Gastrointestinal (GI) symptoms are common during endurance exercise, such as long-distance running and triathlons. In most studies, incidence rates vary from 10 to 81%, depending on factors such as type, duration, and intensity of the exercise studied (8,19,24,26).
The frequency of GI symptoms is much higher during running than during other endurance sports like cycling, rowing, or swimming, where the up and down movements are more limited. In a previous study, it was shown that during a protocol of alternately cycling and running, the number of subjects with one or more GI symptoms increased exponentially with time, whereas running increased the chance for GI symptoms in comparison to cycling (14).
To our knowledge, only one study investigated GI symptoms in relation to exercise of relatively low intensity. Robertson et al. (20) studied six men, who walked daily 37 km on 4 consecutive days. They found no significant increase in fecal blood loss, although no other GI symptoms were studied.
The purpose of the present study was to examine the incidence, duration, and severity of several GI symptoms during exercise of relatively low intensity and long duration, i.e., a 4 days' walk with a daily distance of, on average, 51 km for men and 41 km for women. This yearly event is called the "Nijmeegse Vierdaagse," performed in the surroundings of Nijmegen (The Netherlands).
In addition to exercise, many other factors like age, gender, training status, dehydration, and dietary factors could be associated with GI symptoms (4,8,15,19,26), whereas these symptoms may limit exercise performance (13,24). Therefore, we examined these above mentioned factors in relation to GI symptoms and investigated whether GI symptoms contribute to the drop-out during prolonged walking.
SUBJECTS AND METHODS
Subjects. The study was performed before and during the 1994 "Nijmeegse Vierdaagse." The daily distances were 50.5, 49.6, 48.9, and 53.8 km for men and 40.4, 41.6, 39.4, and 42.6 km for women, respectively. Male subjects started between 4:00 and 4:45 a.m., and female subjects started between 6:00 and 6:45 a.m.
Originally, a questionnaire (see Methods) was sent to a random sample of all 30- to 49-yr-old participants (237 men and 243 women) who were registered at least 10 wk before the event. This sample was stratified according to age and experience. Three age groups were distinguished: 30-36, 37-42 and 43-49 yr. Experience was defined as the number of prior participations in this event, and four subgroups were formed: never, once or twice, three to five, and six or more times. Questionnaires were returned by 269 subjects (response rate 56.0%) from whom 217 subjects were willing to participate in the present study. Further, a diary (see Methods) was sent to the subjects who were willing to participate. Complete responses (questionnaire and two diaries, see Methods) were received from 76 women and 79 men, a response rate of 33.2%. This rate did not differ between the 12 (4 × 3) subgroups. Statistical analysis showed that the respondents (N = 155) did not differ from nonrespondents regarding gender, age, and number of prior participations in the event.
Mean (± SD) age, body weight, and height for the female and male subjects were 40.9 (±5.7) and 39.8 (±5.7) yr, 64.1 (±8.8) and 76.9 (±10.2) kg and 166.9 (±6.3) and 180.2 (±6.2) cm, respectively. After approval of the ethical committee of the Department of Medical Physiology and Sports Medicine, Utrecht University, all subjects were informed of the nature, general purpose, and possible risks of the study, before giving their written consent to participate.
Methods. Data were collected by using a questionnaire before the "Vierdaagse van Nijmegen" and by using a diary before as well as during the event. The questionnaire included questions on personal data (e.g., body weight, height, profession, and health), on the physical activity pattern (type, frequency, and duration of activity at work and during leisure time) during January-June 1994, and on knowledge and attitude toward sports participation and diet.
In the diaries the following data were registered:
- Daily 24-h physical activities (type, frequency, duration, and intensity) according to a modified version of the Bouchard method (3). Based on these data walking speed and energy expenditure were calculated.
- Dietary intake, both during the 3 days preceding the event and during the event, using the dietary record method (25) and a computerized version of the Netherlands nutrient data bank (10). All dietary records were checked by trained dietitians.
- Incidence, duration, and severity of all GI symptoms and other symptoms (see Table 1) occurring both during the 3 days preceding the event and during the event. Duration of symptoms was indicated by the subjects who indicated the beginning and ending of each symptom. Severity of the symptom was evaluated using a 3-point scale (1 = I had a GI symptom, but could continue walking with no time-loss; 2 = the GI symptom delayed my walk temporarily; 3 = the GI symptom was so serious that I had to end my walk).
Finally, body weight was measured to the nearest 0.1 kg on a digital scale, just before and immediately after each walk. Subjects were measured in the same clothes before and after each walk.
Statistical analysis. The relationship between each GI symptom (incidence and duration) and all measured variables was tested using univariate analysis (Mann-Whitney's nonparametric test for independent data and Fisher's exact test) by comparing subjects with and without a particular GI symptom. Significance was accepted at P < 0.05 level. Furthermore, using multivariate analysis (logistic regression analysis), the most important factors related to each GI symptom and related to finishing/not-finishing were established, as well as the relative importance of these factors. Relative importance is given by the exp(B), which indicates the factor by which the odds change when the independent variable increases by one unit (2). All data are presented as means ± standard deviation (SD).
Climate conditions and walking speed. Daily climate conditions were as follows. Relative humidity was highest (90%) during the morning of the 4 days when temperature was relatively low (14-15°C) and lowest in the afternoon (36-39%), when temperature was relatively high (26°C on days 1 and 2, 30°C on days 3 and 4).
Men finished after 5.5 to 12.2 h and women finished after 7.2 to 10.8 h. Mean walking speed on days 1-4 was 6.9 ± 1.0, 6.2 ± 0.7, 6.1 ± 0.8, and 6.5 ± 0.8 km·h−1 for the men (N = 74) and 6.7 ± 0.9, 5.8 ± 0.5, 5.1 ± 0.5 and 5.6 ± 0.6 km·h−1 for the women (N = 72), respectively.
Incidence, duration, and severity of GI-related symptoms. The incidence of each GI symptom among the subjects during the 3 days preceding the event was 1% at the most, and the severity of these symptoms did not influence daily activity.
Incidence of the various GI symptoms during the 4 walking days is shown in Table 1. As vomiting hardly occurred (only once at day 1), this symptom was not depicted. The incidence of all GI symptoms was not significantly different between days.
On average, 24% of the walkers experienced one or more symptoms. The highest number was 29% on day 2, but this was not significantly different from other days (19-27%). Without the symptoms muscle cramps, exhaustion and headache, incidence of "real" GI symptoms ranged from 18% (day 3) to 25% (day 2). Nausea, headache (especially on day 1), and flatulence occurred in 5% or more of the subjects. Flatulence was the most prominent one.
The duration of the various GI symptoms during the 4 days showed a large variability between subjects (see Table 2). Moreover, the frequency distribution of the data was skewed to the right, as only a few subjects had a long duration of GI symptoms. Nausea and flatulence differed significantly in duration between days. In the group of finishers (N = 146), they lasted longer on day 2 than on day 4. Other differences in duration were not significant.
Only a small number of subjects indicated that symptoms were so serious that the walk was delayed (four, five, four, and five subjects on days 1 to 4, respectively). Nine subjects dropped out of whom two subjects indicated that this was due to one or more GI symptoms (see Table 3 and see last paragraph of the results).
Relationships between various GI symptoms and measured variables. Because only a small number of subjects indicated that symptoms were so serious that the walk was delayed or ended, only incidence and duration of GI symptoms were further analyzed. As duration is reflected by continuous numerical values and incidence is not, duration was preferred in further analyses. Of the various factors under study, gender, exercise experience, body weight loss, and dietary intake before and during exercise were related to GI symptoms.
A gender effect was only noted for flatulence on day 1, which lasted significantly longer in the male subjects (37.6 ± 106.8 min) than in the female subjects (7.9 ± 52.2 min).
A significant relationship between the incidence of GI symptoms and experience could not be found for the total group but could be observed for the nonfinishers: six of seven (86%) subjects who experienced one or more GI symptoms on day 1, participated for the first time. In this subgroup the same was found for days 2-4. In comparison, four of 34 (12%) finishers with GI symptoms on day 1 participated for the first time.
Body weight loss is shown in Table 4. The threshold at which subjects suffered significantly more from GI symptoms was about 2.5% body weight loss. On day 1, subjects with a weight loss of 2.5% or more experienced a longer duration of exhaustion (P < 0.05) than subjects with a weight loss less than 2.5% (19.5 ± 30.1 vs 0 min). On day 2, a weight loss above 2.5% was significantly associated with a longer duration of side ache (10.0 ± 34.6 vs 3.9 ± 29.3 min), on day 3 with a longer duration of flatulence (160.0 ± 277.1 vs 26.9 ± 98.8 min), and on day 4 with a longer duration of muscle cramps (13.3 ± 40.0 vs 0.5 ± 4.3 min).
Dietary intake is shown in Table 5. On average, men and women obtained 14.5% of the energy intake from protein, 29.6% from fat and 51.6% from carbohydrates. During the walk, energy intake from carbohydrates was significantly higher and from protein and fat significantly lower than those of the whole day.
Because several dietary components were interrelated, it is difficult to determine which factors are the most important ones for the occurrence of the GI symptoms. For instance, subjects who had a large energy intake also consumed large amounts of protein, carbohydrate, fat, and dietary fiber (Pearson's correlation coefficients were 0.61, 0.90, 0.60, and 0.47, respectively; P < 0.01). Furthermore, subjects who obtain their energy intake (En%) mainly from fat, will generally obtain less energy from carbohydrates (correlation coefficient between En% from carbohydrates and En% from fat = −0.87). We compared dietary factors between subjects with and without GI symptoms. Only symptoms which were present in six or more subjects during the 4 days were further evaluated, e.g., nausea, flatulence, headache, and muscle cramps. The results will be given qualitatively for days 1-4 and are shown in Table 6. For day 1-4, the dietary variable (right column) that is significantly related to a symptom (left column) is shown. For instance, on day 1, subjects with nausea had a significantly lower water intake before the walk than subjects without this symptom.
The results of the multivariate analyses will not be presented, because a correct prediction of all GI symptoms was less than 50%. Therefore, we could not differentiate between underlying factors of primary and secondary importance, and this may explain why so many dietary factors have been identified in relation to GI symptoms (Table 6).
Relationship between finishing/not-finishing and measured variables. As stated in Subjects and Methods, the respondents did not differ from nonrespondents with respect to gender, age, and experience. In the respondent group, 9 subjects were not able to complete the 4 days' walk, indicating a drop-out rate of 5.8% (men 6.3%, women 5.3%). The drop-out rate in the nonrespondents was 9.5%: 12.6% for men and 5.8% for women. Differences between respondents and nonrespondents were significant for the whole group and for men.
Using logistic regression analyses, we compared all measured variables of 9 nonfinishers with those of 146 finishers. Only data from day 1 and from the day of drop-out were analyzed, because on days 2-4 the number of nonfinishers and the number of data were too limited.
The regression model predicted the factor finishing/not finishing on day 1 correct for 98.6% (not finishing 85.7%, finishing 99.3%), and three predictors were found with an exp(B) shown between brackets: 1) occurrence of one or more GI symptoms (0.0069); 2) walking speed (0.0008); 3) relative fiber intake (in g·MJ−1) during the walk (9.0068).
This means that the occurrence of one or more GI symptoms significantly reduced the probability for finishing. An exp(B) of 0.0069 (i.e., 1/145) indicates that the odds of having a GI symptom versus not having a GI symptom is 145 times smaller for finishers than for nonfinishers. This finding is also shown in the univariate analysis. Seven of nine nonfinishers experienced GI symptoms on day 1 (78%) as compared with 41 of 146 (28%) in the group of finishers. The number of nonfinishers with one or more symptoms was relatively higher on all days in comparison with the finishers (P < 0.05). Furthermore, the following symptoms lasted longer in the nonfinishers than in the finishers (P < 0.05): GI cramps on day 1; bloating, exhaustion, and muscle cramps on day 1 and 2; and stomach ache on day 2 and headache on day 3.
On the day of drop-out, the regression model predicts the factor finishing/not finishing correct for 98.6% (not finishing 75.0%, finishing 100.0%) and again three predictors were found, namely occurrence of one or more GI symptom with an exp(B) of 0.0083, i.e., 1/120, walking speed (0.0041), and amount of fat (in g) ingested before the walk (1.1317). This also shows that having one or more GI symptoms is positively associated with drop-out. Further, the effect of GI symptoms on drop-out is of the same order of magnitude as on day 1.
Selection bias. As both the questionnaire and the diary included various topics besides GI symptoms, e.g., personal data, diet, physical activity pattern, and knowledge and attitude toward sports participation and diet, a possible selection bias toward subjects with GI symptoms is not to be expected. However, as we observed a lower rate of subjects who dropped from the race in the male respondents in comparison with the male nonrespondents, it cannot be ruled out that the respondents were more interested in and may have known more about the issues under study. Therefore, the drop-out rate in the respondents may have been underestimated.
Incidence, duration and severity of GI-related symptoms. Research on exercise-related symptoms of the GI tract has mainly been limited to high-intensity sports. During marathons and triathlons, generally 20-50% of the competitors experience one or more GI symptoms, which often may impair exercise performance (4,24). Exercise intensity has been shown to influence the incidence of GI symptoms (5,20,22,23,26). Several factors hypothesized to influence GI symptoms during exercise are dependent of intensity. As the intensity increases up to 70-75% O2max, gastric emptying is delayed (6,9,18) and splanchnic blood flow decreases (21). Also other factors, such as mechanical trauma of the gut (16), intestinal permeability (11), absorption (18), and neuroendocrine changes are affected by exercise intensity.
In the present study, on average 24% of the subjects indicated one or more GI symptoms, but these symptoms were seldomly severe. Only two of nine nonfinishers indicated GI symptoms as a performance limiting factor. Nevertheless, logistic regression analysis revealed that the incidence of GI symptoms was a significant performance limiting factor. An overall lower incidence and severity of GI symptoms during walking may be expected as the intensity of walking is much lower than in marathons or triathlons. This implies that gastric emptying, splanchnic blood flow and the other factors mentioned are probably not affected by exercise to the same extent as during high intensity exercise. An absolute indication of exercise intensity of walking in the present study is not available. However, an average speed of 6.1 km·h−1 indicates an energy expenditure of 18 kJ·kg−1 body weight per hour, whereas running about 3 times faster (approx. 18 km·h−1) costs about 5 times more energy (1).
In addition to exercise intensity, the duration of prolonged exercise is a factor related to GI symptoms (14). In general, no differences in incidence, duration, or severity of GI symptoms were found between days 1 and 4. This indicates that the nonexercise periods between walking days were sufficient for recovery from previous exercise and that a (cumulative) effect of exercise duration on GI symptoms was, therefore, not observed.
Relationships between various GI symptoms and measured variables. Next to intensity, duration, and type of exercise, several other factors have been suggested to be associated with the incidence of GI symptoms (4). These factors, however, have been analyzed only during high-intensity exercise. We found exercise experience, several dietary variables before and during exercise, and body weight loss to be important for GI symptoms during low-intensity exercise with a long duration.
In the present study, age did not influence GI symptoms. It has been suggested that not age itself but the lack of experience may explain the higher incidence in GI symptoms in younger athletes (15). In general, older subjects will have more experience with walking conditions than younger ones. Further, a self-selection may have taken place, whereby walkers who are predisposed to GI symptoms discontinue walking before they ever became experienced walkers (15). We have found that, only for the nonfinishers, subjects without walking experience reported GI symptoms more often than subjects with experience. This suggests that inexperienced subjects were at risk both for drop-out and for GI symptoms, perhaps indicating that they do not have an adequate adaptation to the disruption of the daily routine, and/or a good coping behavior against (mental) stress.
Another factor related to GI symptoms is dietary intake. We found that nearly all GI symptoms, present in six or more subjects, were significantly related to one or more dietary components. However, because of the absence of logistic regression models with an acceptable goodness of fit, it was not possible to determine which dietary components are the most important ones, as they were interrelated. Nevertheless, univariate analyses suggest that subjects consuming a diet with a high contribution of fat and protein experienced GI symptoms more often.
During high-intensity exercise, a significant relationship between dietary intake and GI symptoms has been found in triathletes (17). Intake of large amounts of fat, protein, dietary fiber, and hypertonic beverage before and during a half triathlon induced GI symptoms. Because all these factors decrease gastric emptying, this observation may be based on this phenomenon. Our data revealed not only a relationship between these dietary factors and several GI symptoms, but also for potassium and alcohol intake. In a previous study (14), a relationship between a high serum potassium concentration and nausea was observed. Apparently, both dietary potassium intake and serum potassium levels influence GI symptoms, but the underlying mechanism still remains unexplained.
It is well-known that alcohol consumption can be associated with GI symptoms, such as diarrhea or nausea (7). We observed that alcohol consumption during the evening before the walking day resulted in flatulence, whereas consumption during walking also resulted in flatulence, and also in muscle cramps. Acute and chronic effects of alcohol on the GI tract include increased mucosal permeability, promotion of bacterial overgrowth, altered gut motility, and impaired absorption of water, salt, vitamins, glucose, and fructose (12).
A significant relationship between GI symptoms and body weight loss was found. Rehrer et al. (15) found that more than 3.5% body weight loss after a marathon was associated with an increased incidence of GI symptoms during exercise. We found a threshold of 2.5% body weight loss, which may indicate that the threshold for inducing GI symptoms is a function of both intensity and duration of exercise. Weight loss can mainly be explained by a disbalance between fluid intake and sweat rate. Despite a considerable variability in daily fluid intake (from 200 to 4500 mL), significant relationships with body weight loss and GI symptoms were absent. Apparently, the water intake and water loss by evaporation were not related. Furthermore, subjects who became aware of a relatively high body weight loss during measurement may have adjusted their fluid intake. This may explain why body weight loss was highest on the first day, although inaccuracy in the measurement of weight loss or fluid intake may have played a role as well. For example, body weight was measured with clothes, and if sweat was present in the clothing after the walk, an underestimation of body weight loss could be expected.
We conclude that during walking at relatively low intensity and long duration, GI symptoms do occur, but less frequently and with a lower severity than during prolonged exercise of high intensity. Nevertheless, GI symptoms may impair exercise performance significantly. Especially factors such as walking experience, body weight loss during walking, and several dietary components before and during walking are significantly related to GI symptoms.
On the basis of the findings of the present study, we recommend that to avoid GI symptoms during long-distance walking consumption of water during walking should be emphasized to safeguard a body weight loss under 2.5%. Furthermore, subjects should abstain from a high intake of fat, protein, and especially alcohol during the walking days.