., do you think the longer the daily fast, the greater the chance that one's adaptations to strength training may be compromised?
Dr. Tinsley: I do think it is fair to say that, in most situations, overall energy and protein intake are more important than meal timing or frequency for changes in body composition and exercise adaptations. There are a few caveats though. First, there is often a relationship between energy and nutrient intake and meal timing or frequency. This is evidenced, for example, by the fact that changing meal timing through TRE can lead to a reduction in energy intake. Because of this, a comparison where energy and nutrient intake are held constant but meal frequency or timing is manipulated may be somewhat artificial. Nonetheless, in such a situation, I think the research is clear that energy and nutrient intake is typically more important than timing.
As mentioned previously, the exercise intervention itself is typically the most critical factor when discussing exercise adaptations. The stimulus to elicit adaptation must be sufficient in order to see results. Someone could have the “perfect” nutritional program but see very poor exercise adaptations due to a poor exercise training program. Of course, if someone had an excellent exercise training program but poor nutritional support, adaptations would also be partially compromised. My perspective is that it is hard to argue with the priority of nutritional manipulation, specifically energy restriction, for reducing body mass and body fat. However, gaining lean mass and improving exercise performance are certainly predicated on the exercise stimulus, with optimal nutrition improving these outcomes further.
I do think that longer daily fasting periods increase the likelihood for suboptimal exercise adaptations, but I don’t think we know exactly where that line is. In our studies with feeding windows as short as ~7.5 hours each day, as compared to ~13 hours in a comparison group, we have seen no compromise of lean mass gain, increases in muscle size, or improvements in exercise performance. To date, we don’t know much about how a shorter feeding period, such as 4 hours each day or even OMAD, would affect adaptations. Based on studies examining muscle protein balance, it is reasonable to expect that only have one meal per day is suboptimal and that at least three maximal stimulations of muscle protein synthesis each day (i.e., intakes of ~20+ grams of high-quality protein) would be required to see optimal results. With that said, there are times when this type of research doesn’t directly align with what we see at the whole body or even whole muscle level. So, I think we need more research to tease out how short of a feeding window is too short to see the desired exercise adaptations. Like most items, there would be important contextual factors to consider, including the energy and nutrient composition during the feeding window, the training status of the participants, and the diet before switching to the aggressive TRE program.
Kristi: Are there any populations TRF would not be appropriate for? (For example, athletes with very high energy and/or protein requirements)
Dr. Tinsley: Yes, I do think there are some populations for which TRF would not be ideal. With that said, it also depends on the intensity of the intervention. For example, an athlete with high energy requirements may have a difficult time consuming an adequate number of calories in just 8 hours each day. However, if the athlete is currently eating for 16 hours each day (i.e., from the moment they wake up until bed), it is possible there would be no detriment to slightly reducing the feeding window to 12 to 14 hours. With that said, I typically recommend higher skepticism towards programs lengthening the daily fasting period in those for whom performance is key, particularly those who make their living through sports performance. In my view, the potential risks – even apparently small risks – may outweigh the benefits. While the general population may have more room for trial-and-error, those who are vocational athletes do not. Similarly, those whose primary purpose for their training and nutrition programs is optimal performance should be more cautious to deviate from recommended practices for sports nutrition.
Additional examples of those for whom TRF may not be ideal is those trying to gain weight, those who are pregnant or breastfeeding, those with tendencies towards disordered eating that could be exacerbated by this eating pattern, and those who simply don’t enjoy this pattern of eating.
Where to find Grant Tinsley: